0022-3565/08/3251-27–36$20.00
THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS
Copyright © 2008 by The American Society for Pharmacology and Experimental Therapeutics
JPET 325:27–36, 2008
Vol. 325, No. 1
131904/3316794
Printed in U.S.A.
Activation and Inhibition of Adenylyl Cyclase Isoforms
by Forskolin Analogs
Cibele Pinto, Dan Papa,1 Melanie Hübner, Tung-Chung Mou, Gerald H. Lushington,
and Roland Seifert
Department of Pharmacology and Toxicology (C.P., D.P.) and Molecular Graphics and Molecular Modeling Laboratory (G.H.L.),
University of Kansas, Lawrence, Kansas; Department of Pharmacology and Toxicology, University of Regensburg,
Regensburg, Germany (M.H., R.S.); and Center for Biomolecular Structure and Dynamics, University of Montana,
Missoula, Montana (T.-C.M.)
Numerous receptors for hormones and neurotransmitters
couple to the G-protein Gs␣ to stimulate adenylyl cyclase
(AC). AC catalyzes the conversion of ATP into the second
messenger cAMP, which regulates numerous body functions
including memory and learning, heart contractility, respiration, and lipolysis (Sunahara et al., 1996; Tang and Hurley,
1998; Hanoune and Defer, 2001). Traditionally, this signal
transduction cascade has been targeted pharmacologically by
receptor agonists or antagonists and by inhibitors of phosphodiesterases, catalyzing cAMP degradation (Iwatsubo et
al., 2006).
AC is an interesting pharmacological target for several
reasons. First, in many tissues, several receptors couple to
This study was supported by the Deutsche Forschungsgemeinschaft (Research Grant Se 529/5-1 to R.S.). M.H. is the recipient of a predoctoral fellowship of the Elite Graduate Student Program of the Free State of Bavaria.
1
Current affiliation: Biopharmaceutical Analysis, Aptuit, Kansas City,
Missouri.
Article, publication date, and citation information can be found at
http://jpet.aspetjournals.org.
doi:10.1124/jpet.107.131904.
dideoxy-forskolin, and, particularly, BODIPY-forskolin. In contrast,
BODIPY-forskolin acted as partial agonist at the other ACs. 1-Deoxy-forskolin analogs were devoid of agonistic activity at ACs but
antagonized the effects of FS in a mixed competitive/noncompetitive manner. At purified catalytic AC subunits, BODIPY-forskolin
acted as weak partial agonist/strong partial antagonist. Molecular
modeling revealed that the BODIPY group rotates promiscuously
outside of the FS-binding site. Collectively, ACs are not uniformly
activated and inhibited by FS and FS analogs, demonstrating the
feasibility to design isoform-selective FS analogs. The two- and
multiple-state models, originally developed to conceptualize ligand effects at G-protein-coupled receptors, can be applied to
ACs to explain certain experimental data.
Gs␣ and hence AC (Birnbaumer et al., 1990). Thus, AC integrates the input from several receptors and is localized at a
central position in the signaling cascade. Second, a therapeutic problem with receptor agonists is desensitization, resulting in a loss of drug efficacy during long-term treatment
(Penn et al., 2000). Third, mammalian cells express nine AC
isoforms with distinct regulatory properties and tissue distribution (Hanoune and Defer, 2001). Fourth, the genes for
several AC isoforms were successfully knocked out in mice,
and the resulting distinct phenotypes point to unique physiological functions of each AC isoform (Hanoune and Defer,
2001).
There are two principal approaches to manipulate ACs
pharmacologically. First, the catalytic site can be targeted by
nucleotides that inhibit AC either competitively or noncompetitively (Dessauer et al., 1999; Gille et al., 2004; Iwatsubo
et al., 2006; Mou et al., 2006). Recombinant AC1 (predominantly expressed in the brain) and AC5 (the major AC isoform in the heart) are inhibited more potently by certain
competitive inhibitors than AC2 (also predominantly ex-
ABBREVIATIONS: AC, adenylyl cyclase; FS, forskolin; Sf9, Spodoptera frugiperda; GTP␥S, guanosine 5⬘-[␥-thio]triphosphate; DMB, 7-deacetyl7-(N-methylpiperazino-␥-butyryloxy); DMSO, dimethyl sulfoxide; 6A7DA, 6-acetyl-7-deacetyl; 7DA, 7-deacetyl; 9d, 9-deoxy; 1d, 1-deoxy; 1,9dd,
1,9-dideoxy; 7DA1,9dd, 7-deacetyl-1,9-dideoxy; 7DA1d-FS, 7-deacetyl-1-deoxy.
27
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ABSTRACT
Adenylyl cyclase (AC) isoforms 1 to 9 are differentially expressed
in tissues and constitute an interesting drug target. ACs 1 to 8 are
activated by the diterpene, forskolin (FS). It is unfortunate that
there is a paucity of AC isoform-selective activators. To develop
such compounds, an understanding of the structure/activity relationships of diterpenes is necessary. Therefore, we examined the
effects of FS and nine FS analogs on ACs 1, 2, and 5 expressed
in Spodoptera frugiperda insect cells. Diterpenes showed the
highest potencies at AC1 and the lowest potencies at AC2. We
identified full agonists, partial agonists, antagonists, and inverse
agonists, i.e., diterpenes that reduced basal AC activity. Each AC
isoform exhibited a distinct pharmacological profile. AC2 showed
the highest basal activity of all AC isoforms and highest sensitivity
to inverse agonistic effects of 1-deoxy-forskolin, 7-deacetyl-1,9-
28
Pinto et al.
Materials and Methods
Materials. Baculoviruses encoding ACs 1, 2, and 5 were a gift
from Drs. G. Gilman and R. K. Sunahara (University of Texas Southwestern Medical Center, Dallas, TX). Wild-type baculovirus was
prepared using the BaculoGOLD transfection kit (BD PharMingen,
San Diego, CA). Sf9 insect cells were from the American Type Cell
Culture Collection (Rockville, MD). Recombinant cytosolic C1a domain from canine AC5, referred to as C1, the C2a domain from rat
AC2, referred to as C2, and Gs␣ were purified as described previously
(Tesmer et al., 2002). Gs␣ was activated with guanosine 5⬘-[␥-thio]triphosphate (GTP␥S) as described previously (Tesmer et al., 2002).
FS was from LC Laboratories (Woburn, MA). DMB-FS was from
Calbiochem (La Jolla, CA). BODIPY, BODIPY-FS, and SF 900 II
medium were from Invitrogen (Carlsbad, CA). All other FS analogs
were from Sigma-Aldrich (St. Louis, MO). Stock solutions of FS, FS
analogs, and BODIPY (10 mM each) were prepared in DMSO and
stored at ⫺20°C. Dilutions of FS analogs were prepared in such a
way that in all AC assays, a final DMSO concentration of 3% (v/v)
was achieved. [␣-32P]ATP (800 Ci/mmol) was purchased from
PerkinElmer (Wellesley, MA). Neutral alumina (super I, WN-6) was
from Sigma. Fetal bovine serum was from Atlas Biologicals (Ft.
Collins, CO).
Cell Culture and Membrane Preparation. Cell culture and
membrane preparation were performed as described previously
(Seifert et al., 1998). In brief, Sf9 cells were cultured in SF 900 II
medium supplemented with 5% (v/v) fetal bovine serum and 0.1
mg/ml gentamicin. High-titer baculovirus stocks were generated
through two sequential amplification steps as described previously
(Seifert et al., 1998). In each amplification step, the supernatant
fluid was harvested and stored under light protection at 4°C. For
membrane preparation, Sf9 cells (3.0 ⫻ 106 cells/ml) were infected
with correspondent baculovirus encoding different mammalian ACs
(1:100 dilutions of high-titer virus stocks unless stated otherwise)
and cultured for 48 h. For experiments shown in Table 2, we also
used 1:10 and 1:300 dilutions of virus stocks. Membranes expressing
each construct and membranes from uninfected Sf9 cells were prepared as described previously (Seifert et al., 1998). In brief, cells
were harvested, and cell suspensions were centrifuged for 10 min at
1000g at 4°C. Pellets were resuspended in 10 ml of lysis buffer (1 mM
EDTA, 0.2 mM phenylmethylsulfonyl fluoride, 10 g/ml leupeptin,
and 10 g/ml benzamide, pH 7.4). Thereafter, cells were lysed with
20 to 25 strokes using a Dounce homogenizer. The resultant cell
fragment suspension was centrifuged for 5 min at 500g and 4°C to
sediment nuclei. The cell membrane-containing supernatant suspension was transferred into 30-ml tubes and centrifuged for 20 min at
30,000g and 4°C. The supernatant fluid was discarded, and cell
pellets were resuspended in buffer consisting of 75 mM Tris/HCl,
12.5 mM MgCl2, and 1 mM EDTA, pH 7.4. Membrane aliquots of 0.5
to 1 ml were prepared and stored at ⫺80°C, and protein concentration for each membrane preparation was determined using the BioRad DC protein assay kit (Bio-Rad, Hercules, CA).
AC Activity Assay. AC activity was determined essentially as
described in the literature (Gille et al., 2004). Before experiments,
membranes were sedimented by a 15-min centrifugation at 4°C and
15,000g and resuspended in 75 mM Tris/HCl, pH 7.4. Reaction
mixtures (50-l final volume) contained 30 g of membrane protein,
40 M ATP/Mn2⫹ plus 10 mM MnCl2 and FS or FS analogs at
various concentrations in the presence of 3% (v/v) DMSO. With the
exception of some experimental conditions shown in Fig. 2, reaction
mixtures also contained 10 M GTP␥S. After a 2-min preincubation
at 37°C, reactions were initiated by adding 20 l of reaction mixture
containing (final) 1.0 to 1.5 Ci/tube [␣-32P]ATP, 0.1 mM cAMP, and
a regenerating system consisting of 2.7 mM mono(cyclohexyl)ammonium phosphoenolpyruvate, 0.125 IU of pyruvate kinase, and 1 IU of
myokinase. Reactions were conducted for 20 min at 37°C and were
terminated by adding 20 l of 2.2 N HCl. Denatured protein was
precipitated by a 1-min centrifugation at 25°C and 15,000g. The
supernatant fluid (65 l) was applied onto disposable columns filled
with 1.3 g of neutral alumina. [32P]cAMP was separated from
[␣-32P]ATP by elution of [32P]cAMP with 4 ml of 0.1 M ammonium
acetate, pH 7.0. Recovery of [32P]cAMP was ⬃80% as assessed with
[3H]cAMP as standard. [32P]cAMP was determined by liquid scintillation counting using Ecolume scintillation cocktail (Fisher Scientific
Co., Pittsburgh, PA). Data were analyzed by nonlinear regression
using the Prism 4.02 program (GraphPad Software, Inc., San Diego,
CA). In experiments with purified AC catalytic subunits, reaction
mixtures contained 3 nM C1, 15 nM C2, 51 nM GTP␥S-activated Gs␣
and 1.0 Ci/tube [␣-32P]ATP, 0.1 mM cAMP, 100 mM KCl, and 25
mM HEPES/NaOH, pH 7.4. Reactions were conducted for 20 min at
30°C and were terminated by adding 20 l of 2.2 N HCl (Mou et al.,
2006).
Molecular Modeling. The crystal structure 1CJK of mammalian
AC subunits C1 and C2 was chosen as a template for modeling the
dynamics of BODIPY-FS interaction with the enzyme (Tesmer et al.,
1999). The structure was edited in SYBYL (The Tripos Associates,
St. Louis, MO) as follows. The nucleotide C1/C2 inhibitor was deleted
from the nucleotide-binding site, the BODIPY group was covalently
attached to FS according to a conformation that was chosen so as to
initially avoid direct clashes with receptor residues (the initial
BODIPY orientation pointed outwards toward the mouth of the
receptor), and protons were added to both ligand and receptor ac-
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pressed in brain) (Gille et al., 2004), and selective AC5 inhibitors are of potential value in the treatment of heart
failure (Rottlaender et al., 2007).
Second, the diterpene, forskolin (FS), isolated from the roots
of the Indian plant Coleus forskohlii, is a very effective activator
of ACs 1 to 8 but not of AC9 (Seamon and Daly, 1986; Tang and
Hurley, 1998). FS binds to a defined hydrophobic pocket close to
the catalytic site of AC (Tesmer et al., 1997; Tang and Hurley,
1998). FS is of interest for the treatment of various disorders,
including heart failure, bronchial asthma, obesity, and glaucoma, but the development of FS analogs as drugs has been
hampered by the unavailability of isoform-selective FS analogs
(Seamon and Daly, 1986; Laurenza et al., 1989; Robbins et al.,
1996; Onda et al., 2001). Another major problem in the field is
the fact that we still do not know whether the FS-binding site in
ACs is of any physiological relevance and whether there is an
endogenous physiological ligand for this site. Very recently, FS
has been identified in the cyst fluid of patients with polycystic
kidney disease, but it is unknown whether FS was produced
endogenously or administered exogenously by FS-containing
herbal medicines (Putnam et al., 2007). Finally, FS is a very
hydrophobic compound, and because of limitations of test systems with respect to organic solvent compatibility, it was often
difficult to generate saturated concentration/response curves
for diterpenes, rendering calculation of ligand potencies and
efficacies ambiguous.
The aim of the present study was to systematically characterize the effects of FS and nine FS analogs on AC. Figure
1 shows the structures of FS and the FS analogs studied. FS
analogs differ from each other in the OH substitution of C1
and C9, acetyl substitution at C7 or C6, and the type of
substituent at C7. We determined the catalytic activity of
mammalian AC isoforms 1, 2, and 5 expressed in Spodoptera
frugiperda (Sf9) insect cell membranes. As a control, we also
studied the endogenous insect cell AC. Moreover, we examined the interaction of one particularly interesting FS analog,
BODIPY-FS, with purified catalytic subunits of AC and conducted molecular modeling studies with this analog.
Adenylyl Cyclases and Forskolin Analogs
29
cording to an assumed pH of 7.2 (aspartate and glutamate residues
left as anionic and lysine and arginine residues specified as cationic).
The complex structure was dynamically equilibrated in SYBYL for
40 ps using the Tripos molecular force field (Clark et al., 1989),
Gasteiger-Marsili charges (Gasteiger and Marsili, 1980), a nonboding distance cutoff of 8.0 Å, a background dielectric constant of 78.2,
Downloaded from jpet.aspetjournals.org at ASPET Journals on October 19, 2016
Fig. 1. Structures of FS and FS analogs. A, FS; B, 1,9dd-FS; C, 1d-FS; D,
9-d-FS; E, 7DA-FS; F, 6A7DA-FS; G,
7DA1,9dd-FS; H, 7DA1d-FS; I, DMBFS; J, BODIPY-FS.
30
Pinto et al.
Fig. 2. Analysis of AC activities in membranes from uninfected Sf9 cells and membranes from wild-type baculovirus-, AC1-, AC2-, and AC5 virus-infected Sf9 cells. AC
activity was determined as described under Materials and
Methods. Each assay tube contained membranes (30 g
protein/tube), [␣-32P]ATP (1.0 –1.5 Ci/tube), 40 M unlabeled ATP, Mn2⫹ (10 mM) and 3% (v/v) DMSO (basal),
GTP␥S (10 M), FS (300 M), or GTP␥S (10 M) plus FS
(300 M). Data shown are the mean values ⫾ S.D. of three
experiments performed in duplicate with two separate
membrane preparations. wt-virus, wild-type baculovirus.
Results
Expression and Activation of ACs 1, 2, and 5 in Sf9
Insect Cells. Sf9 cells express an as-yet unidentified membrane AC (Tang et al., 1991; Seifert et al., 1998; Gille et al.,
2004). Thus, when using Sf9 cells as an expression system for
mammalian ACs, we had to ensure that the activities in
membranes from cells infected with AC-encoding baculoviruses were well above the endogenous AC activity of the
insect cells. In agreement with previous data (Tang et al.,
1991), infection of Sf9 cells with a baculovirus not encoding a
mammalian AC, in our case a wild-type virus encoding no
recombinant mammalian protein at all, reduced maximal AC
activities in Sf9 membranes under various experimental con-
ditions by more than 50% (Fig. 2). Thus, the AC activity in
membranes from wild-type virus-infected cells constitutes
the true background of endogenous enzyme activity for analysis of mammalian ACs. Infection of Sf9 cells with baculoviruses encoding AC 1, 2, or 5 resulted in basal AC activities
7 to 25-fold higher than in membranes from wild-type virusinfected cells. The FS-stimulated activities were up to 12-fold
higher in membranes from AC virus-infected cells than in
membranes from wild-type virus-infected cells. Thus, Sf9
cells are a sensitive expression system for analyzing the
effects of FS analogs on mammalian ACs. It should be noted
that the FS-stimulated activities were similar for ACs 1, 2,
and 5, pointing to similar AC expression levels, whereas ACs
1 and 2 exhibited higher basal activities than AC5. High
basal activity of AC2 had also already been noted in a previous study (Pieroni et al., 1995).
In membranes from uninfected Sf9 cells, the direct G-protein
activator GTP␥S substantially enhanced AC activity. This stimulation is explained by an activation of the endogenous Gs␣-like
G-protein of the insect cells (Seifert et al., 1998). The addition of
GTP␥S to FS-containing tubes resulted in an additive stimulation of AC in membranes from uninfected Sf9 cells and wildtype virus-infected cells. The stimulatory effects of GTP␥S in
TABLE 1
Potencies and efficacies of FS and FS analogs for activation/inhibition of AC isoforms
AC activity in membranes from uninfected Sf9 cells (control) and Sf9 cells expressing mammalian membranous ACs 1, 2, and 5 was determined as described under Materials
and Methods. Each assay tube contained membrane (30 g protein/tube), 关␣-32P兴ATP (1.0 –1.5 Ci/tube), 40 M unlabeled ATP, Mn2⫹ (10 mM) and GTP␥S (10 M) for
maximal activation of insect Gs␣, 3% (v/v) DMSO, and increasing concentrations of FS or different FS analogs. Data were analyzed by nonlinear regression and best fitted
to sigmoidal concentration/response curves. Relative stimulatory efficacies for each analog were determined by dividing the fitted maximal stimulation obtained for the
analog by the maximum stimulation obtained by treatment of different ACs with 300 M FS and 10 M GTP␥S and are expressed in percentages. The maximal AC activities
stimulated by FS plus GTP␥S in Sf9 membranes expressing ACs 1, 2, and 5 and uninfected membranes were 144 ⫾ 4.8, 155 ⫾ 8.9, 167 ⫾ 3.5, and 44.6 ⫾ 1.6 pmol/mg/min,
respectively. Potencies (EC50 values given in M) of FS and FS analogs were obtained by nonlinear regression analysis of the data. Data shown are mean values ⫾ S.D. of
three to six independent experiments with four membrane preparations performed in duplicates.
Control
AC1
AC2
AC5
Diterpene
FS
DMB-FS
7DA-FS
6A7DA-FS
9d-FS
1d-FS
1,9dd-FS
7DA1,9dd-FS
7DA1d-FS
BODIPY-FS
EC50
Efficacy
EC50
Efficacy
EC50
Efficacy
EC50
Efficacy
2.0 ⫾ 0.6
12.0 ⫾ 2.8*
34.5 ⫾ 7.8**
5.3 ⫾ 2.0
92.5 ⫾ 12.1*a
Ineffective
Ineffective
Ineffective
Ineffective
2.9 ⫾ 0.2
100
84.6 ⫾ 6.4
81.2 ⫾ 9.2
85.5 ⫾ 10.8
72.0 ⫾ 6.2*a
Ineffective
Ineffective
Ineffective
Ineffective
50.6 ⫾ 11.8**
0.7 ⫾ 0.1
2.6 ⫾ 0.7*
3.4 ⫾ 1.6*
0.8 ⫾ 0.3
3.6 ⫾ 1.3*
Ineffective
Ineffective
Ineffective
Ineffective
0.7 ⫾ 0.1
100
78.5 ⫾ 6.1*
88.0 ⫾ 1.0
83.5 ⫾ 6.1
83.5 ⫾ 9.2
Ineffective
Ineffective
Ineffective
Ineffective
62.0 ⫾ 1.4**
8.7 ⫾ 1.1
27.6 ⫾ 4.0**
143 ⫾ 59.2**a
13.3 ⫾ 2.1*
250 ⫾ 70.7**a
3.5 ⫾ 0.7*
Ineffective
80.6 ⫾ 9.1**
Ineffective
1.2 ⫾ 1.1
100
43.5 ⫾ 2.1**
86.0 ⫾ 7.2a
64.6 ⫾ 10.2*
84.0 ⫾ 6.0a
⫺15.6 ⫾ 2.0**
Ineffective
⫺18.3 ⫾ 9.1**
Ineffective
⫺33.3 ⫾ 1.5**
4.8 ⫾ 1.7
48.0 ⫾ 9.6**a
63.3 ⫾ 10.6**a
7.4 ⫾ 2.8
65.3 ⫾ 9.0**a
Ineffective
Ineffective
Ineffective
Ineffective
2.3 ⫾ 1.1
100
94.9 ⫾ 1.2a
63.2 ⫾ 4.4**a
103.3 ⫾ 10.6
78.6 ⫾ 9.7*a
Ineffective
Ineffective
Ineffective
Ineffective
48.2 ⫾ 1.6**
a
For those FS analogs not reaching saturation of the concentration/response curves, the actual stimulatory effect at a concentration of 300 M was used for calculation
of efficacy. Extrapolated EC50 values marked with an ⬙a⬙ were derived from concentration/response curves that did not reach saturation. Statistical comparisons between FS
and different analogs were performed using the Student’s t test.
* p ⬍ 0.05.
** p ⬍ 0.01.
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and initial atomic velocities as assigned according to a Boltzmann
distribution suitable to 298.15K. The resulting relaxed structure was
then subjected to a 100-ps analysis run under the same conditions,
with the final velocities of the equilibration run taken as initial
velocities for the analysis run. The resulting trajectory was analyzed
visually in SYBYL to qualitatively assess the conformational stability of the bound BODIPY-FS ligand.
Statistics. Statistical comparisons in Tables 1 and 2 were performed using the Student’s t test. Differences were considered as
statistically significant with p ⬍ 0.05 (ⴱ) and p ⬍ 0.01 (ⴱⴱ).
Adenylyl Cyclases and Forskolin Analogs
31
TABLE 2
Effects of BODIPY-FS on the activity of AC2 expressed with various virus titers
Sf9 membranes were infected with AC2 virus at the titers given in the table. Membranes were prepared after a 48- h infection. AC activity was determined as described under
Materials and Methods. Each assay tube contained a specific membrane (30 g protein/tube), 关␣-32P兴ATP (1.0 –1.5 Ci/tube), 40 M unlabeled ATP, Mn2⫹ (10 mM) and
GTP␥S (10 M), 3% (v/v) DMSO, and increasing concentrations (10 nM–30 M) of BODIPY-FS. Data were analyzed by nonlinear regression and best fitted to sigmoidal
concentration/response curves. Data shown are mean values ⫾ S.D. of three independent experiments in duplicate.
Virus Titer
AC Activity (Basal)
AC Activity (BODIPY-FS)
pmol/mg/min
1:10
1:100
1:300
44.1 ⫾ 2.6
40.1 ⫾ 3.9
32.5 ⫾ 3.1
27.7 ⫾ 0.9
23.1 ⫾ 2.1
20.7 ⫾ 1.3
EC50
%
mM
⫺37.2 ⫾ 1.7
⫺42.4 ⫾ 2.9
⫺36.3 ⫾ 1.9
1.1 ⫾ 0.5
0.6 ⫾ 0.4
0.5 ⫾ 0.2
In general, among all ACs studied, diterpenes exhibited
the highest potencies at AC1. The order of potency at AC1
was BODIPY-FS ⬃ FS ⬃ 6A7DA-FS ⬎ DMB-FS ⬃7DA-FS ⬃
9d-FS. The order of efficacy at AC1 was FS ⬎ 7DA-FS ⬃
6A7DA-FS ⬃ 9d-FS ⬎ DMB-FS ⬎ BODIPY-FS.
The pharmacological profile of AC2 differed considerably
from the profiles of the other ACs. Specifically, with two
exceptions (DMB-FS and BODIPY-FS), the potencies of diterpenes at AC2 were lower than at the other ACs. For example,
FS activated AC2 12-fold less potently than AC1. The order of
potency of stimulatory diterpenes at AC2 was FS ⬎ 6A7DAFS ⬎ DMB-FS ⬎ 7DA-FS ⬎ 9d-FS, and the order of efficacy
was FS ⬎ 7DA-FS ⬃ 9dFS ⬎ 6A7DA-FS ⬎ DMB-FS. 1d-FS,
7DA1,9dd-FS, and BODIPY-FS exhibited inhibitory effects
on basal activity in membranes expressing AC2. Among
these compounds, BODIPY-FS was the most potent and efficacious compound.
We also addressed the question of whether the inhibitory
effect of BODIPY-FS depends on the expression level and
basal activity of AC2. To address this question, we infected
Sf9 cells with various titers of AC2 virus (Table 2). In fact,
with decreasing virus titer, basal activity in AC2-expressing
membranes decreased. However, the relative inhibitory effect of BODIPY-FS did not decrease with decreasing virus
Fig. 3. Effects of FS and FS analogs on the catalytic activity of membranous ACs. AC activity was
determined as described under Materials and
Methods. Each assay tube contained a specific
membrane (30 g protein/tube), [␣-32P]ATP (1.0 –
1.5 Ci/tube), 40 M unlabeled ATP, Mn2⫹ (10
mM) and GTP␥S (10 M), 3% (v/v) DMSO, and
increasing concentrations (300 nM–300 M) of different diterpenes as indicated on the abscissa.
Shown are saturation curves of the relative stimulatory effects of FS and different FS analogs on
the catalytic activity of insect cell AC (A) and AC1
(B), AC2 (C), and AC5 (D). f, FS; Œ, 7DA-FS; ,
6A7DA-FS; F, 9d-FS; ⽧, DMB-FS; ‚, 1d-FS; 䡺,
1,9dd-FS; E, 7DA1,9dd-FS. Data were analyzed by
nonlinear regression and best fitted to sigmoidal
concentration/response curves. The AC activities
stimulated by FS plus GTP␥S in Sf9 membranes
expressing ACs 1, 2, and 5 and uninfected membranes were 140 ⫾ 0.1, 153 ⫾ 12.9, 166 ⫾ 1.5, and
45.8 ⫾ 2.1 pmol/mg/min, respectively. Basal AC
activities were 38.1 ⫾ 7.7, 40.4 ⫾ 5.5, 11.4 ⫾ 0.1,
and 2.0 ⫾ 0.9 pmol/mg/min, respectively. Data
shown are mean values ⫾ S.D. of two to three
independent experiments with a single membrane
preparation performed in duplicate. Similar results were obtained with four different membrane
preparations.
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membranes expressing ACs 1, 2, and 5 were very small. Moreover, GTP␥S did not enhance or only minimally enhanced the
stimulatory effect of FS in membranes expressing ACs 1, 2, and
5. The small or absent stimulatory effects of GTP␥S in membranes expressing ACs 1, 2, and 5 are probably due to an
intrinsically low expression of the Gs␣-like G-protein of the
insect cells (Seifert et al., 1998) that is further decreased by the
infection per se. However, to ensure optimal conditions for AC
activation in all types of membranes studied, we routinely included GTP␥S (10 M) in all further experiments.
Effects of FS and FS Analogs on Catalytic Activity of
Membrane ACs. Figure 3 shows representative concentration/response curves for the effects of FS and FS analogs on
the activity of insect cell AC, AC1, AC2, and AC5, and Fig. 4
shows a comparison of the effects of FS and BODIPY-FS on
catalytic activity. Table 1 provides a summary of the effects
of diterpenes on ACs.
Diterpenes activated insect cell AC in the order of potency
FS ⬎ BODIPY-FS ⬎ 6A7DA-FS ⬎ DMB-FS ⬎ 7DA-FS ⬎
9d-FS. With the exception of AC2, all 1d-FS derivatives (1dFS, 1,9dd-FS, 7DA1,9dd-FS, and 7DA1d-FS) exhibited neither stimulatory nor inhibitory effects on ACs. At insect cell
AC, the order of efficacy of diterpenes was FS ⬎ 6A7DA-FS ⬃
DMB-FS ⬃ 7DA-FS ⬎ 9d-FS ⬎ BODIPY-FS.
Relative Inhibitory Effect of BODIPY-FS
32
Pinto et al.
titer. In addition, the potency of BODIPY-FS remained unchanged over the range of virus titers studied. We tried to
express AC2 even at lower levels than those shown in Table
2, but those efforts failed. It is, unfortunately, a well known
weakness of the Sf9 insect cell expression system that it does
not allow for reliable control recombinant protein expression
(Gille and Seifert, 2003).
At AC5, diterpenes stimulated catalysis in the order of
potency BODIPY-FS ⬎ FS ⬎ 6A7DA-FS ⬎ DMB-FS ⬎ 7DA-
FS ⬃ 9d-FS, and the order of efficacy was FS ⬃ 6A7DA-FS ⬃
DMB-FS ⬎ 9d-FS ⬎ 7DA-FS ⬎ BODIPY-FS.
Inhibition of the Stimulatory Effects of FS on AC
Catalysis by Various FS Analogs. Figure 5 shows concentration/response curves for FS at insect cell AC and ACs 1, 2,
and 5 in the absence and presence of 1,9dd-FS, 1dFS, and
7DA1,9dd-FS in a fixed concentration (100 M). The 1d-FS
derivatives shifted the concentration/response curves to the
right, but they also reduced the maximum stimulatory effect
Fig. 5. Inhibition of the stimulatory effects of FS on AC
catalysis by various FS analogs. AC activity was determined as described under Materials and Methods. Each
assay tube contained a specific membrane (30 g protein/
tube), [␣-32P]ATP (1.0 –1.5 Ci/tube), 40 M unlabeled
ATP, Mn2⫹ (10 mM) and GTP␥S (10 M), 3% (v/v) DMSO,
and increasing concentrations (300 nM–300 M) of FS in
the presence of a single fixed concentration (100 M each)
of various other diterpenes. A, insect cell AC; B, AC1; C,
AC2; D, AC5. f, FS alone; Œ, FS ⫹ 1,9dd-FS (100 M); ⽧,
FS ⫹ 1d-FS (100 M); F, FS ⫹ 7DA1,9dd-FS (100 M).
Relative stimulatory effects (efficacies) were determined by
dividing the fitted maximal stimulation obtained for each
experimental condition by the maximum stimulation obtained by treatment of each individual membranous ACs
with 300 M FS and GTP␥S (10 M). The AC activities
stimulated by FS plus GTP␥S in Sf9 membranes expressing ACs 1, 2, and 5 and uninfected membranes were 139 ⫾
0.5, 155 ⫾ 7.9, 171 ⫾ 1.8, and 44.8 ⫾ 1.1 pmol/mg/min,
respectively. Basal AC activities were 39.0 ⫾ 3.1, 41.2 ⫾
3.4, 10.5 ⫾ 0.3, and 2.3 ⫾ 0.3 pmol/mg/min, respectively.
Data were analyzed by nonlinear regression and best fitted
to sigmoidal concentration/response curves. Data shown
are mean values ⫾ S.D. of two independent experiments of
two membrane preparations performed in duplicate.
Downloaded from jpet.aspetjournals.org at ASPET Journals on October 19, 2016
Fig. 4. Effects of FS and BODIPY-FS on the catalytic activity of membranous ACs. AC activity was
determined as described under Materials and Methods. Each assay tube contained a specific membrane
(30 g protein/tube), [␣-32P]ATP (1.0 –1.5 Ci/tube),
40 M unlabeled ATP, Mn2⫹ (10 mM) and GTP␥S
(10 M), 3% (v/v) DMSO, and increasing concentrations (10 nM–300 M) of FS or BODIPY-FS as indicated on the abscissa. Shown are saturation
curves of the relative stimulatory effects of FS (f)
and the relative stimulatory or inhibitory effects of
BODIPY-FS (䡺) on the catalytic activity of insect
cell (A), AC1 (B), AC2 (C), and AC5 (D). Data were
analyzed by nonlinear regression and best fitted to
sigmoidal concentration/response curves. The AC
activities stimulated by FS plus GTP␥S in Sf9 membranes expressing ACs 1, 2, and 5 and uninfected
membranes were 143 ⫾ 0.9, 155 ⫾ 4.1, 169 ⫾ 5.7,
and 42.1 ⫾ 2.2 pmol/mg/min, respectively. Basal AC
activities were 39.9 ⫾ 1.7, 43.1 ⫾ 3.5, 10.4 ⫾ 0.4,
and 2.1 ⫾ 0.3 pmol/mg/min, respectively. Data
shown are mean values ⫾ S.D. of two independent
experiments of one membrane preparation performed in duplicate. Similar results were obtained
with four different membrane preparations.
Adenylyl Cyclases and Forskolin Analogs
ible with the assumption that the interaction of the FS moiety
with the receptor is intact and that promiscuous sampling of the
BODIPY moiety with various receptor residues contributes positively to ligand potency.
Discussion
Crystallographic studies showed that diterpenes bind to a
hydrophobic cleft formed by the C1 and C2 catalytic subunits
of AC opposite to the catalytic site (Tesmer et al., 1997; Tang
and Hurley, 1998). In addition to hydrophobic interactions,
diterpenes form hydrogen bonds with ACs, namely between
the C1-OH group and the backbone oxygen of Val506 (AC5
numbering), the C7-acetyl group and Ser942 (AC2 numbering), as well as the C11-OH group and Ser508 (AC5 numbering). The amino acids interacting with FS are highly conserved among the FS-sensitive AC isoforms 1 to 8 (Tang and
Hurley, 1998). Thus, it was not unexpected that in all ACs
studied, deletion of the C7-acetyl group resulted in a decrease
in potency and, to a lesser extent, in a decrease of efficacy of
diterpenes (Table 1). The hydrogen bond between the C1-OH
group and Val506 is crucial for AC activation because its
deletion, alone or in combination with other structural modifications, resulted in ineffective FS analogs devoid of stimulatory effects.
The reduced potency and efficacy of DMB-FS relative to FS
at all ACs (Table 1) suggests that the DMB substituent, in
contrast to the BODIPY substituent (Fig. 7), exhibits unfavorable interactions with the enzyme. DMB-FS was introduced as water-soluble FS analog to facilitate experiments
with intact cells (Laurenza et al., 1987), but given its pharmacological properties, DMB-FS is not a true substitute for
FS.
BODIPY-FS was introduced as a fluorescent probe for localization of ACs in intact cells (Liu et al., 1998; Takahashi et
al., 2002), but its functional interaction with AC has not yet
been examined. In BODIPY-FS, the small C7-acetyl group is
substituted by the large BODIPY group connected to C7 of
the diterpene ring through a long linker (Fig. 1, A and J). The
affinity of BODIPY-FS for various ACs including C1/C2 is
similar to, or even higher than, that of FS (Table 1; Fig. 6).
Molecular modeling revealed that the BODIPY group faces
Fig. 6. Effects of FS and BODIPY-FS on the catalytic activity of purified catalytic AC subunits C1/C2. AC activity was determined as described under
Materials and Methods. Each assay tube contained reaction mixtures of 3 nM C1, 15 nM C2, 51 nM GTP␥S-activated Gs␣, 1.0 Ci/tube [␣-32P]ATP,
0.1 mM cAMP, 100 mM KCl and 25 mM HEPES/NaOH, pH 7.4, 3% (v/v) DMSO, and increasing concentrations (10 nM–300 M) of FS or BODIPY-FS
as indicated on the abscissa. AC activities are referred to the C1 concentration in the assay. A, concentration-response curves for the stimulatory
effects of FS (f), BODIPY-FS (Œ), and BODIPY dye (). B, magnification of the data for BODIPY-FS (Œ) and BODIPY dye () shown in A. C,
concentration-response curves for the inhibitory effects of BODIPY-FS (Œ) and BODIPY dye (f) on AC activity stimulated by 3 M FS. AC activity
in the presence of 3 M FS amounted to 386 ⫾ 21 nmol/mg/min. Data were analyzed by nonlinear regression and best fitted to sigmoidal
concentration/response curves. Data shown are mean values ⫾ S.D. of three independent experiments in duplicate.
Downloaded from jpet.aspetjournals.org at ASPET Journals on October 19, 2016
of FS by 40 to 70%, i.e., 1d-FS analogs displayed mixed
competitive/noncompetitive antagonism.
Interaction of C1/C2 with FS and BODIPY-FS.
BODIPY-FS exhibits very different pharmacological properties, depending on the specific AC isoform studied (Fig. 4;
Table 1). Because purified AC catalytic subunits C1/C2 have
been successfully used as a model system for examining
ligand/enzyme interactions by catalysis studies and molecular modeling (Mou et al., 2006), we assessed the interaction of
C1/C2 with FS and BODIPY-FS. As a control, we also studied
the free dye, BODIPY. Basal catalysis with C1/C2 was very
low, but FS effectively stimulated the enzyme with an EC50
of 9.1 ⫾ 2.3 M (Fig. 6A). BODIPY-FS acted as a very weak
partial agonist (efficacy ⬎ 5% of that of FS) (Fig. 6A), but due
to the very low basal activity in the system and the large
stimulatory effect of FS, the small stimulatory effect of
BODIPY-FS could also be clearly distinguished (Fig. 6B).
BODIPY-FS activated catalysis with a potency of 2.5 ⫾ 0.3
M, i.e., BODIPY-FS was more potent than FS. The free
BODIPY dye was ineffective, indicating that the link between the FS- and BODIPY moieties was important for the
pharmacological activities of BODIPY-FS. As was predicted
from the weak partial agonism of BODIPY-FS, the FS analog
acted as a strong partial antagonist (IC50, 0.8 ⫾ 0.3 M) to
inhibit FS-stimulated catalysis (Fig. 6C). The IC50 of
BODIPY-FS for partial antagonism (Fig. 6C) fits well to its
EC50 for partial agonism (Fig. 6B). The BODIPY dye itself
did not inhibit FS-stimulated catalysis (Fig. 6C).
Molecular Modeling of the Interaction of C1/C2 with
BODIPY-FS. Our 100-ps dynamic simulation of BODIPY-FS
bound to C1/C2 showed that the FS moiety of this ligand
remained very firmly bound to the native FS-binding site, in
that FS atom positions in the final conformer resolved in the
simulation vary by less than 0.5 Å root-mean-squared deviation relative to those present in the original crystal structure (Fig. 7). However, the BODIPY moiety was found to
sample promiscuously its degrees of freedom, transiting a
more than 150° arc during the course of a 60-ps time space
during which time it experienced brief coupling with a broad
range of receptor residues. In view of the fact that BODIPY-FS
is a similarly potent ligand as, or even more potent ligand than,
FS (Figs. 4 and 5; Table 1), our AC activity results are compat-
33
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Pinto et al.
Fig. 7. Molecular modeling of the interaction of the catalytic AC subunits C1/C2 with BODIPY-FS. Modeling studies were performed as described under Materials and Methods. Variations in structure of BODIPY-FS bound to C1/C2
over the course of a 100-ps molecular dynamics simulation
are shown. AC residues are rendered according to secondary structure (yellow, -sheets; magenta, ␣-helices; cyan,
strands/turns) and ligand as CPK-colored (cyan, H; gray, C;
blue, N; red, O) sticks.
tives do not bind to AC (Seamon et al., 1984). However, the
inhibitory effects of 1d-FS and 7DA1,9dd-FS on basal AC2
activity (Fig. 3C; Table 1) indicate that this is not true. These
findings prompted us to explore the hypothesis that 1d-FS
derivatives inhibit the stimulatory effects of FS on catalysis.
In fact, 1d-FS, 1,9dd-FS, and 7DA1,9dd-FS inhibited FSstimulated catalysis at all four ACs studied in a mixed competitive/noncompetitive manner (Fig. 5). These data clearly
show that in contrast to previously held view (Laurenza et
al., 1989), 1d-FS derivatives bind to ACs. An explanation for
this apparent discrepancy is that previous [3H]FS-binding
studies with cell membranes actually did not monitor AC but
rather more abundant FS-binding proteins such as ion channels and glucose transporters (Laurenza et al., 1989). This
explanation is supported by the high-affinity [3H]FS binding
and the low-affinity FS-activation of AC (Table 1; Fig. 6)
(Seamon et al., 1984). It is intriguing that C. forskohlii extracts that are popular over-the-counter drugs for weight
reduction do not only contain FS but also 1d-FS derivatives
(Ding and Staudinger, 2005). Thus, antagonism between FS
and 1d-FS derivatives with respect to AC activation may
actually annihilate the desired weight-reducing effect (Fig. 5)
but not the undesired effects of diterpenes on hepatic enzyme
induction, potentially resulting in drug interactions (Ding
and Staudinger, 2005). This may also explain the paucity of
controlled clinical studies demonstrating efficacy of C. forskohlii extracts in obesity.
Crystallographic studies identified a single FS-binding site
in AC (Tesmer et al., 1997; Tang and Hurley, 1998). The mixed
competitive/noncompetitive interaction between FS and 1d-FS
derivatives is explained by the fact that those ligands are very
lipophilic, impairing free ligand exchange at the FS site. Difficulties to obtain equilibrium conditions at the FS-binding site
were also observed for purified catalytic AC subunits (Dessauer
et al., 1997). Thus, the mixed competitive/noncompetitive interaction between FS and 1d-FS derivatives may reflect hemiequilibrium conditions (Kenakin et al., 2006) rather than the existence of a second FS-binding site.
AC2 exhibited distinct pharmacological properties. First,
and in agreement with Pieroni et al. (1995), AC2 displayed
Downloaded from jpet.aspetjournals.org at ASPET Journals on October 19, 2016
outside of the FS site and promiscuously interacts with several amino acids surrounding this area (Fig. 7). These interactions probably contribute to the high ligand affinity for
ACs. Given the differential effects of BODIPY-FS on various
ACs, it is also possible that these promiscuous interactions
are different for various ACs. The fortuitous identification of
BODIPY-FS as a potent FS analog opens novel opportunities
for ligand design, be it fluorescent or nonfluorescent ligands.
It is unusual that a fluorescent compound is a lead structure
for ligand design.
Considering the highly conserved diterpene-binding site in
ACs (Tang and Hurley, 1998), it was unexpected that various
ACs responded differently to the prototypical diterpene, FS,
i.e., FS activated AC1 up to 12-fold more potently than ACs 2
and 5 (Table 1). These differences were unmasked because
the use of a sufficiently high concentration of organic solvent
[3% (v/v) DMSO], allowing effective solubilization of diterpenes. In addition, we used Mn2⫹ and GTP␥S to fully activate
AC (Sunahara et al., 1997; Gille et al., 2004). Together, these
experimental conditions facilitated generation of saturated
concentration/response curves for several diterpenes (Figs.
3– 6). A previous study did not reveal differences in FS potency between AC2 and AC5 (Onda et al., 2001). It is noteworthy that Onda et al. (2001) did not obtain saturated
concentration/response curves. This could be due to the use of
Mg2⫹ in their study and/or insufficient diterpene solubilization because the solvent conditions were not provided.
Our data suggest that amino acids other than those immediately in contact with the diterpene have an impact on
diterpene affinity. In fact, there are significant structural
differences between ACs in the transmembrane domains
(Sunahara et al., 1996; Tang and Hurley, 1998) that may
influence diterpene potency indirectly by modulating the
relative mobility of the cytosolic catalytic sites. An approach to further address this issue is the construction and
expression of chimeric ACs in which the transmembrane
domains are exchanged.
In agreement with previous data (Laurenza et al., 1989),
1d-FS derivatives did not activate AC (Table 1). Based on
[3H]FS-binding studies, it was assumed that 1d-FS deriva-
Adenylyl Cyclases and Forskolin Analogs
Acknowledgments
We thank Dr. Andreas Gille (University of Heidelberg, Germany)
for helpful discussions, Astrid Seefeld and Gertraud Wilberg for
expert technical assistance, and the reviewers for helpful critique.
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the highest basal activity (Fig. 2). Second, except for DMB-FS
and BODIPY-FS, AC2 exhibited the lowest affinity for diterpenes (Table 1). Third, and most remarkably, several 1d-FS
derivatives and BODIPY-FS exhibited prominent inhibitory
effects on AC2 that were not observed with another AC
isoform. Thus, by analogy to the two-state model of receptor
activation (Seifert and Wenzel-Seifert, 2002), AC2 could be
considered as a constitutively active “FS receptor,” with the
FS site isomerizing between an inactive (R) state and an
active (R*) state. FS stabilizes the R* state, increasing catalysis, whereas 1d-FS derivatives and BODIPY-FS stabilize
the R state and decrease the high basal catalysis rate of AC2.
These findings also raise the question whether the elusive
endogenous ligand for the FS site (Laurenza et al., 1989;
Putnam et al., 2007) is an inverse agonist.
Although the two-state model can explain the inhibitory
effects of certain diterpenes on basal AC2 activity, this model
is insufficient at explaining all pharmacological effects of
diterpenes. In particular, AC1 exhibited only a slightly lower
basal catalysis rate than AC2, but at this AC, the most
effective AC2 inverse agonist, BODIPY-FS, was actually a
rather strong partial agonist and not an inverse agonist (Fig.
5; Table 1). At C1/C2, BODIPY-FS behaved as a strong partial antagonist (Fig. 6). Thus, BODIPY-FS is reminiscent of a
protean receptor ligand (Kenakin, 2001). In addition, the
two-state model predicts that receptors with high constitutive activity exhibit increased potency and efficacy of partial
agonists relative to a receptor with low constitutive activity
(Seifert and Wenzel-Seifert, 2002). The two-state model also
predicts that with increasing receptor expression level, the
abundance of the active (R*) state increases, increasing basal
effector activity and the efficacy of inverse agonists. Indeed,
with increasing AC2 virus titer during the infection, the
corresponding basal AC activities increased (Table 2). However, there was no increase in the efficacy of BODIPY-FS as
a result of increased virus titer, expression, and activity.
Moreover, a comparison of the potencies and efficacies of
partial FS agonists at AC2, exhibiting high basal activity,
and AC5, exhibiting rather low basal activity (Fig. 2), clearly
shows that this prediction of the two-state model is not fulfilled. Thus, a multistate model in which a given diterpene
stabilizes a ligand-specific conformation in each AC isoform
with distinct catalysis-activating or -inhibiting properties is
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(Kenakin, 2001, 2002; Seifert and Wenzel-Seifert, 2002; Kobilka, 2007).
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is conceivable that selective AC1 activators could be valuable
drugs for the treatment of Alzheimer’s disease.
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Address correspondence to: Dr. Roland Seifert, Department of Pharmacology and Toxicology, University of Regensburg, Universitätsstrasse 31,
D-93053 Regensburg, Germany. E-mail: roland.seifert@chemie.uniregensburg.de
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