Obesity
Perspective
COVID-19 AND OBESITY
The Role of Adipocytes and Adipocyte-Like Cells
in the Severity of COVID-19 Infections
Ilja L. Kruglikov1 and Philipp E. Scherer2
Coronavirus disease-2019 (COVID-19), caused by the highly pathogenic severe acute respiratory syndrome
coronavirus 2 (SARS-CoV-2), demonstrates high morbidity and mortality caused by development of a severe
acute respiratory syndrome connected with extensive pulmonary fibrosis. In this Perspective, we argue that
adipocytes and adipocyte-like cells, such as pulmonary lipofibroblasts, may play an important role in the
pathogenic response to SARS-CoV-2. Expression of angiotensin-converting enzyme 2 (the functional receptor for SARS-CoV) is upregulated in adipocytes of patients with obesity and diabetes, which turns adipose
tissue into a potential target and viral reservoir. This may explain why obesity and diabetes are potential
comorbidities for COVID-19 infections. Similar to the recently established adipocyte-myofibroblast transition, pulmonary lipofibroblasts located in the alveolar interstitium and closely related to classical adipocytes
demonstrate the ability to transdifferentiate into myofibroblasts that play an integral part of pulmonary fibrosis. This may significantly increase the severity of the local response to SARS-CoV-2 in the lung. To reduce
the severity and mortality associated with COVID-19, we propose to probe for the clinical response to thiazolidinediones, peroxisome proliferator activated receptor γ agonists that are well-known antidiabetic drugs.
Thiazolidinediones are able to stabilize lipofibroblasts in their “inactive” state, preventing the transition to
myofibroblasts and thereby reducing the development of pulmonary fibrosis and stimulating its resolution.
Obesity (2020) 28, 1187-1190.
Introduction
Coronavirus disease-2019 (COVID-2019) caused by the highly pathogenic severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)
demonstrates high morbidity and mortality. Progressive consolidation of
the lung leading to severe acute respiratory syndrome is recognized as
the most common complication in this disease. One of the main reasons
for this pulmonary consolidation is an extensive pulmonary fibrosis (PF).
PF is likely to be present before the onset of other typical symptoms of
a viral infection. General prevalence and incidence of PF increase with
age, and PF is especially high in patients over 65 years of age (1). It also
has a sexually dimorphic incidence rate, with higher prevalence in males
than in females; there is also strong reason to believe that the prevalence
of PF in old patients has steadily increased in recent years (1).
PF has been observed in the lungs of patients infected with SARS-CoV
virus (2). This virus shares a high genetic homology with SARS-CoV-2.
Autopsy of affected cases have demonstrated the appearance of fibrous
tissue in alveolar spaces already within the first week of SARS development, with subsequent interstitial and air space fibrosis during the second
week and dense septal and alveolar fibrosis during the third week (2),
findings further confirmed by computed tomography scans (3). There is
broad clinical evidence demonstrating that viral infections are a risk factor for PF (4). Moreover, both viral infections and aging were strongly
associated cofactors in PF in this study as well (4). Furthermore, a positive correlation was established between the duration of the illness caused
by SARS-CoV and the degree of interstitial fibrosis (5).
Pathophysiologically, infection with SARS-CoV induces the expression of transforming growth factor β (TGF-β) and facilitates its signaling activity. In contrast, infection suppresses the angiotensin-converting
enzyme 2 (ACE2), which is the functional receptor that SARS-CoV
exploits for cell entry. At the same time, ACE2 acts as a negative regulator of pulmonary fibrosis (2). ACE2 is expressed in several different
organs, including in the lungs. Under normal conditions, this enzyme
is anchored in the plasma membrane of cells. SARS-CoV binds to
ACE2, causes endocytosis, and traffics the virus/ACE complex to endosomes. Interestingly, the affinity between ACE2 and SARS-CoV-2 is
approximately 10- to 20-fold higher than the affinity between ACE2
and SARS-CoV (6). Because SARS-CoV/CoV-2 both strongly interact with ACE2, the cells expressing ACE are likely to be linked to the
progression of PF. In this Perspective, we discuss the role of adipocytes
and, more importantly, adipocyte-like cells in COVID-19 and describe
a possible adjuvant therapy to reduce the severity of this disease.
Role of Adipocyte-Like Lung Cells in
Pathophysiology of PF
The pathophysiology of pulmonary fibrosis is not fully elucidated.
However, this condition is known to be a typical component of systemic sclerosis, which is a multisystem fibrotic disorder affecting the
skin, lungs, and some other internal organs. In experimental models,
systemic sclerosis can be reliably induced by application of bleomycin.
1
Scientific Department, Wellcomet GmbH, Karlsruhe, Germany 2 Touchstone Diabetes Center, Department of Internal Medicine, University of Texas Southwestern
Medical Center, Dallas, Texas, USA. Correspondence: Philipp E. Scherer (philipp.scherer@utsouthwestern.edu)
© 2020 The Obesity Society. Received: 8 April 2020; Accepted: 23 April 2020; Published online 10 June 2020. doi:10.1002/oby.22856
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This induction is connected with the appearance of biosynthetically
highly active myofibroblasts in affected tissues. Myofibroblasts excessively produce extracellular matrix components, hence modifying the
lung structure and negatively affecting gas exchange. Whereas the presence of myofibroblasts in PF is well established, it is still a matter of
debate where these myofibroblasts originate from. Possible origins for
these pulmonary myofibroblasts have been postulated, such as interstitial fibroblasts, pericytes, mesothelial, and epithelial cells (7).
In cutaneous fibrosis, which is also a typical manifestation of systemic
sclerosis, the appearance of myofibroblasts is mainly connected with
the transdifferentiation of dermal adipocytes located near the interface
of the dermis-subcutis known as adipocyte-myofibroblast transition
(8). Very recently, we formally demonstrated that transdifferentiation
of mature adipocytes into myofibroblasts takes place through an intermediate step, dedifferentiation of mature adipocytes into adipocyte-derived preadipocytes (9), which means that the accumulation of these
dedifferentiated adipocytes in affected tissue areas must precede the
development of fibrosis (Figure 1, top). In the skin, this process is
dependent on the hair follicle cycle, and a causal involvement of TGF-β
is highly likely. Further connecting pulmonary and cutaneous fibrosis
is the fact that the TGF-β/Smad pathway has been found to be critically
involved in PF as well (2).
Because both cutaneous fibrosis and PF share some important features and because they are both typical components of systemic
sclerosis, we propose that they present similar pathophysiological
pathways linked to an adipogenic-myogenic transition. Therefore,
we invoke the pulmonary lipofibroblasts (LiF) as the local lung
equivalent for PF in a similar fashion that the dermal adipocyte is the
source for dermal sclerosis (10).
LiFs carry characteristic lipid droplets and express high levels of perilipin-2. They are located in the alveolar interstitium adjacent to type
2 alveolar epithelial cells (AEC2) and assist these cells in surfactant production. Approximately 2% of AEC2 cells express ACE2 and
Figure 1 Top: Increased expression of ACE2 (the SARS-CoV-2 receptor) on obese and diabetic adipose tissue. This
dysfunctional adipose tissue also displays increased fibrosis, at least in part because of a adipocyte-myofibroblast
transition. PPARγ agonists (thiazolidinediones [TZDs]) along with adiponectin (a TZD target as well) are potently
antifibrotic and restore functional adipose tissue. Bottom: Lipofibroblasts are the local adipocyte equivalent in the
lung and also display the ability to dedifferentiate into myofibroblasts that contribute in an integral way to pulmonary
fibrosis. Similar to adipose tissue, TZDs have the potential to act on the myofibroblasts and partially convert them
back to lipofibroblasts. In that role, TZDs and adiponectin act as antifibrotic agents as well and have the potential to
restore a higher degree of functionality in lung tissue.
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Obesity
Perspective
COVID-19 AND OBESITY
therefore represent the biggest pool of ACE2-expressing cells in
the lungs (11). Hence, these cells are the most probable target for
SARS-CoV/CoV-2. LiFs demonstrate the ability to transdifferentiate
into myofibroblasts in vitro in response to hyperoxia and some other
factors, and this transition is thought to be the key event in bronchopulmonary dysplasia (12). The appearance of myofibroblasts derived
from LiFs was recently confirmed in vivo in mice (13). Whereas the
LiF-myofibroblast transition is typically observed in lung fibroblasts
during PF formation, the reverse process, i.e., a myofibroblast-LiF
transition, can be observed during the resolution of fibrosis (13)
(Figure 1, bottom). These authors additionally reported a reduction
of LiF differentiation markers and a significant increase in fibrotic
markers in end-stage PF. This is consistent with a disappearance of
LiFs and their replacement by fibrotic tissue.
LiFs were first established and investigated in neonatal lungs of
rodents, whereas their existence in humans was for a long time a topic
of controversy (14). Whereas some authors reported the existence of
these cells in adult humans, others did not confirm these findings. Only
through the use of the neutral lipid fluorescence stain LipidTOX could
the existence of resident LiFs in the vicinity of AEC2 in human adult
lungs be demonstrated (13). Their established existence in human
lungs promotes LiFs from a hypothetically postulated entity to an
important player in PF. It is not known whether the pulmonary lipofibroblast is a manifestation of ectopic fat deposition. If so, PF could
be a similar pathophysiology as nonalcoholic steatohepatitis; in other
words, it could be abnormal fat distribution outside conventional adipose tissue that is the driving force for the lipid deposition in these
cells. These are certainly questions that need to be further addressed.
ACE2 is widely expressed in adipocytes and enriched in adipocytes
of individuals with obesity and type 2 diabetes. Very little is known
about the ACE2 expression in LiFs. Consequently, the question of
whether SARS-CoV/CoV-2 can directly influence these cells and
thereby promote enhanced LiF-myofibroblast transition should be
urgently investigated experimentally. Given their close transcriptional relationship to adipocytes, LiFs are likely candidates for ACE2
expression. If ACE2 is indeed found to be present on LiFs, then these
cells may be important targets for the prevention or reduction of
COVID-19–associated PF.
Potential Experimental Approaches to
Reduce Severity of COVID-19
If we assume that SARS-CoV/CoV-2 can directly enter the cell
and influence the LiF transcriptional program, one way to reduce
PF may be the use of PPARγ agonists, which demonstrate potent
antifibrotic effects attenuating myofibroblast differentiation and disrupting TGF-β signaling. Induction of PPARγ leads to an effective
reduction of tissue and organ fibrotic disease, including pulmonary
fibrosis (15). This effect is at least partially mediated by adiponectin
(16). The level of adiponectin in circulation is inversely proportional
to adipose mass, is significantly reduced in patients with systemic
sclerosis, and is negatively correlated with severity of this disease
(17). Circulating levels of adiponectin can be effectively elevated,
for example, through application of thiazolidinediones (TZDs),
which are PPARγ agonists and established antidiabetic agents (18).
Indeed, application of one of these TZDs (rosiglitazone) significantly increased the expression of perilipin-2 in LiFs, reinforcing
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the adipogenic phenotype of human lung fibroblasts (13) (Figure 1,
bottom). Recently, it was shown that another antidiabetic drug, metformin, accelerated resolution of pulmonary fibrosis by inducing
transdifferentiation of myofibroblasts into lipofibroblasts (19). This
is also consistent with the finding that metformin decreased the risk
of mortality associated with chronic lower respiratory diseases in diabetic patients (16). Whether the application of TZDs and metformin
modulates ACE2 expression in adipose tissue and whether TZDs can,
through the modulation of ACE2, influence the infectivity of SARSCoV should be investigated in future research.
Conclusion
Interaction of both adipocytes themselves and adipose-like cells with
SARS-CoV/CoV-2 is involved in pathophysiology of COVID-19.
Adipose tissue can serve as a viral reservoir, whereas transdifferentiation of pulmonary lipofibroblasts into myofibroblasts can contribute
to the development of PF and thus is likely to influence the clinical
severity of COVID-19. Application of TZDs and metformin as an adjuvant therapy in COVID-19 patients may be a worthwhile approach
to reduce the development of PF and thus attenuate the severity of the
course of disease. This TZD-based intervention could be started early
on upon developing symptoms of COVID-19. While issues around the
long-term use of TZDs have been raised regarding cardiovascular side
effects, these issues have been resolved and deemed nonsignificant. The
biggest concern with TZDs is a moderate weight gain; the short-term
use proposed here should not be an issue in this respect either. Patients
with type 2 diabetes may benefit a great deal from this approach but
would have to be monitored carefully for any signs of hypoglycemia
because of enhanced insulin sensitivity.
In addition, because obesity is such a major risk factor for a negative
clinical prognosis (20,21), weight loss per se is a viable strategy as well,
even though this is clearly hard to achieve in the absence of any pharmacological interventions.O
Acknowledgments
We would like to thank Dr. Yu Aaron An, Christy Gliniak, Yingfeng
Deng, and Leon Straub for their help with database analysis. We also
would like to thank Richard Howdy from VisuallyMedical for the generation of the Figure.
Funding agencies: PES is supported by NIH grants R01-DK55758, R01-DK099110,
RC2-DK118620, P01-DK088761, and P01-AG051459. PES is also supported by an
unrestricted grant from the Novo Nordisk Research Foundation.
Disclosure: ILK is the managing partner of Wellcomet GmbH. Wellcomet GmbH
provided support in the form of salaries for ILK but did not have any additional role in
decision to publish or preparation of the manuscript. The commercial affiliation of ILK
with Wellcomet GmbH does not alter the adherence to all journal policies on sharing
data and materials.
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