Caveolins/caveolae protect adipocytes from fatty acid-mediated lipotoxicity

Ceramides are fuel gauges on the drive to cardiometabolic disease

Ceramides are signals of fatty acid excess that accumulate when a cell's energetic needs have been met and its nutrient storage has reached capacity. As these sphingolipids accrue, they alter the metabolism and survival of cells throughout the body including in the heart, liver, blood vessels, skeletal muscle, brain, and kidney. These ceramide actions elicit the tissue dysfunction that underlies cardiometabolic diseases such as diabetes, coronary artery disease, metabolic-associated steatohepatitis, and heart failure. Here, we review the biosynthesis and degradation pathways that maintain ceramide levels in normal physiology and discuss how the loss of ceramide homeostasis drives cardiometabolic pathologies. We highlight signaling nodes that sense small changes in ceramides and in turn reprogram cellular metabolism and stimulate apoptosis. Finally, we evaluate the emerging therapeutic utility of these unique lipids as biomarkers that forecast disease risk and as targets of ceramide-lowering interventions that ameliorate disease.

Mass Spectrometry-Based Lipidomics of Brown Adipose Tissue and Plasma of New-Born Lambs Subjected to Short-Term Cold Exposure

During cold exposure, brown adipose tissue (BAT) holds the key mechanism in the generation of heat, thus inducing thermogenic adaptation in response to cooler environmental changes. This process can lead to a major lipidome remodelling in BAT, where the increase in abundance of many lipid classes plays a significant role in the thermogenic mechanisms for heat production. This study aimed to identify different types of lipids, through liquid chromatography-mass spectrometry (LC-MS), in BAT and plasma during a short-term cold challenge (2-days), or not, in new-born lambs. Fifteen new-born Romney lambs were selected randomly and divided into three groups: Group 1 (n = 3) with BAT and plasma obtained within 24 h after birth, as a control; Group 2 (n = 6) kept indoors for two days at an ambient temperature (20-22 °C) and Group 3 (n = 6) kept indoors for two days at a cold temperature (4 °C). Significant differences in lipid composition of many lipid categories (such as glycerolipids, glycerophospholipids, sphingolipids and sterol lipids) were observed in BAT and plasma under cold conditions, compared with ambient conditions. Data obtained from the present study suggest that short-term cold exposure induces profound changes in BAT and plasma lipidome composition of new-born lambs, which may enhance lipid metabolism via BAT thermogenic activation and adipocyte survival during cold adaptation. Further analysis on the roles of these lipid changes, validation of potential biomarkers for BAT activity, such as LPC 18:1 and PC 35:6, should contribute to the improvement of new-born lamb survival. Collectively, these observations help broaden the knowledge on the variations of lipid composition during cold exposure.

Lipodistrophy: a paradigm for understanding the consequences of "overloading" adipose tissue

Lipodystrophies have been recognized since at least the nineteenth century and, despite their rarity, tended to attract considerable medical attention because of the severity and somewhat paradoxical nature of the associated metabolic disease that so closely mimics that of obesity. Within the last 20 yr most of the monogenic subtypes have been characterized, facilitating family genetic screening and earlier disease detection as well as providing important insights into adipocyte biology and the systemic consequences of impaired adipocyte function. Even more recently, compelling genetic studies have suggested that subtle partial lipodystrophy is likely to be a major factor in prevalent insulin-resistant type 2 diabetes mellitus (T2DM), justifying the longstanding interest in these disorders. This progress has also underpinned novel approaches to treatment that, in at least some patients, can be of considerable therapeutic benefit.

Loss of Caveolin-1 Is Associated with a Decrease in Beta Cell Death in Mice on a High Fat Diet

Elevated free fatty acids (FFAs) impair beta cell function and reduce beta cell mass as a consequence of the lipotoxicity that occurs in type 2 diabetes (T2D). We previously reported that the membrane protein caveolin-1 (CAV1) sensitizes to palmitate-induced apoptosis in the beta pancreatic cell line MIN6. Thus, our hypothesis was that CAV1 knock-out (CAV1 KO) mice subjected to a high fat diet (HFD) should suffer less damage to beta cells than wild type (WT) mice. Here, we evaluated the in vivo response of beta cells in the pancreatic islets of 8-week-old C57Bl/6J CAV1 KO mice subjected to a control diet (CD, 14% kcal fat) or a HFD (60% kcal fat) for 12 weeks. We observed that CAV1 KO mice were resistant to weight gain when on HFD, although they had high serum cholesterol and FFA levels, impaired glucose tolerance and were insulin resistant. Some of these alterations were also observed in mice on CD. Interestingly, KO mice fed with HFD showed an adaptive response of the pancreatic beta cells and exhibited a significant decrease in beta cell apoptosis in their islets compared to WT mice. These in vivo results suggest that although the CAV1 KO mice are metabolically unhealthy, they adapt better to a HFD than WT mice. To shed light on the possible signaling pathway(s) involved, MIN6 murine beta cells expressing (MIN6 CAV) or not expressing (MIN6 Mock) CAV1 were incubated with the saturated fatty acid palmitate in the presence of mitogen-activated protein kinase inhibitors. Western blot analysis revealed that CAV1 enhanced palmitate-induced JNK, p38 and ERK phosphorylation in MIN6 CAV1 cells. Moreover, all the MAPK inhibitors partially restored MIN6 viability, but the effect was most notable with the ERK inhibitor. In conclusion, our results suggest that CAV1 KO mice adapted better to a HFD despite their altered metabolic state and that this may at least in part be due to reduced beta cell damage. Moreover, they indicate that the ability of CAV1 to increase sensitivity to FFAs may be mediated by MAPK and particularly ERK activation.

Cavin-1/PTRF mediates insulin-dependent focal adhesion remodeling and ameliorates high-fat diet-induced inflammatory responses in mice

Cavin-1/polymerase I and transcript release factor (PTRF) is a requisite component of caveolae, small plasma membrane invaginations that are highly abundant in adipocytes. Cavin-1 is a dynamic molecule whose dissociation from caveolae plays an important role in mechanoprotection and rRNA synthesis. In the former situation, the acute dissociation of cavin-1 from caveolae allows cell membrane expansion that occurs upon insulin-aided lipid uptake into the fat cells. Cavin-1 dissociation from caveolae and membrane flattening alters the cytoskeleton and the interaction of plasma membrane proteins with the extracellular matrix through interactions with focal adhesion structures. Here, using cavin-1 knockout mice, subcellular fractionation, and immunoblotting methods, we addressed the relationship of cavin-1 with focal adhesion complexes following nutritional stimulation. We found that cavin-1 is acutely translocated to focal complex compartments upon insulin stimulation, where it regulates focal complex formation through an interaction with paxillin. We found that loss of cavin-1 impairs focal complex remodeling and focal adhesion formation and causes a mechanical stress response, concomitant with activation of proinflammatory and senescence/apoptosis pathways. We conclude that cavin-1 plays key roles in dynamic remodeling of focal complexes upon metabolic stimulation. This mechanism also underlies the crucial role of caveolae in the long-term healthy expansion of the adipocyte.

Caveolae: Structure, Function, and Relationship to Disease

The plasma membrane of eukaryotic cells is not a simple sheet of lipids and proteins but is differentiated into subdomains with crucial functions. Caveolae, small pits in the plasma membrane, are the most abundant surface subdomains of many mammalian cells. The cellular functions of caveolae have long remained obscure, but a new molecular understanding of caveola formation has led to insights into their workings. Caveolae are formed by the coordinated action of a number of lipid-interacting proteins to produce a microdomain with a specific structure and lipid composition. Caveolae can bud from the plasma membrane to form an endocytic vesicle or can flatten into the membrane to help cells withstand mechanical stress. The role of caveolae as mechanoprotective and signal transduction elements is reviewed in the context of disease conditions associated with caveola dysfunction.

Changes in the plasma membrane in metabolic disease: impact of the membrane environment on G protein-coupled receptor structure and function

Drug development targeting GPCRs often utilizes model heterologous cell expression systems, reflecting an implicit assumption that the membrane environment has little functional impact on these receptors or on their responsiveness to drugs. However, much recent data have illustrated that membrane components can have an important functional impact on intrinsic membrane proteins. This review is directed toward gaining a better understanding of the structure of the plasma membrane in health and disease, and how this organelle can influence GPCR structure, function and regulation. It is important to recognize that the membrane provides a potential mode of lateral allosteric regulation of GPCRs and can affect the effectiveness of drugs and their biological responses in various disease states, which can even vary among individuals across the population. The type 1 cholecystokinin receptor is reviewed as an exemplar of a class A GPCR that is affected in this way by changes in the plasma membrane.

LINKED ARTICLES

This article is part of a themed section on Molecular Pharmacology of GPCRs. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.21/issuetoc.

De Novo Sphingolipid Biosynthesis Is Required for Adipocyte Survival and Metabolic Homeostasis

Sphingolipids are a diverse class of essential cellular lipids that function as structural membrane components and as signaling molecules. Cells acquire sphingolipids by both de novo biosynthesis and recycling of exogenous sphingolipids. The individual importance of these pathways for the generation of essential sphingolipids in differentiated cells is not well understood. To investigate the requirement for de novo sphingolipid biosynthesis in adipocytes, a cell type with highly regulated lipid metabolism, we generated mice with an adipocyte-specific deletion of Sptlc1 Sptlc1 is an obligate subunit of serine palmitoyltransferase, the enzyme responsible for the first and rate-limiting step of de novo sphingolipid biosynthesis. These mice, which initially developed adipose tissue, exhibited a striking age-dependent loss of adipose tissue accompanied by evidence of adipocyte death, increased macrophage infiltration, and tissue fibrosis. Adipocyte differentiation was not affected by the Sptlc1 deletion. The mice also had elevated fasting blood glucose, fatty liver, and insulin resistance. Collectively, these data indicate that de novo sphingolipid biosynthesis is required for adipocyte cell viability and normal metabolic function and that reduced de novo sphingolipid biosynthesis within adipocytes is associated with adipocyte death, adipose tissue remodeling, and metabolic dysfunction.

Knockdown of PTRF ameliorates adipocyte differentiation and functionality of human mesenchymal stem cells

Healthy expansion of human adipose tissue requires mesenchymal stem cells (hMSC) able to proliferate and differentiate into mature adipocytes. Hence, characterization of those factors that coordinate hMSC-to-adipocyte transition is of paramount importance to modulate the adipose tissue expansion. It has been previously reported that the adipogenic program of hMSC can be disrupted by upregulating caveolar proteins, and polymerase I and transcript release factor (PTRF) is an integral component of caveolae, highly expressed in adipose tissue. Here, we hypothesized that the role of PTRF in adipocyte functionality might stem from an effect on hMSC. To test this hypothesis, we isolated hMSC from the subcutaneous fat depot. We found an upregulated expression of the PTRF associated with decreased adipogenic potential of hMSC, likely due to the existence of senescent adipocyte precursors. Employing short hairpin RNA-based constructs to stably reduce PTRF, we were able to restore insulin sensitivity and reduced basal lipolysis and leptin levels in human adipocytes with high levels of PTRF. Additionally, we pinpointed the detrimental effect caused by PTRF on the adipose tissue to the existence of senescent adipocyte precursors unable to proliferate and differentiate into adipocytes. This study provides evidence that impaired adipocyte functionality can be corrected, at least partially, by PTRF downregulation and warrants further in vivo research in patients with dysfunctional adipose tissue to prevent metabolic complications.

AGPAT2 is essential for postnatal development and maintenance of white and brown adipose tissue

Objective

Characterize the cellular and molecular events responsible for lipodystrophy in AGPAT2 deficient mice.

Methods

Adipose tissue and differentiated MEF were assessed using light and electron microscopy, followed by protein (immunoblots) and mRNA analysis (qPCR). Phospholipid profiling was determined by electrospray ionization tandem mass spectrometry (ESI-MS/MS).

Results

In contrast to adult Agpat2^−/− mice, fetuses and newborn Agpat2^−/− mice have normal mass of white and brown adipose tissue. Loss of both the adipose tissue depots occurs during the first week of postnatal life as a consequence of adipocyte death and inflammatory infiltration of the adipose tissue. At the ultrastructural level, adipose tissue of newborn Agpat2^−/− mice is virtually devoid of caveolae and has abnormal mitochondria and lipid droplets. Autophagic structures are also abundant. Consistent with these findings, differentiated Agpat2^−/− mouse embryonic fibroblasts (MEFs) also have impaired adipogenesis, characterized by a lower number of lipid-laden cells and ultrastructural abnormalities in lipid droplets, mitochondria and plasma membrane. Overexpression of PPARγ, the master regulator of adipogenesis, increased the number of Agpat2^−/− MEFs that differentiated into adipocyte-like cells but did not prevent morphological abnormalities and cell death. Furthermore, differentiated Agpat2^−/− MEFs have abnormal phospholipid compositions with 3-fold increased levels of phosphatidic acid.

Conclusion

We conclude that lipodystrophy in Agpat2^−/− mice results from postnatal cell death of adipose tissue in association with acute local inflammation. It is possible that AGPAT2 deficient adipocytes have an altered lipid filling or a reduced capacity to adapt the massive lipid availability associated with postnatal feeding.

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Post weaning Agpat2^−/− mice are lipodystrophic.

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However, they are born with normal mass of white and brown adipose tissue.

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Adipose tissue in Agpat2^−/− mice undergoes postnatal inflammatory cell death.

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Differentiated Agpat2^−/− MEFs recapitulate abnormalities of Agpat2^−/− adipocytes.

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Abnormal phospholipid composition might underlies lipodystrophy in Agpat2^−/− mice.

Hormone responsiveness of cultured Sertoli cells obtained from adult rats after their rapid isolation under less harsh conditions

During adulthood, testicular Sertoli cells (Sc) coordinate all stages of germ cell (Gc) development involved in sperm production. However, our understanding about the functions of adult Sc is limited because of the difficulties involved in the process of isolating these cells from the adult testis, mainly because of the presence of large number of advanced Gc which interfere with Sc isolation at this age. Most of our knowledge about Sc function are derived from studies which used pre-pubertal rat Sc (18 ± 2-day old) as it is easy to isolate and culture Sc at this age. To this end, we established a less time consuming and less harsh procedure of isolating Sc from adult (60 days of age) rat testis for facilitating research on Sc-mediated regulation of spermatogenesis during adulthood. The cells were isolated using collagenase digestion at higher temperature, reducing the exposure time of cells to the enzyme. Step-wise digestion with intermittent removal of small clusters of tissue helped in increasing the yield of Sc. Isolated Sc were cultured and treated with FSH and testosterone (T) to evaluate their hormone responsiveness in terms of lactate, E2 , cAMP production. Adult Sc were found to be active and produced high amounts of lactate in a FSH-independent manner. FSH-mediated augmentation of cAMP and E2 production by adult Sc was less as compared with that by pre-pubertal Sc obtained from 18-day-old rats. Androgen-binding ability of adult Sc was significantly higher than pre-pubertal Sc. Although T treatment remarkably augmented expression of Claudin 11, it failed to augment lactate production by adult Sc. This efficient and rapid procedure for isolation and culture of functionally viable adult rat Sertoli cells may pave the way for determining their role in regulation and maintenance of spermatogenesis.

Caveolae, lipid droplets, and adipose tissue biology: pathophysiological aspects

Adipocytes are specialized cells that function to store energy in the form of lipids, predominantly triglycerides (TGs), and as a regulatory system contributing to metabolic homoeostasis through the production and secretion of hormones and cytokines. The regulation of lipid homeostasis by adipose tissue is an important aspect of whole-body metabolism. Owing to the central nature of adipose tissue in lipid metabolism, dysregulation has wide-ranging effects, contributing to disorders as diverse as diabetes, cardiovascular disease, cancer, and neurodegeneration. Excess lipids are stored in specialized organelles called lipid droplets (LDs). The surface of the lipid droplet can be considered a highly regulated membrane domain that both protects the contents of the LD from unregulated lipolysis and the cell from the cytotoxic effects of elevated free fatty acids. The surface of the LD is coated with a variety of regulatory proteins, either resident or transiently associated, including enzymes involved in the breakdown of TG, lipid transport proteins, and cofactors. Recent studies have begun to unravel the range of LD-associated proteins and to define their functional significance. Importantly, the involvement of LD proteins in pathophysiological disorders is beginning to be understood. This review will outline recent advances in defining the diversity of LD-associated proteins and their links to metabolic disorders including the integral membrane protein, caveolin-1 (CAV1). Analysis of the role of CAV1 in adipose tissue has highlighted the interconnectedness between the regulation of lipid storage and the function of the adipocyte plasma membrane.

The cell biology of fat expansion

Adipose tissue is a complex, multicellular organ that profoundly influences the function of nearly all other organ systems through its diverse metabolite and adipokine secretome. Adipocytes are the primary cell type of adipose tissue and play a key role in maintaining energy homeostasis. The efficiency with which adipose tissue responds to whole-body energetic demands reflects the ability of adipocytes to adapt to an altered nutrient environment, and has profound systemic implications. Deciphering adipocyte cell biology is an important component of understanding how the aberrant physiology of expanding adipose tissue contributes to the metabolic dysregulation associated with obesity.

Regulation of adipocyte lipolysis

In adipocytes the hydrolysis of TAG to produce fatty acids and glycerol under fasting conditions or times of elevated energy demands is tightly regulated by neuroendocrine signals, resulting in the activation of lipolytic enzymes. Among the classic regulators of lipolysis, adrenergic stimulation and the insulin-mediated control of lipid mobilisation are the best known. Initially, hormone-sensitive lipase (HSL) was thought to be the rate-limiting enzyme of the first lipolytic step, while we now know that adipocyte TAG lipase is the key enzyme for lipolysis initiation. Pivotal, previously unsuspected components have also been identified at the protective interface of the lipid droplet surface and in the signalling pathways that control lipolysis. Perilipin, comparative gene identification-58 (CGI-58) and other proteins of the lipid droplet surface are currently known to be key regulators of the lipolytic machinery, protecting or exposing the TAG core of the droplet to lipases. The neuroendocrine control of lipolysis is prototypically exerted by catecholaminergic stimulation and insulin-induced suppression, both of which affect cyclic AMP levels and hence the protein kinase A-mediated phosphorylation of HSL and perilipin. Interestingly, in recent decades adipose tissue has been shown to secrete a large number of adipokines, which exert direct effects on lipolysis, while adipocytes reportedly express a wide range of receptors for signals involved in lipid mobilisation. Recently recognised mediators of lipolysis include some adipokines, structural membrane proteins, atrial natriuretic peptides, AMP-activated protein kinase and mitogen-activated protein kinase. Lipolysis needs to be reanalysed from the broader perspective of its specific physiological or pathological context since basal or stimulated lipolytic rates occur under diverse conditions and by different mechanisms.

An association of metabolic syndrome constellation with cellular membrane caveolae

Metabolic syndrome (MetS) is a cluster of metabolic abnormalities that can predispose an individual to a greater risk of developing type-2 diabetes and cardiovascular diseases. The cluster includes abdominal obesity, dyslipidemia, hypertension, and hyperglycemia - all of which are risk factors to public health. While searching for a link among the aforementioned malaises, clues have been focused on the cell membrane domain caveolae, wherein the MetS-associated active molecules are colocalized and interacted with to carry out designated biological activities. Caveola disarray could induce all of those individual metabolic abnormalities to be present in animal models and humans, providing a new target for therapeutic strategy in the management of MetS.

Pleiotropic effects of cavin-1 deficiency on lipid metabolism

Mice and humans lacking caveolae due to gene knock-out or inactivating mutations of cavin-1/PTRF have numerous pathologies including markedly aberrant fuel metabolism, lipodystrophy, and muscular dystrophy. We characterized the physiologic/metabolic profile of cavin-1 knock-out mice and determined that they were lean because of reduced white adipose depots. The knock-out mice were resistant to diet-induced obesity and had abnormal lipid metabolism in the major metabolic organs of white and brown fat and liver. Epididymal white fat cells from cavin-1-null mice were small and insensitive to insulin and β-adrenergic agonists resulting in reduced adipocyte lipid storage and impaired lipid tolerance. At the molecular level, the lipolytic defects in white fat were caused by impaired perilipin phosphorylation, and the reduced triglyceride accumulation was caused by decreased fatty acid uptake and incorporation as well as the virtual absence of insulin-stimulated glucose transport. The livers of cavin-1-null mice were mildly steatotic and did not accumulate more lipid after high-fat feeding. The brown adipose tissues of cavin-1-null mice exhibited decreased mitochondria protein expression, which was restored upon high fat feeding. Taken together, these data suggest that dysfunction in fat, muscle, and liver metabolism in cavin-1-null mice causes a pleiotropic phenotype, one apparently identical to that of humans lacking caveolae in all tissues.

Adipocyte size and cellular expression of caveolar proteins analyzed by confocal microscopy

Caveolae are abundant in adipocytes and are involved in the regulation of lipid accumulation, which is the main volume determinant of these cells. We have developed and applied a confocal microscopic technique for measuring individual cellular expression of the caveolar proteins cavin-1 and caveolin-1 along with the size of individual adipocytes. The technique was applied on collagenase isolated adipocytes from ad libitum fed Sprague-Dawley rats of different age (4–26 wk) and weight (103–629 g). We found that cellular expression of caveolar proteins was variable (SD of log expression in the range from 0.25 to 0.65). Regression analysis of protein expression on adipocyte size revealed that the expression of the caveolar proteins cavin-1 and caveolin-1 on adipocytes from individual rats was tightly related to adipocyte cell surface area (mean coefficient of regression was 0.83 for cavin and 0.77 for caveolin), indicating that caveolar density was the same in membranes from all cells within a biopsy. This intrinsic relation remained unchanged with animal age, but adipocytes from animals with increasing age showed a decrease in mean expression of caveolar proteins per unit cell surface. The different relation between adipocyte size and cellular expression levels of caveolar proteins within and between individuals of different age shows that caveolar density is an age-sensitive characteristic of adipocytes.

Role of adipose specific lipid droplet proteins in maintaining whole body energy homeostasis

Excess or insufficient lipid storage in white adipose tissue lipid droplets is associated with dyslipidemia, insulin resistance and increased risk for diabetes type 2. Thus, maintenance of adipose lipid droplet growth and function is critical to preserve whole body insulin sensitivity and energy homeostasis. Progress in understanding biology of lipid droplets has underscored the role of proteins that interact with lipid droplets. Here, we review the current knowledge of adipose specific lipid droplet proteins, which share unique functions controlling adipocyte lipid storage, limiting lipid spill-over and lipotoxic effects thought to contribute to disease. This article is part of a Special Issue entitled: Modulation of Adipose Tissue in Health and Disease.

Caveolin-1 deficiency leads to increased susceptibility to cell death and fibrosis in white adipose tissue: characterization of a lipodystrophic model

Caveolin-1 (CAV1) is an important regulator of adipose tissue homeostasis. In the present study we examined the impact of CAV1 deficiency on the properties of mouse adipose tissue both in vivo and in explant cultures during conditions of metabolic stress. In CAV1(-/-) mice fasting caused loss of adipose tissue mass despite a lack of hormone-sensitive lipase (HSL) phosphorylation. In addition, fasting resulted in increased macrophage infiltration, enhanced deposition of collagen, and a reduction in the level of the lipid droplet protein perilipin A (PLIN1a). Explant cultures of CAV1(-/-) adipose tissue also showed a loss of PLIN1a during culture, enhanced secretion of IL-6, increased release of lactate dehydrogenase, and demonstrated increased susceptibility to cell death upon collagenase treatment. Attenuated PKA-mediated signaling to HSL, loss of PLIN1a and increased secretion of IL-6 were also observed in adipose tissue explants of CAV1(+/+) mice with diet-induced obesity. Together these results suggest that while alterations in adipocyte lipid droplet biology support adipose tissue metabolism in the absence of PKA-mediated pro-lipolytic signaling in CAV1(-/-) mice, the tissue is intrinsically unstable resulting in increased susceptibility to cell death, which we suggest underlies the development of fibrosis and inflammation during periods of metabolic stress.

Caveolin-1 deficiency leads to increased susceptibility to cell death and fibrosis in white adipose tissue: characterization of a lipodystrophic model

Caveolin-1 (CAV1) is an important regulator of adipose tissue homeostasis. In the present study we examined the impact of CAV1 deficiency on the properties of mouse adipose tissue both in vivo and in explant cultures during conditions of metabolic stress. In CAV1^−/− mice fasting caused loss of adipose tissue mass despite a lack of hormone-sensitive lipase (HSL) phosphorylation. In addition, fasting resulted in increased macrophage infiltration, enhanced deposition of collagen, and a reduction in the level of the lipid droplet protein perilipin A (PLIN1a). Explant cultures of CAV1^−/− adipose tissue also showed a loss of PLIN1a during culture, enhanced secretion of IL-6, increased release of lactate dehydrogenase, and demonstrated increased susceptibility to cell death upon collagenase treatment. Attenuated PKA-mediated signaling to HSL, loss of PLIN1a and increased secretion of IL-6 were also observed in adipose tissue explants of CAV1^+/+ mice with diet-induced obesity. Together these results suggest that while alterations in adipocyte lipid droplet biology support adipose tissue metabolism in the absence of PKA-mediated pro-lipolytic signaling in CAV1^−/− mice, the tissue is intrinsically unstable resulting in increased susceptibility to cell death, which we suggest underlies the development of fibrosis and inflammation during periods of metabolic stress.

Cholesterol depletion in adipocytes causes caveolae collapse concomitant with proteosomal degradation of cavin-2 in a switch-like fashion

Caveolae, little caves of cell surfaces, are enriched in cholesterol, a certain level of which is required for their structural integrity. Here we show in adipocytes that cavin-2, a peripheral membrane protein and one of 3 cavin isoforms present in caveolae from non-muscle tissue, is degraded upon cholesterol depletion in a rapid fashion resulting in collapse of caveolae. We exposed 3T3-L1 adipocytes to the cholesterol depleting agent methyl-β-cyclodextrin, which results in a sudden and extensive degradation of cavin-2 by the proteasome and a concomitant movement of cavin-1 from the plasma membrane to the cytosol along with loss of caveolae. The recovery of cavin-2 at the plasma membrane is cholesterol-dependent and is required for the return of cavin-1 from the cytosol to the cell surface and caveolae restoration. Expression of shRNA directed against cavin-2 also results in a cytosolic distribution of cavin-1 and loss of caveolae. Taken together, these data demonstrate that cavin-2 functions as a cholesterol responsive component of caveolae that is required for cavin-1 localization to the plasma membrane, and caveolae structural integrity.