Caveolin-1 expression and cavin stability regulate caveolae dynamics in adipocyte lipid store fluctuation

Adipocytes specialized in the storage of energy as fat are among the most caveolae-enriched cell types. Loss of caveolae produces lipodystrophic diabetes in humans, which cannot be reversed by endothelial rescue of caveolin expression in mice, indicating major importance of adipocyte caveolae. However, how caveolae participate in fat cell functions is poorly understood. We investigated dynamic conditions of lipid store fluctuations and demonstrate reciprocal regulation of caveolae density and fat cell lipid droplet storage. We identified caveolin-1 expression as a crucial step in adipose cell lines and in mice to raise the density of caveolae, to increase adipocyte ability to accommodate larger lipid droplets, and to promote cell expansion by increased glucose utilization. In human subjects enrolled in a trial of 8 weeks of overfeeding to promote fattening, adipocyte expansion response correlated with initial caveolin-1 expression. Conversely, lipid mobilization in cultured adipocytes to induce lipid droplet shrinkage led to biphasic response of cavin-1 with ultimate loss of expression of cavin-1 and -3 and EHD2 by protein degradation, coincident with caveolae disassembly. We have identified the key steps in cavin/caveolin interplay regulating adipocyte caveolae dynamics. Our data establish that caveolae participate in a unique cell response connected to lipid store fluctuation, suggesting lipid-induced mechanotension in adipocytes

The lipoatrophic caveolin-1 deficient mouse model reveals autophagy in mature adipocytes

Adipose tissue lipoatrophy caused by caveolin gene deletion in mice is not linked to defective adipocyte differentiation. We show that adipose tissue development cannot be rescued by endothelial specific caveolin-1 re-expression, indicating primordial role of caveolin in mature adipocytes. Partial or total caveolin deficiency in adipocytes induced broad protein expression defects, including but not limited to previously described downregulation of insulin receptor. Global alterations in protein turnover, and accelerated degradation of long-lived proteins were found in caveolin-deficient adipocytes. Lipidation of endogenous LC3 autophagy marker and distribution of GFP-LC3 into aggregates demonstrated activated autophagy in the absence of caveolin-1 in adipocytes. Furthermore, electron microscopy revealed autophagic vacuoles in caveolin-1 deficient but not control adipocytes. Surprisingly, significant levels of lipidated LC3-II were found around lipid droplets of normal adipocytes, maintained in nutrient-rich conditions or isolated from fed mice, which do not display autophagy. Altogether, these data indicate that caveolin deficiency induce autophagy in adipocytes, a feature that is not a physiological response to fasting in normal fat cells. This likely resulted from defective insulin and lipolytic responses that converge in chronic nutrient shortage in adipocytes lacking caveolin-1. This is the first report of a pathological situation with autophagy as an adaptative response to adipocyte failure

Regulated association of caveolins to lipid droplets during differentiation of 3T3-L1 adipocytes.

Caveolins, structural protein coats of caveolae primarily involved in membrane-related functions, have also been found associated to lipid droplets (LD), specialized organelles for fat storage. In the present study, we wanted to delineate the main features that govern the presence of caveolin-1 on adipocyte lipid droplets. Using either morphological or biochemical approaches, we found caveolins to associate to LD in 3T3-L1 adipocytes during their late maturation phase. The time course of this association could be modulated by constitutive activation of src-kinase, suggesting that the specific enrichment of caveolins in enlarged LD results from an active pathway rather than trapping of caveolins to lipid storage organelle acting as a passive sink. The fat cell size dependence of the association of organized caveolins on adipocytes LD suggests a role for these proteins in the long-term handling of lipid stores

Caveolin-1 but not cavin overexpression produces “supersized” adipocytes with larger lipid droplets in adipocytes

Caveolin-1 cooperates with the recently described cavin protein family to form membrane caveolae structures, which are particularly abundant in adipocytes. Caveolin-1 is also present in adipocytes as a lipid droplet associated pool, whose function remains uncertain. Caveolin-1 deficiency leads to lipoatrophy in mice models and human subjects, suggesting a critical role in the storage of lipids. We report here on retroviral overexpression caveolin-1-RFP, cavin-1-GFP or cavin-3-GFP in 3T3-L1 adipocytes. All three cell lines differentiated normally, contained a higher number of caveolae, and displayed ameliorated maximal insulin response, which is known to occur in caveolae. Only caveolin-1 overexpressing adipocytes, but not cavin-1 or -3, accumulated significantly larger lipid droplets than control lines containing empty retroviral vectors. Also, when injected into nude mice, all cell lines induced the formation of newly generated fat pads, but those produced from caveolin-1-RFP preadipocytes contained larger adipocytes, indicating a specific impact of caveolin-1 but not other cavins on lipid storage in vivo. Together our data demonstrate a specific role of lipid droplet caveolin pool, independent of caveolae, to modulate lipid droplet expansibility

Plasma Membrane Subdomain Compartmentalization Contributes to Distinct Mechanisms of Ceramide Action on Insulin Signaling

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Measurements and modeling of type-I and type-II ELMs heat flux to the DIII-D divertor

Type-I and type-II edge-localized-modes (ELMs) heat flux profiles measured at the DIII-D divertor feature a peak in the vicinity of the strike-point and a plateau in the scrape-off-layer (SOL), which extends to the first wall. The plateau is present in attached and detached divertors and it is found to originate with plasma bursts upstream in the SOL. The integrated ELM heat flux is distributed at ∼65% in the peak and ∼35% in this plateau. The parallel loss model, currently used at ITER to predict power loads to the walls, is benchmarked using these results in the primary and secondary divertors with unprecedented constraints using experimental input data for ELM size, radial velocity, energy, electron temperature and density, heat flux footprints and number of filaments. The model can reproduce the experimental near-SOL peak within ∼20%, but cannot match the SOL plateau. Employing a two-component approach for the ELM radial velocity, as guided by intermittent data, the full radial heat flux profile can be well matched. The ELM-averaged radial velocity at the separatrix, which explains profile widening, increases from ∼0.2 km s−1 in attached to ∼0.8 km s−1 in detached scenarios, as the ELM filaments’ path becomes electrically disconnected from the sheath at the target. The results presented here indicate filaments fragmentation as a possible mechanism for ELM transport to the far-SOL and provide evidence on the beneficial role of detachment to mitigate ELM flux in the divertor far-SOL. However, these findings imply that wall regions far from the strike points in future machines should be designed to withstand significant heat flux, even for small-ELM regimes

Lipid droplet analysis in caveolin-deficient adipocytes: alterations in surface phospholipid composition and maturation defects.

Caveolins form plasmalemnal invaginated caveolae. They also locate around intracellular lipid droplets but their role in this location remains unclear. By studying primary adipocytes that highly express caveolin-1, we characterized the impact of caveolin-1 deficiency on lipid droplet proteome and lipidome. We identified several missing proteins on the lipid droplet surface of caveolin-deficient adipocytes and showed that the caveolin-1 lipid droplet pool is organized as multi-protein complexes containing cavin-1, with similar dynamics as those found in caveolae. On the lipid side, caveolin deficiency did not qualitatively alter neutral lipids in lipid droplet, but significantly reduced the relative abundance of surface phospholipid species: phosphatidylserine and lysophospholipids. Caveolin-deficient adipocytes can form only small lipid droplets, suggesting that the caveolin-lipid droplet pool might be involved in lipid droplet size regulation. Accordingly, we show that caveolin-1 concentration on adipocyte lipid droplets positively correlated with lipid droplet size in obese rodent models and human adipocytes. Moreover, rescue experiments by caveolin- green fluorescent protein in caveolin-deficient cells exposed to fatty acid overload demonstrated that caveolin-coated lipid droplets were able to grow larger than caveolin-devoid lipid droplets. Altogether, these data demonstrate that the lipid droplet-caveolin pool impacts on phospholipid and protein surface composition of lipid droplets and suggest a functional role on lipid droplet expandability