Anil Chawla

Signal transmission by many cell surface receptors results in the activation of phosphoinositide (PI) 3-kinases that phosphorylate the 3' position of polyphosphoinositides. From a screen for mouse proteins that bind phosphoinositides, the protein GRP1was identified. GRP1 binds phosphatidylinositol-3,4,5-trisphosphate [PtdIns(3,4, 5)P3] through a pleckstrin homology (PH) domain and displays a region of high sequence similarity to the yeast Sec7 protein. The PH domain of the closely related protein cytohesin-1, which, through its Sec7 homology domain, regulates integrin beta2 and catalyzes guanine nucleotide exchange of the small guanine nucleotide-binding protein ARF1, was also found to specifically bind PtdIns(3,4,5)P3. GRP1 and cytohesin-1 appear to connect receptor-activated PI 3-kinase signaling pathways with proteins that mediate biological responses such as cell adhesion and membrane trafficking.

Storage of energy as triglyceride in large adipose-specific lipid droplets is a fundamental need in all mammals. Efficient sequestration of fat in adipocytes also prevents fatty acid overload in skeletal muscle and liver, which can impair insulin signaling. Here we report that the Cide domain-containing protein Cidea, previously thought to be a mitochondrial protein, colocalizes around lipid droplets with perilipin, a regulator of lipolysis. Cidea-GFP greatly enhances lipid droplet size when ectopically expressed in preadipocytes or COS cells. These results explain previous findings showing that depletion of Cidea with RNAi markedly elevates lipolysis in human adipocytes. Like perilipin, Cidea and the related lipid droplet protein Cidec/FSP27 are controlled by peroxisome proliferator-activated receptor gamma (PPARgamma). Treatment of lean or obese mice with the PPARgamma agonist rosiglitazone markedly up-regulates Cidea expression in white adipose tissue (WAT), increasing lipid deposition. Strikingly, in both omental and s.c. WAT from BMI-matched obese humans, expression of Cidea, Cidec/FSP27, and perilipin correlates positively with insulin sensitivity (HOMA-IR index). Thus, Cidea and other lipid droplet proteins define a novel, highly regulated pathway of triglyceride deposition in human WAT. The data support a model whereby failure of this pathway results in ectopic lipid accumulation, insulin resistance, and its associated comorbidities in humans.

Fat-specific protein (FSP)27/Cidec is most highly expressed in white and brown adipose tissues and increases in abundance by over 50-fold during adipogenesis. However, its function in adipocytes has remained elusive since its discovery over 15 years ago. Here we demonstrate that FSP27/Cidec localizes to lipid droplets in cultured adipocytes and functions to promote lipid accumulation. Ectopically expressed FSP27-GFP surrounds lipid droplets in 3T3-L1 adipocytes and colocalizes with the known lipid droplet protein perilipin. Immunostaining of endogenous FSP27 in 3T3-L1 adipocytes also confirmed its presence on lipid droplets. FSP27-GFP expression also markedly increases lipid droplet size and enhances accumulation of total neutral lipids in 3T3-L1 preadipocytes as well as other cell types such as COS cells. Conversely, RNA interference-based FSP27/Cidec depletion in mature adipocytes significantly stimulates lipolysis and reduces the size of lipid droplets. These data reveal FSP27/Cidec as a novel adipocyte lipid droplet protein that negatively regulates lipolysis and promotes triglyceride accumulation.

Lipodystrophic syndromes are characterized by adipose tissue deficiency. Although rare, they are of considerable interest as they, like obesity, typically lead to ectopic lipid accumulation, dyslipidaemia and insulin resistant diabetes. In this paper we describe a female patient with partial lipodystrophy (affecting limb, femorogluteal and subcutaneous abdominal fat), white adipocytes with multiloculated lipid droplets and insulin-resistant diabetes, who was found to be homozygous for a premature truncation mutation in the lipid droplet protein cell death-inducing Dffa-like effector C (CIDEC) (E186X). The truncation disrupts the highly conserved CIDE-C domain and the mutant protein is mistargeted and fails to increase the lipid droplet size in transfected cells. In mice, Cidec deficiency also reduces fat mass and induces the formation of white adipocytes with multilocular lipid droplets, but in contrast to our patient, Cidec null mice are protected against diet-induced obesity and insulin resistance. In addition to describing a novel autosomal recessive form of familial partial lipodystrophy, these observations also suggest that CIDEC is required for unilocular lipid droplet formation and optimal energy storage in human fat.

Signal transmission by insulin involves tyrosine phosphorylation of a major insulin receptor substrate (IRS-1) and exchange of Ras-bound guanosine diphosphate for guanosine triphosphate. Proteins containing Src homology 2 and 3 (SH2 and SH3) domains, such as the p85 regulatory subunit of phosphatidylinositol-3 kinase and growth factor receptor-bound protein 2 (GRB2), bind tyrosine phosphate sites on IRS-1 through their SH2 regions. Such complexes in COS cells were found to contain the heterologously expressed putative guanine nucleotide exchange factor encoded by the Drosophila son of sevenless gene (dSos). Thus, GRB2, p85, or other proteins with SH2-SH3 adapter sequences may link Sos proteins to IRS-1 signaling complexes as part of the mechanism by which insulin activates Ras.