Glucagon, the counter-regulatory hormone to insulin, is secreted from pancreatic ␣ cells in response to low blood glucose. To examine the role of glucagon in glucose homeostasis, mice were generated with a null mutation of the glucagon receptor (Gcgr ؊/؊ ). These mice display lower blood glucose levels throughout the day and improved glucose tolerance but similar insulin levels compared with control animals. Gcgr ؊/؊ mice displayed supraphysiological glucagon levels associated with postnatal enlargement of the pancreas and hyperplasia of islets due predominantly to ␣ cell, and to a lesser extent, ␦ cell proliferation. In addition, increased proglucagon expression and processing resulted in increased pancreatic glucogen-like peptide 1 (GLP-1) (1-37) and GLP-1 amide (1-36 amide) content and a 3-to 10-fold increase in circulating GLP-1 amide. Gcgr ؊/؊ mice also displayed reduced adiposity and leptin levels but normal body weight, food intake, and energy expenditure. These data indicate that glucagon is essential for maintenance of normal glycemia and postnatal regulation of islet and ␣ and ␦ cell numbers. Furthermore, the lean phenotype of Gcgr ؊/؊ mice suggests glucagon action may be involved in the regulation of whole body composition.

Caveolae organelles and caveolin-1 protein expression are most abundant in adipocytes and endothelial cells. Our initial report on mice lacking caveolin-1 (Cav-1) demonstrated a loss of caveolae and perturbations in endothelial cell function. More recently, however, observation of the Cav-1-deficient cohorts into old age revealed significantly lower body weights, as compared with wild-type controls. These results suggest that Cav-1 null mice may have problems with lipid metabolism and/or adipocyte functioning. To test this hypothesis directly, we placed a cohort of wild-type and Cav-1 null mice on a high fat diet. Interestingly, despite being hyperphagic, Cav-1 null mice show overt resistance to diet-induced obesity. As predicted, adipocytes from Cav-1 null null mice lack caveolae membranes. Early on, a lack of caveolin-1 selectively affects only the female mammary gland fat pad and results in a near complete ablation of the hypo-dermal fat layer. There are also indications of generalized adipose tissue pathology. With increasing age, a systemic decompensation in lipid accumulation occurs resulting in dramatically smaller fat pads, histologically reduced adipocyte cell diameter, and a poorly differentiated/hypercellular white adipose parenchyma. To gain mechanistic insights into this phenotype, we show that, although serum insulin, glucose, and cholesterol levels are entirely normal, Cav-1 null mice have severely elevated triglyceride and free fatty acid levels, especially in the postprandial state. However, this build-up of triglyceriderich chylomicrons/very low density lipoproteins is not due to perturbed lipoprotein lipase activity, a major culprit of isolated hypertriglyceridemia. The lean body phenotype and metabolic defects observed in Cav-1 null mice are consistent with the previously proposed functions of caveolin-1 and caveolae in adipocytes. Our results show for the first time a clear role for caveolins in systemic lipid homeostasis in vivo and place caveolin-1/ caveolae as major factors in hyperlipidemias and obesity.

A growing body of evidence suggests that muscle cell caveolae may function as specialized membrane microdomains in which the dystrophin-glycoprotein complex and cellular signaling molecules reside. Caveolin-3 (Cav-3) is the only caveolin family member expressed in striated muscle cell types (cardiac and skeletal). Interestingly, skeletal muscle fibers from Cav-3 (؊/؊) knockout mice show a number of myopathic changes, consistent with a mild-to-moderate muscular dystrophy phenotype. However, it remains unknown whether a loss of Cav-3 affects the phenotypic behavior cardiac myocytes in vivo. Here, we present a detailed characterization of the hearts of Cav-3 knock-out mice. We show that these mice develop a progressive cardiomyopathic phenotype. At four months of age, Cav-3 knock-out hearts display significant hypertrophy, dilation, and reduced fractional shortening, as revealed by gated cardiac MRI and transthoracic echocardiography. Histological analysis reveals marked cardiac myocyte hypertrophy, with accompanying cellular infiltrates and progressive interstitial/peri-vascular fibrosis. Interestingly, loss of Cav-3 expression in the heart does not change the expression or the membrane association of the dystrophin-glycoprotein (DG) complex. However, a marker of the DG complex, ␣-sarcoglycan, was specifically excluded from lipid raft domains in the absence of Cav-3. Because activation of the Ras-p42/44 MAPK pathway in cardiac myocytes can drive cardiac hypertrophy, we next assessed the activation state of this pathway using a phospho-specific antibody probe. We show that p42/44 MAPK (ERK1/2) is hyperactivated in hearts derived from Cav-3 knock-out mice. These results are consistent with previous in vitro data demonstrating that caveolins may function as negative regulators of the p42/44 MAPK cascade. Taken together, our data argue that loss of Cav-3 expression is sufficient to induce a molecular program leading to cardiac myocyte hypertrophy and cardiomyopathy.

Recently, development of a caveolin-1-deficient (Cav-1 null) mouse model has allowed the detailed analysis of caveolin-1's function in the context of a whole animal. Interestingly, we now report that the hearts of Cav-1 null mice are markedly abnormal, despite the fact that caveolin-1 is not expressed in cardiac myocytes. However, caveolin-1 is abundantly expressed in the nonmyocytic cells of the heart, i.e., cardiac fibroblasts and endothelia. Quantitative imaging studies of Cav-1 null hearts demonstrate a significantly enlarged right ventricular cavity and a thickened left ventricular wall with decreased systolic function. Histological analysis reveals myocyte hypertrophy with interstitial/perivascular fibrosis. Because caveolin-1 is thought to act as a negative regulator of the p42/44 MAP kinase cascade, we performed Western blot analysis with phospho-specific antibodies that only recognize activated ERK1/2. As predicted, the p42/44 MAP kinase cascade is hyperactivated in Cav-1 null heart tissue (i.e., interstitial fibrotic lesions) and isolated cardiac fibroblasts. In addition, endothelial and inducible nitric oxide synthase levels are dramatically upregulated. Thus loss of caveolin-1 expression drives p42/44 MAP kinase activation and cardiac hypertrophy.