Loss of Adipocyte STAT5 Confers Increased Depot-Specific Adiposity in Male and Female Mice That Is Not Associated With Altered Adipose Tissue Lipolysis

Novel insights into regulators and functional modulators of adipogenesis

Adipogenesis is a process that differentiates new adipocytes from precursor cells and is tightly regulated by several factors, including many transcription factors and various post-translational modifications. Recently, new roles of adipogenesis have been suggested in various diseases. However, the molecular mechanisms and functional modulation of these adipogenic genes remain poorly understood. This review summarizes the regulatory factors and modulators of adipogenesis and discusses future research directions to identify novel mechanisms regulating adipogenesis and the effects of adipogenic regulators in pathological conditions. The master adipogenic transcriptional factors PPARγ and C/EBPα were identified along with other crucial regulatory factors such as SREBP, Kroxs, STAT5, Wnt, FOXO1, SWI/SNF, KLFs, and PARPs. These transcriptional factors regulate adipogenesis through specific mechanisms, depending on the adipogenic stage. However, further studies related to the in vivo role of newly discovered adipogenic regulators and their function in various diseases are needed to develop new potent therapeutic strategies for metabolic diseases and cancer.

Mechanisms of hepatic fatty acid oxidation and ketogenesis during fasting

Fasting is part of many weight management and health-boosting regimens. Fasting causes substantial metabolic adaptations in the liver that include the stimulation of fatty acid oxidation and ketogenesis. The induction of fatty acid oxidation and ketogenesis during fasting is mainly driven by interrelated changes in plasma levels of various hormones and an increase in plasma nonesterified fatty acid (NEFA) levels and is mediated transcriptionally by the peroxisome proliferator-activated receptor (PPAR)α, supported by CREB3L3 (cyclic AMP-responsive element-binding protein 3 like 3). Compared with men, women exhibit higher ketone levels during fasting, likely due to higher NEFA availability, suggesting that the metabolic response to fasting shows sexual dimorphism. Here, we synthesize the current molecular knowledge on the impact of fasting on hepatic fatty acid oxidation and ketogenesis.

Adipocyte STAT5 (signal transducer and activator of transcription 5) is not required for glucocorticoid-induced metabolic dysfunction

Excess glucocorticoid (GC) signaling in adipose tissue is a key driver of insulin resistance and hepatic steatosis, but underlying mechanisms have not been fully elucidated. Signal transducer and activator of transcription 5 (STAT5) signaling in adipocytes has also been implicated in the progression of similar metabolic disturbances. Although STAT5 has been shown to interact with the glucocorticoid receptor (GR) in many cell types including adipocytes, the relevance of the STAT5/GR complex has not been investigated in adipocytes. Adult male and female adipocyte-specific STAT5 knockout (STAT5AKO) and floxed mice were given corticosterone (CORT) or vehicle in their drinking water for 1 wk and examined for differences in their metabolic responses to GC excess. CORT-induced lipolysis, insulin resistance, and changes in body composition were comparable between genotypes and in both sexes. Adipocyte STAT5 is not necessary for GC-mediated progression of metabolic disease.NEW & NOTEWORTHY Both STAT5 and glucocorticoid receptor contribute to metabolic processes and type 2 diabetes, in large part, due to their functions in adipocytes. These two transcription factors can form a complex and function together. Our novel studies determined the role of adipocyte STAT5 in glucocorticoid-induced diabetes. We observed that STAT5 in adipocytes is not needed for glucocorticoid-induced diabetes.

Growth hormone stimulates lipolysis in mice but not in adipose tissue or adipocyte culture

The inhibitory effect of growth hormone (GH) on adipose tissue growth and the stimulatory effect of GH on lipolysis are well known, but the mechanisms underlying these effects are not completely understood. In this study, we revisited the effects of GH on adipose tissue growth and lipolysis in the lit/lit mouse model. The lit/lit mice are GH deficient because of a mutation in the GH releasing hormone receptor gene. We found that the lit/lit mice had more subcutaneous fat and larger adipocytes than their heterozygous lit/+ littermates and that these differences were partially reversed by 4-week GH injection. We also found that GH injection to the lit/lit mice caused the mature adipose tissue and adipocytes to reduce in size. These results demonstrate that GH inhibits adipose tissue growth at least in part by stimulating lipolysis. To determine the mechanism by which GH stimulates lipolysis, we cultured adipose tissue explants and adipocytes derived from lit/lit mice with GH and/or isoproterenol, an agonist of the beta-adrenergic receptors. These experiments showed that whereas isoproterenol, expectedly, stimulated potent lipolysis, GH, surprisingly, had no effect on basal lipolysis or isoproterenol-induced lipolysis in adipose tissue explants or adipocytes. We also found that both isoproterenol-induced lipolysis and phosphorylation of hormone-sensitive lipase were not different between lit/lit and lit/+ mice. Taken together, these results support the conclusion that GH has lipolytic effect in mice but argue against the notion that GH stimulates lipolysis by directly acting on adipocytes or by enhancing β-adrenergic receptors-mediated lipolysis.

Adipocyte-Specific Laminin Alpha 4 Deletion Preserves Adipose Tissue Health despite Increasing Adiposity

Laminins are heterotrimeric glycoproteins with structural and functional roles in basement membranes. The predominant laminin alpha chain found in adipocyte basement membranes is laminin α4 (LAMA4). Global LAMA4 deletion in mice leads to reduced adiposity and increased energy expenditure, but also results in vascular defects that complicate the interpretation of metabolic data. Here, we describe the generation and initial phenotypic analysis of an adipocyte-specific LAMA4 knockout mouse (Lama4^AKO). We first performed an in-silico analysis to determine the degree to which laminin α4 was expressed in human and murine adipocytes. Next, male Lama4^AKO and control mice were fed chow or high-fat diets and glucose tolerance was assessed along with serum insulin and leptin levels. Adipocyte area was measured in both epididymal and inguinal white adipose tissue (eWAT and iWAT, respectively), and eWAT was used for RNA-sequencing. We found that laminin α4 was highly expressed in human and murine adipocytes. Further, chow-fed Lama4^AKO mice are like control mice in terms of body weight, body composition, and glucose tolerance, although they have larger eWAT adipocytes and lower insulin levels. High-fat-fed Lama4^AKO mice are fatter and more glucose tolerant when compared to control mice. Transcriptionally, the eWAT of high-fat fed Lama4^AKO mice resembles that of chow-fed control mice. We conclude from these findings that adipocyte-specific LAMA4 deletion is protective in an obesogenic environment, even though overall adiposity is increased.

Oncostatin M Induces Lipolysis and Suppresses Insulin Response in 3T3-L1 Adipocytes

Oncostatin M (OSM) is an immune cell-derived cytokine that is upregulated in adipose tissue in obesity. Upon binding its receptor (OSMR), OSM induces the phosphorylation of the p66 subunit of Src homology 2 domain-containing transforming protein 1 (SHC1), called p66Shc, and activates the extracellular signal-related kinase (ERK) pathway. Mice with adipocyte-specific OSMR deletion (Osmr^FKO) are insulin resistant and exhibit adipose tissue inflammation, suggesting that intact adipocyte OSM–OSMR signaling is necessary for maintaining adipose tissue health. How OSM affects specific adipocyte functions is still unclear. Here, we examined the effects of OSM on adipocyte lipolysis. We treated 3T3-L1 adipocytes with OSM, insulin, and/or inhibitors of SHC1 and ERK and measured glycerol release. We also measured phosphorylation of p66Shc, ERK, and insulin receptor substrate-1 (IRS1) and the expression of lipolysis-associated genes in OSM-exposed 3T3-L1 adipocytes and primary adipocytes from control and Osmr^FKO mice. We found that OSM induces adipocyte lipolysis via a p66Shc-ERK pathway and inhibits the suppression of lipolysis by insulin. Further, OSM induces phosphorylation of inhibitory IRS1 residues. We conclude that OSM is a stimulator of lipolysis and inhibits adipocyte insulin response. Future studies will determine how these roles of OSM affect adipose tissue function in health and disease.