What we talk about when we talk about fat

Perivascular adipose tissue in vascular function and disease: a review of current research and animal models

Perivascular adipose tissue (PVAT), long assumed to be nothing more than vessel-supporting connective tissue, is now understood to be an important, active component of the vasculature, with integral roles in vascular health and disease. PVAT is an adipose tissue with similarities to both brown and white adipose tissue, although recent evidence suggests that PVAT develops from its own precursors. Like other adipose tissue depots, PVAT secretes numerous biologically active substances that can act in both autocrine and paracrine fashion. PVAT has also proven to be involved in vascular inflammation. Although PVAT can support inflammation during atherosclerosis via macrophage accumulation, emerging evidence suggests that PVAT also has antiatherosclerotic properties related to its abilities to induce nonshivering thermogenesis and metabolize fatty acids. We here discuss the accumulated knowledge of PVAT biology and related research on models of hypertension and atherosclerosis.

Brown adipose tissue derived VEGF-A modulates cold tolerance and energy expenditure

We recently reported that local overexpression of VEGF-A in white adipose tissue (WAT) protects against diet-induced obesity and metabolic dysfunction. The observation that VEGF-A induces a “brown adipose tissue (BAT)-like” phenotype in WAT prompted us to further explore the direct function of VEGF-A in BAT. We utilized a doxycycline (Dox)-inducible, brown adipocyte-specific VEGF-A transgenic overexpression model to assess direct effects of VEGF-A in BAT in vivo. We observed that BAT-specific VEGF-A expression increases vascularization and up-regulates expression of both UCP1 and PGC-1α in BAT. As a result, the transgenic mice show increased thermogenesis during chronic cold exposure. In diet-induced obese mice, introducing VEGF-A locally in BAT rescues capillary rarefaction, ameliorates brown adipocyte dysfunction, and improves deleterious effects on glucose and lipid metabolism caused by a high-fat diet challenge. These results demonstrate a direct positive role of VEGF-A in the activation and expansion of BAT.

Improved methodologies for the study of adipose biology: insights gained and opportunities ahead

Adipocyte differentiation and function have become areas of intense focus in the field of energy metabolism; however, understanding the role of specific genes in the establishment and maintenance of fat cell function can be challenging and complex. In this review, we offer practical guidelines for the study of adipocyte development and function. We discuss improved cellular and genetic systems for the study of adipose biology and highlight recent insights gained from these new approaches.

Role of histone acetyltransferases and histone deacetylases in adipocyte differentiation and adipogenesis

Adipogenesis is a complex process strictly regulated by a well-established cascade that has been thoroughly studied in the last two decades. This process is governed by complex regulatory networks that involve the activation/inhibition of multiple functional genes, and is controlled by histone-modifying enzymes. Among such modification enzymes, histone acetyltransferases (HATs) and histone deacetylases (HDACs) play important roles in the transcriptional regulation and post-translational modification of protein acetylation. HATs and HDACs have been shown to respond to signals that regulate cell differentiation, participate in the regulation of protein acetylation, mediate transcription and post-translation modifications, and directly acetylate/deacetylate various transcription factors and regulatory proteins. In this paper, we review the role of HATs and HDACs in white and brown adipocyte differentiation and adipogenesis, to expand our knowledge on fat formation and adipose tissue biology.

Global O-GlcNAc Levels Modulate Transcription of the Adipocyte Secretome during Chronic Insulin Resistance

Increased flux through the hexosamine biosynthetic pathway and the corresponding increase in intracellular glycosylation of proteins via O-linked β-N-acetylglucosamine (O-GlcNAc) is sufficient to induce insulin resistance (IR) in multiple systems. Previously, our group used shotgun proteomics to identify multiple rodent adipocytokines and secreted proteins whose levels are modulated upon the induction of IR by indirectly and directly modulating O-GlcNAc levels. We have validated the relative levels of several of these factors using immunoblotting. Since adipocytokines levels are regulated primarily at the level of transcription and O-GlcNAc alters the function of many transcription factors, we hypothesized that elevated O-GlcNAc levels on key transcription factors are modulating secreted protein expression. Here, we show that upon the elevation of O-GlcNAc levels and the induction of IR in mature 3T3-F442a adipocytes, the transcript levels of multiple secreted proteins reflect the modulation observed at the protein level. We validate the transcript levels in male mouse models of diabetes. Using inguinal fat pads from the severely IR db/db mouse model and the mildly IR diet-induced mouse model, we have confirmed that the secreted proteins regulated by O-GlcNAc modulation in cell culture are likewise modulated in the whole animal upon a shift to IR. By comparing the promoters of similarly regulated genes, we determine that Sp1 is a common cis-acting element. Furthermore, we show that the LPL and SPARC promoters are enriched for Sp1 and O-GlcNAc modified proteins during insulin resistance in adipocytes. Thus, the O-GlcNAc modification of proteins bound to promoters, including Sp1, is linked to adipocytokine transcription during insulin resistance.