Abstract
Obesity is one of the top preventable causes of death in the United States, as it can lead to cardiovascular disease, type II diabetes, and cancer. Visceral adipose tissue (VAT) accumulation and inflammation directly link to metabolic dysfunction and obesity-associated disease, and act as predictors of obesity-associated mortality. As such, identification of novel targets to limit diet-induced VAT accumulation has the potential to impact morbidity and mortality.
Inflammatory cells, as well as the cytokines they produce, play a large role in obesity-related diseases. High-fat diet (HFD) results in the induction of expression of proinflammatory genes in both mice and humans, such as monocyte chemoattractant protein-1 (MCP-1). MCP-1 is a potent chemotactic factor for monocytes. Once infiltrated into the adipose tissue in the advanced stages of obesity, macrophages participate in the inflammatory pathways that are activated in obese adipose tissue, including insulin signaling, toll-like receptor (TLR) activation, and the nuclear factor kappa B (NFκB) pathway. While it is evident that macrophages are the main producers of MCP-1 during later stages of obesity, it is less clear where the initial obesity-induced increases in MCP-1 are derived.
The helix-loop-helix (HLH) family of transcription factors is a highly conserved group that plays a role in the differentiation and growth of a variety of cell types. Inhibitor of Differentiation 3 (Id3) belongs to the HLH transcription factor family, and protein levels increase in the stromavascular fraction (SVF) of visceral adipose tissue during obesity. Global deletion of Id3 attenuates HFD-induced obesity, seen by decreased body weight and attenuated expansion of VAT. However, it is yet to be determined whether Id3 plays a role in obesity-induced metabolic perturbations or inflammation within VAT.
MCP-1 intracellular staining determined that CD45-CD31-Ter119-CD34+CD29+Sca-1+ adipocyte progenitor cells (AdPCs) within VAT are the first cells to produce MCP-1 after initiation of HFD. Production of MCP-1 was limited to the CD24- subpopulation of AdPCs, those that have been previously demonstrated to be further committed to the adipocyte lineage. 1 week of HFD results in an increase in the number of CD24- AdPCs, primarily via proliferation. Human AdPCs, identified as CD45-CD31-CD34+CD44+CD90+ cells, also expressed MCP-1, with higher expression in omental adipose tissue compared to subcutaneous adipose tissue. Additionally, high surface levels of CD44 on AdPCs marked the most abundant producers of MCP-1, in both murine and human VAT.
Id3 was identified as a critical regulator of AdPC proliferation, which may be dependent on Id3-induced repression of the p21Cip1 promoter. Id3-deficient mice had fewer CD24- AdPCs, as well as reduced MCP-1 levels and attenuated accumulation of M1 macrophages within the VAT. Id3-/- mice also had improved glucose uptake during glucose tolerance tests (GTTs) as well as enhanced insulin-stimulated phosphorylation of AKT. Adoptive transfer of Id3+/+ AdPCs to Id3-/- mice resulted in increased MCP-1 secretion and significantly enhanced M1/M2 macrophage ratio in VAT. Adoptive transfer also resulted in increased weight gain and worsened glucose tolerance. However, adoptive transfer of Id3-/- AdPCs to Id3-/- mice had no effect on these parameters, leading us to believe that AdPCs must express Id3 to have MCP-1-mediated increases in M1 macrophages and glucose intolerance.
