Abstract 374: Inhibitor of Differentiation 3 Promotes High Fat--Induced Expansion of Committed Inflammatory Adipocyte Progenitor Cells

Background: Global deletion of helix-loop-helix (HLH) transcription factor Inhibitor of Differentiation 3 (Id3) attenuates high fat diet (HFD) induced obesity and adipose tissue inflammation. Specifically, Id3 -/- mice have less epididymal adipose tissue (eAT), fewer proinflammatory macrophages in eAT after HFD, and attenuated HFD-induced levels of MCP-1 mRNA and protein in eAT. It is currently unclear how Id3 is regulating MCP-1 or macrophage accumulation in eAT. Methods & Results: Id3 -/- mice and Id3 +/+ littermates were fed 60% HFD (n=7, 5) or standard chow (n=6, 7) for 1 week. Intracellular levels of MCP-1 in stromavascular fraction (SVF) cells from eAT were analyzed via flow cytometry. HFD increased the number of MCP-1 hi cells in Id3 +/+ mice compared to chow controls (8.66 vs 2.36 x10 4 , p<0.05), but there was no increase in Id3 -/- mice (3.28 vs 1.53 x10 4 , n.s.). Phenotyping of the SVF by flow cytometry revealed that MCP-1 hi cells were mostly adipocyte progenitor cells (AdPCs) (78.8±6.8%), recently characterized as CD31 - CD45 - Ter119 - CD34 + CD29 + Sca-1 + . Interestingly, HFD induced an Id3-dependent increase in AdPCs. It was recently shown that CD24 - AdPCs are more committed to the adipocyte lineage than CD24 + AdPCs. CD24 + AdPCs lacked MCP-1 expression, but committed CD24 - AdPCs expressed MCP-1. To determine proliferation of AdPCs, BrdU was i.p. injected during the 1 week HFD ( Id3 +/+ n=11, Id3 -/- n=7). Id3 +/+ mice had a HFD-induced increase in proliferating AdPCs (11.71% vs 5.71%, p<0.01), while, Id3 -/- mice lacked an increase (7.96% vs 6.69%, n.s.). Finally, OP-9 and 3T3-L1 preadipocyte cell lines were transfected with a p21 Cip1 luciferase promoter construct. Co-transfection with Id3 reduced p21 Cip1 promoter activation by 2-fold (p<0.01). Conclusion: Committed AdPCs are the predominant source of MCP-1 in the early stages of high fat feeding, and diet-induced expansion of these AdPCs is dependent on Id3. Id3 inhibition of p21 Cip1 promoter activation in AdPCs may be one potential mechanism mediating this effect. This novel evidence of the inflammatory properties of AdPCs and the role that Id3 plays in their expansion during obesity may lead to new therapeutic targets for the treatment of diet-induced adipose inflammation and its metabolic consequences.