Patho-physiological conditions with high oxidative stress, such as conditions associated with increased denatured heme-proteins, are associated with enhanced adipogenic response. This effect predominantly manifests as adipocyte hypertrophy characterized by dysfunctional, pro-inflammatory adipocytes exhibiting reduced expression of anti-inflammatory hormone, adiponectin. To understand how increased levels of cellular heme, a pro-oxidant molecule, modulates adipogenesis; the following study was designed to evaluate effects of heme on adipogenesis in human mesenchymal stem cells (hMSCs) and mouse pre-adipocytes (3T3L1). Experiments were conducted in the absence and in the presence of a superoxide dismutase (SOD) mimetic (tempol, 100 µM). Heme (10 µM) increased (P<0.05) adipogenesis in hMSCs and mouse pre-adipocytes, where tempol alone (100 µmol/L) attenuated adipogenesis in these cells (P<0.05). Tempol also reversed heme-induced increase in adipogenesis in both hMSCs and mouse pre-adipocytes (P<0.05). In addition, heme exposed 3T3L1 exhibited reduced (P<0.05) expression of transcriptional regulator-sirtuin 1 (Sirt1), along with, increased (P<0.05) expression of adipogenic markers peroxisome proliferators-activated receptor-gamma (PPARγ), C/EBPα, and aP2. These effects of heme were rescued (P<0.05) in cells concurrently treated with heme and tempol (P<0.05) and prevented in cells over-expressing Sirt1. Taken together, our results indicate that heme-induced oxidative stress inhibits Sirt1, thus un-inhibiting adipogenic regulators such as PPARγ and C/EBPα; which in turn induce increased adipogenesis along with adipocyte hypertrophy in pre-adipocytes. Anti-oxidant induced offsetting of these effects of heme supports the role of heme-dependent oxidative stress in mediating such events.
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