TNF alpha induces and insulin inhibits caspase 3-dependent adipocyte apoptosis

Regulation of fat cell number by apoptosis is proposed to be part of a norm al physiological cycle in adipose growth and development. To investigate th is process, cultured rat adipocytes were treated with various concentration s of tumor necrosis factor alpha (TNF alpha) and/or insulin to determine th e roles of these factors in adipocyte apoptosis. The cells were analyzed by flow cytometry using a TUNEL assay. TNF alpha increased adipocyte apoptosi s in a dose-dependent fashion. TNF alpha-mediated apoptosis was detectable within 6 h of treatment and continued to increase with time. Decreasing med ia insulin concentration from 8.5 to 0.85 nM resulted in increased adipocyt e apoptosis, whereas high doses of insulin protected adipocytes from TNF al pha-induced apoptosis. TNF alpha-activated apoptosis was accompanied by an increase in caspase 3 activity and could be inhibited by a caspase S-specif ic inhibitor. These data suggest that adipose tissue cell number is regulat ed, in part, by an apoptotic signaling pathway that involves TNF alpha, ins ulin, and caspase 3. (C) 2001 Academic Press.