A prominent functional change during differentiation of lutein cells from follicular thecal and granulosa cells is an enhanced production and secretion of progestins. The regulation of this process is not fully understood but may be associated with the expression of transcription factors which activate genes, products of which are involved in pathways of the cholesterol and lipid metabolism. As peroxisome proliferator-activated receptors (PPARs) play a role in both pathways, we were interested in the expression of PPAR , a PPAR form which is involved in adipogenic differentiation. First, we were able to show the expression of PPAR in bovine lutein cells (day 12 of the ovarian cycle) at the mRNA and protein level by imaging, flow cytometry and blot analysis, and secondly a role of PPAR in the secretion of progesterone. The cells (24 h culture) responded dose dependently by increasing progesterone secretion (up to 1·5-fold of the basal level) to an endogenous ligand of PPAR , 15-deoxy-12,14 prostaglandin J 2 (15-dPGJ 2 ) and to the thiazolidinedione ciglitizone. Aurintricarboxylic acid (ATA) was found to reduce the intracellular PPAR level and to promote cell cycle progress, indicating that ATA can be used as a tool for experimental changes of PPAR proteins in intact cells and for studying the physiological consequences. The ATA-mediated decrease of PPAR was accompanied by reduced progesterone production and a progression of the cell cycle, suggesting a function of PPAR in both processes. The response to ATA was abrogated by a high dose (>490 nM) of 15-dPGJ 2 , suggesting that 15-dPGJ 2 exerts its effect on steroidogenic activity via PPAR and that the 15-dPGJ 2 -PPAR system plays a role in the maintenance of a differentiated quiescent stage in lutein cells.

Nuclear peroxisome proliferator-activated receptor gamma (PPARgamma) is the target of antidiabetogenic thiazolidinediones (TZD). However, recent studies failed to show that TZD has an effect in vitro on insulin-regulated glucose uptake in skeletal muscles, the major site of glucose disposal. The potential effects of TZD on cells adjacent to skeletal muscles are not well characterized but may be involved in TZD's actions. Hence, we studied these cells from mice treated with the carrier and with the TZD ciglitazone (9 nmol/g body weight). The cells were typified by lipid enrichment (floating adipocytes and macrophages), by the ectopic expression of cellular fibronectin (fibroblasts), fibronectin and PPARgamma (preadipocytes), PPARgamma and CD11b/Mac-1 (active macrophages) as revealed by flow cytometry and immunoblotting. The glucose transporter 4 proteins (GLUT4) and the uptake of glucose and long-chain fatty acids (LCFA) were determined flow cytometrically using fluorescent derivatives of glucose (NBDG) and LCFA (C16-Bodipy). The expression of tumor necrosis factor alpha (TNFalpha) in CD11b/Mac-1-positive and CD11b/Mac-1-negative cells separated by magnetic immunobeads was analyzed. The results showed that TZD treatment upregulated GLUT4 expression, and increased insulin-regulated NBDG uptake and C16-Bodipy binding and influx, at the same time as increasing the quantity of PPARgamma-expressing fibroblasts; this indicates the development of the preadipocyte phenotype. In contrast, TZD lowered the number of adipocytes (0.6-fold compared to the carrier-treated control) perhaps through an action of TNFalpha from CD11b- and PPARgamma-expressing macrophages. The data suggest that the regulatory effects of TZD on energy homeostasis involve two major targets: the PPARgamma-positive fibroblasts whose adipogenic program is promoted, and CD11b-PPARgamma-expressing macrophages which become cytotoxic and fibrogenic because of the effects of TNFalpha on neighboring adipocytes and fibroblasts, respectively.