TNF-ALPHA-MEDIATED INHIBITION AND REVERSAL OF ADIPOCYTE DIFFERENTIATION IS ACCOMPANIED BY SUPPRESSED EXPRESSION OF PPAR-GAMMA WITHOUT EFFECTS ON PREF-1 EXPRESSION

Tumor necrosis factor alpha (TNF alpha) is a polypeptide hormone with pleiotropic effects on cellular proliferation and differentiation. To investigate how TNF alpha inhibits and reverses adipocyte differentiat ion, we studied the expression of two factors involved in the adipocyt e differentiation process. Peroxisome proliferator-activated receptor gamma (PPAR gamma) is a positive regulator of adipogenesis, whereas pr eadipocyte factor 1 (Pref-1) inhibits adipocyte differentiation. The e xpression patterns of both PPAR gamma and Pref-1 change during early s tages of adipocyte differentiation. Decreased expression of Pref-1 and increased expression of PPAR gamma occur 1 day and 2 days, respective ly, after 3T3-L1 cells reach confluence. During TNF alpha-mediated inh ibition of adipocyte differentiation, PPAR gamma messenger RNA (mRNA) expression stays at low levels. In contrast. TNF alpha treatment has n o Effect on the normal decrease in Pref-1 gene expression that occurs during adipogenesis. We observed that certain cytokine and growth fact ors [such as TNF alpha, basic fibroblast growth factor, transforming g rowth factor beta, and protein kinase C-activating agents plus calcium ionophore], when added to differentiated adipocytes, cause rapid down -regulation of PPAR gamma mRNA expression with concomitant decrease in adipocyte-specific gene expression but fail to increase Pref-1 mRNA e xpression. Moreover, addition of TNF alpha to fully differentiated adi pocytes results in the rapid disappearance of PPAR gamma protein expre ssion and the rapid loss of PPAR gamma DNA-binding activity. Therefore , Pref-1 seems to function as a nonreversible molecular checkpoint who se expression is insensitive to TNF alpha-generated signals, whereas P PAR gamma expression remains sensitive to TNF alpha at all stages of t he adipogenesis program. Our results support the notion that dediffere ntiated adipocytes and preadipocytes are not identical, though they sh are many similar morphological and gene expression patterns.