The adipogenic transcription factor peroxisome proliferator-activated receptor-gamma (PPARgamma) plays a pivotal role in the regulation of whole body insulin sensitivity. Little, however, is known about hormonal and molecular modulation of PPARgamma gene expression. Therefore, we investigated the temporal and conditional expression of PPARgamma in a serum-free model of 3T3-L1 adipocyte differentiation. We show here that expression of PPARgamma depends on the full set of known adipogenic stimuli and not on a single hormone/agent. Unexpectedly, an indeed marked but only transient peak of PPARgamma expression (39 +/-5-fold increase over basal on day 3 after hormonal stimulation) occurs during serum-free adipose conversion. To our knowledge, this finding is novel and probably remained hidden until now because of the common use of serum-containing preadipocyte culture systems. We conclude that maintenance, but not induction, of PPARgamma gene expression in vitro must be achieved by one or more still unknown serum component(s). (C) 2002 Elsevier Science (USA). All rights reserved.