AIMS/HYPOTHESIS: Thiazolidinediones are new oral antidiabetic drugs that activate the nuclear receptor PPARgamma. Our aim was to identify potential target genes of PPARgamma in the human adipocyte in order to clarify how thiazolidinediones improve insulin sensitivity. METHODS: The effect of BRL 49653 (Rosiglitazone) on the mRNA expression of insulin receptor, insulin receptor substrate-1, p85alpha, p110alpha and p110beta subunits of phosphatidylinositol 3-kinase, Glut 4 and hormone sensitive lipase was examined in isolated adipocytes. Target mRNA levels were determined by RT-competitive PCR. RESULTS: The BRL 49653 (1 micromol/l) increased the mRNA concentrations of p85alphaPI-3 K (264 +/- 46 vs 161 +/- 31 amol/microg total RNA, p = 0.003) whithout affecting the expression of the other mRNAs of interest. This effect was dose-dependent (K0.5 = 5 nmol/l) and was reproduced by a specific activator of RXR, indicating that it was probably mediated by the PPARgamma/RXR heterodimer. The BRL 49653 also increased the amount of p85alphaPI-3K protein in adipose tissue explants (71 +/- 19%). In addition, BRL 49653 produced a more than twofold increase in insulin stimulation of phosphatidylinositol 3-kinase activity and significantly enhanced the antilipolytic action of insulin. CONCLUSION/INTERPRETATION: This work demonstrates that the gene of p85alphaPI-3K is probably a target of PPARgamma and that thiazolidinediones can improve insulin action in normal human adipocytes. Although the precise mechanism of action of BRL 49653 on PI3-Kinase activity is not completely clear, these findings improve our understanding of the insulin-sensitizing effects of the thiazolidinediones, possible drugs for the treatment of Type II (non-insulin-dependent) diabetes mellitus.