Stage-specific effects of a thiazolidinedione on proliferation, differentiation and PPARgamma mRNA expression in 3T3-L1 adipocytes.

To clarify the target phase of thiazolidinediones, which are ligands for peroxisome proliferator-activated receptor (PPAR)gamma, during adipocyte differentiation, the effects of a thiazolidinedione, pioglitazone, on every stage during the course of adipocyte differentiation were investigated. Pioglitazone did not affect the cellular protein content and [3H]thymidine incorporation into preconfluent 3T3-L1 preadipocytes. Induction of differentiation of confluent 3T3-L1 preadipocytes with insulin, dexamethasone and isomethylbutylxanthine for 48 h resulted in 30% inhibition of [3H]thymidine incorporation into the cells and 354% increase in cellular protein content. Pioglitazone at 1 microM accelerated the increase in cellular protein content by 33% and the inhibition in the [3H]thymidine incorporation by 12%. Pioglitazone, when added from the start of the induction stage, dose-dependently enhanced cellular triglyceride accumulation, and both basal and insulin-stimulated glucose transporting activity producing only a slight increase in the ratio of insulin stimulation to basal glucose transporting activity. In mature adipocytes, however, pioglitazone did not enhance either of the transporting activities. PPARgamma messenger RNA (mRNA) levels estimated by a semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) increased during the course of adipocyte differentiation. Although pioglitazone dose-dependently up-regulated PPARgamma mRNA levels in postconfluent preadipocytes without induction, it down-regulated them in mature adipocytes. Thus, a PPARgamma agonist, pioglitazone, arrested the growth, and increased protein content and PPARgamma mRNA levels in postconfluent preadipocytes, followed by commitment and hypertrophy of 3T3-L1 cells without changing insulin sensitivity, whereas it failed to stimulate glucose transporting activities and down-regulated PPARgamma mRNA expression in mature adipocytes.