The PPARγ ligand ciglitazone regulates androgen receptor activation differently in androgen-dependent versus androgen-independent human prostate cancer cells.

The androgen receptor (AR) regulates growth and progression of androgen-dependent as well as androgen-independent prostate cancer cells. Peroxisome proliferator-activated receptor gamma (PPARγ) agonists have been reported to reduce AR activation in androgen-dependent LNCaP prostate cancer cells. To determine whether PPARγ ligands are equally effective at inhibiting AR activity in androgen-independent prostate cancer, we examined the effect of the PPARγ ligands ciglitazone and rosiglitazone on C4-2 cells, an androgen- independent derivative of the LNCaP cell line. Luciferase-based reporter assays and Western blot analysis demonstrated that PPARγ ligand reduced dihydrotestosterone (DHT)-induced increases in AR activity in LNCaP cells. However, in C4-2 cells, these compounds increased DHT-induced AR driven luciferase activity. In addition, ciglitazone did not significantly alter DHT-mediated increases in prostate specific antigen (PSA) protein or mRNA levels within C4-2 cells. siRNA-based experiments demonstrated that the ciglitazone-induced regulation of AR activity observed in C4-2 cells was dependent on the presence of PPARγ. Furthermore, overexpression of the AR corepressor cyclin D1 inhibited the ability of ciglitazone to induce AR luciferase activity in C4-2 cells. Thus, our data suggest that both PPARγ and cyclin D1 levels influence the ability of ciglitazone to differentially regulate AR signaling in androgen-independent C4-2 prostate cancer cells.

Thiazolidinediones regulate expression of cell cycle proteins in human prostate cancer cells via PPARgamma-dependent and PPARgamma-independent pathways.

Thiazolidinediones (TZDs) are peroxisome proliferator activated receptor gamma (PPARgamma) ligands that have been reported to reduce proliferation of human prostate cancer cells. However, the mechanisms by which TZDs inhibit prostate cancer cell proliferation are not fully understood. In addition, it is not known if the anti-proliferative effects of TZDs require activation of PPARgamma or are mediated by PPARgamma-independent pathways. The goals of this study were to assess whether TZDs regulate expression of proteins that control the transition from G1 to S phase of the cell cycle and define the role of PPARgamma in these TZD-induced responses in androgen-independent human prostate cancer cell lines. Western blot analysis revealed that growth inhibitory concentrations of the TZDs rosiglitazone and ciglitazone induced expression of the cyclin dependent kinase inhibitor p21 and decreased cyclin D1 levels in the androgen independent PC-3 cell line. Phosphorylation of retinoblastoma protein at Serine 780 was also reduced in PC-3 cells exposed to ciglitazone. Furthermore, growth inhibitory concentrations of ciglitazone increased p21 and lowered cyclin D1 expression within C4-2 cells. PPARgamma-directed siRNAs inhibited the ability of rosiglitazone to regulate expression of cyclin D1 and p21. However, knockdown of PPARgamma did not significantly reduce ciglitazone-induced alterations in cyclin D1 and p21. Furthermore PPARgamma siRNA did not prevent inhibition of PC-3 cell proliferation by either TZD. Thus, activation of PPARgamma is involved in rosiglitazone-induced alterations in cell cycle protein expression. However, the alterations in protein expression and proliferation induced by ciglitazone occur primarily via PPARgamma-independent signaling pathways.