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.

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 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 both PPARγ and cyclin D1 levels influence the ability of ciglitazone to differentially regulate AR signaling in androgen-independent C4-2 prostate cancer cells.

Nuclear receptors are a superfamily of ligand-activated transcription factors that play critical roles in the regulation of normal biological processes and several disease states. Of the nuclear receptors expressed within the prostate, the androgen receptor (AR) promotes the differentiation of prostatic epithelial cells and stimulates production of enzymes needed for liquefaction of semen. Multiple forms of AR also promote the growth of both early and late stage prostate cancers. As a result, drugs that target the AR signaling pathway are routinely used to treat patients with advanced forms of prostate cancer. Data also suggest that a second member of the nuclear receptor superfamily, the peroxisome proliferator activated receptor gamma (PPARγ), is a tumor suppressor that regulates growth of normal prostate and prostate cancers. Recent studies indicate there is a bidirectional interaction between AR and PPARγ, with each receptor influencing the expression and/or activity of the other within prostatic tissues. In this review, we examine how AR and PPARγ each regulate the growth and development of normal prostatic epithelial cells and prostate cancers. We also discuss interactions between the AR and PPARγ signaling pathways and how those interactions may influence prostate biology.

Calreticulin (CRT) is a protein found mainly in the endoplasmic reticulum (ER) that maintains calcium levels and controls protein folding, but has recently been found at the cell surface, cytoplasm, and in the extracellular matrix. CRT participates in multiple physiological processes such as gene expression, the immune response, and cancer. Calreticulin has been shown to autoacetylate with the binding of preferred ligand 7,8‐diacetoxy‐4‐methylcoumarin (DAMC). This project aims to develop a chemical biology approach to investigate importance of CRT acylating abilities on its nonendoplasmic reticulum functions by targeting the downstream substrates of CRT acetylation. Our goal was to use CRT to transfer a pentynoyl tag (using a novel ligand, DPeMC) to its substrates, which can then be used as a handle for protein identification. Molecular modelling using available data in the literature was used to approximate the binding interface between CRT and the acylation ligands. Molecular Operating Environment (MOE) software was used to perform sequence alignment, simulated annealing, positional refinement, and blind docking of acylated coumarins with the CRT model. Docking studies pointed to the P domain as the most thermodynamically and sterically favourable region for acylated coumarin binding with tryptophan residue 200 within the active site on CRT. Absorption and fluorescence spectra of all coumarin compounds in ethanol:PBS (1:9 v/v) solution were investigated. Stern–Volmer quenching constant (KSV), binding constant (K), and number of binding sites (n) of each coumarin compound with CRT was determined. Our studies demonstrated that acyl coumarin compounds bind to CRT using a dynamic quenching mechanism, bind to a single binding site on the P domain, and that the protein–ligand interaction is spontaneous and exothermic.