Peroxisome proliferator-activated receptor γ inhibits follicular and anaplastic thyroid carcinoma cells growth by upregulating p21Cip1/WAF1 gene in a Sp1-dependent

Peroxisome proliferator-activated receptor gamma (PPAR gamma) has been demonstrated to be antineoplastic against various human tumors. The aim of this study was to delineate the molecular mechanism underlying PPAR gamma ligand rosiglitazone (BRL) antiproliferative effects in follicular WRO and anaplastic FRO human thyroid carcinoma cells. BRIL upregulated the p21(Cip1/WAF1) levels in the two thyroid cancer cells, while did not modify the p53 protein content. Different evidences indicate that the p21(Cip1/WAF1) upregulation by BRL requires a functional PPAR gamma, since it was reversed by silencing PPAR gamma and pretreatment with GW9662, an irreversible PPAR gamma antagonist. Transient transfection assays showed that BRIL triggered the transcriptional activity of p21(Cip1/WAF1) promoter gene in a p53-independent way, being a p21(Cip1/WAF1) promoter construct deleted in the p53 sites still activated by BRL. The Sp1 inhibitor mithramycin silenced the p21(Cip/WAF1) promoter activity suggesting an important role of Sp1 in mediating BRIL activation. The electrophoretic mobility shift and chromatin immunoprecipitation (ChIP) assays evidenced a functional interaction between PPAR gamma and Sp1 in regulating p21(Cip1/WAF1). Intriguingly, ChIP analysis revealed in the p21(Cip1/WAF1) gene promoter an increased recruitment of the RNA Pol II associated with an increased histone H3 acetylation and a reduced H3 methylation. The biological event, consistent with PPAR gamma-induced WRO and FRO cell growth inhibition, was reversed by p21(Cip1/WAF1) antisense oligonucleotides and was confirmed by increasing the PPAR gamma expression, suggesting a crucial role exerted by p21(Cip1/WAF1) in PPAR gamma action. Our results further candidate BRL as a potential agent able to inhibit tumor progression of follicular and anaplastic thyroid carcinoma.