Phorbol 12-myristate 13-acetate inhibits FRO anaplastic human thyroid cancer cell proliferation by inducing cell cycle arrest in G1/S phase:: Evidence for an effect mediated by PKCδ

Phorbol 12-myristate 13-acetate (PMA) is known to affect a variety of cellular processes, including cell proliferation, differentiation, and migration. PMA has been shown to promote anti proliferative and antimigratory effects in manytypes of cancer cells, Our findings show that PMA induced a strong antiproliferative effect in two anaplastic (FRO and ARO) and one follicular (ML-1) thyroid cancer cell lines, and increased the fraction of FRO cells in G1 phase of the cell cycle. The fractions in the S and G2 phases were decreased. Moreover, PMA evoked a significant increase in the levels of the cell cycle regulators p21(Waf1/Cip1) and P27(Kip1). The levels of cyclin D3 and the cyclin-dependent kinases cdk4 and cdk6 decreased, as did the phosphorylation of the Rb-protein. PMA did not induce apoptosis. PMA stimulated the translocation of protein kinase C (PKC) alpha, beta I and delta isoforms to the cell membrane. PKC delta small interfering RNA attenuated the PMA-induced anti proliferative effect and prevented the upregulation of p21(Waf1/Cip1) and p(27Kip1). Pro-longed stimulation with PMA decreased the phosphorylation of mitogen-activated protein (MAP) kinase. PMA also decreased the phosphorylation of Akt and evoked a biphasic change in the phosphorylation of the forkhead box class-O protein (FOXO): an increase in phosphorylation, followed by a dephosphorylation. In addition, PMA inhibited FRO, ARO and ML-1 cell migration toward serum. The inactive phorbol ester analog 4 alpha-phorbol and the diacylglycerol analog 1,2-dioctanoyl-sn-glycerol were without an effect on proliferation and migration. The results indicate that PMA is an effective inhibitor of thyroid cancer cell proliferation and migration by a mechanism involving PKC-MAP kinase/Akt and FOXO signaling. (C) 2008 Elsevier Ireland Ltd. All rights reserved.