Enzastaurin a protein kinase Cbeta-selective inhibitor, inhibits the growth of SCLC and NSCLC cell lines

Abstract

13138 Background: PKCβ is a member of the PKC family of serine-threonine protein kinases involved in tumor cell proliferation and apoptosis. Inhibition of PKCs induced differentiation and enhanced chemotherapy. PKCβ activation is required for tumor-induced angiogenesis. The PKCβ-selective inhibitor enzastaurin, originally developed as an antiangiogenic agent, inhibited tumor cell proliferation in prostate, colon and glioblastoma cell lines in vitro. In NSCLC lines, enhanced phosphorylation and altered PKC expression was demonstrated. In SCLC lines specific PKC isoforms were associated with cisplatin resistance. Methods: The growth inhibitory effects of enzastaruin were evaluated by 6-day MTT assays; the cell cycle effects by FACS analysis; the effects on downstream phophorylated signaling molecules by western blotting. Results: Enzastaurin inhibited the growth of 11 SCLC lines (IC50s 3–10 μM) and 4 NSCLC cell lines (IC50s 3–10 μM). An increase of 7–31% of cells in the G2/M phase of the cell cycle compared to untreated control was observed following 48 hour exposure to the IC50 dose of enzastaurin in both SCLC and NSCLC cell lines. PKCβ has been shown to phosphorylate both GSK3β and Akt. A 24-hour IC50 enzastaurin exposure significantly reduced phosphorylation of GSK3β (Ser9) in both SCLC and NSCLC lines. No changes were observed in phospho-AKT (Thr308) in either SCLC or NSCLC cell lines. Phospho-ribosomal protein S6 (Ser240/244) was also reduced in both SCLC and NSCLC cell lines. Potential synergy was studied between enzastaurin and pemetrexed in SCLC and NSCLC lines and the results were analyzed using the Calcusyn Program by Chou and Talalay. Synergistic (CI <1) to additive interactions were observed between pemetrexed (IC20–70) and enzastaurin (≤ IC50) in both SCLC lines (N = 3) and NSCLC lines (N = 2). Conclusions: We conclude that enzastaruin produces in vitro growth inhibition of SCLC and NSCLC cell lines through inhibition of GSK3β ser9 phosphorylation and has synergistic growth inhibition with pemetrexed. [Table: see text]