Enzastaurin (LY317615), an Oral Protein Kinase C β Inhibitor, Induces Apoptosis in Multiple Myeloma Cell Lines.

The protein kinase C (PKC) family consists of 11 serine/threonine protein kinase isoforms that are involved with cell proliferation and differentiation, gene transcription, and tumor-induced angiogenesis. The PKC pathway has been shown to play a role in the regulation of cell growth in several hematologic malignancies. However, in multiple myeloma (MM), the role of the PKC pathway has not been extensively studied. Enzastaurin (LY317615), an acyclic bisindolylmaleimide, is an oral inhibitor of the PKC β isozyme. Enzastaurin has been reported to induce apoptosis and suppress proliferation in the HCT116 colon cancer cell line by inhibiting the AKT pathway. The objective of this study was to assess the efficacy of Enzastaurin in inducing apoptosis in MM cell lines and to investigate possible mechanisms of apoptosis. A spectrum of MM cell lines, with unique characteristics (dexamethasone-sensitive, dexamethasone-resistant, chemotherapy-sensitive, chemotherapy-resistant) were treated with Enzastaurin. There is evidence of cell death in all cell lines at clinically significant concentrations (1–3μM) after 72 hours of treatment. The dexamethasone-sensitive MM1.S cell line was used to further assess the effect of Enzastaurin in the presence of dexamethasone, insulin-like growth factor-1 (IGF-1), and IL-6. IGF-1 and IL-6 are potent growth factors for MM and have been observed to blunt the cytotoxic activity of dexamethasone. Dexamethasone and Enzastaurin appear to have an additive effect in the induction of apoptosis. In addition, while IGF-1 slightly decreases the effect of Enzastaurin, IL-6 has no effect. Enzastaurin treatment also induces a decrease in GSK3β and AKTSerine473 phosphorylation. GSK3β phosphorylation is thought to be a reliable pharmacodynamic marker for Enzastaurin activity. In conclusion, these data indicate that Enzastaurin induces apoptosis in MM cells and suppression of the AKT pathway may be one of the mechanisms by which Enzastaurin exerts its anti-myeloma activity.