The Oral PKC-β Inhibitor Enzastaurin (LY317615) Suppress Phosphorylation and Induces Apoptosis in Multiple Myeloma Cell Lines by Inhibition of AKT Pathway.

The PKC pathway has been shown to play a role in the regulation of cell proliferation in several hematologic malignancies. In this study we tested the oral PKC-β inhibitor, Enzastaurin (LY317615 - Eli Lilly) for its therapeutic efficacy in Multiple Myeloma (MM). We first analyzed PKC-β I and II expression by Western blot in a panel of 19 human MM cell lines, showing that 9 cell lines express either 1 or both isoforms. We next examined the growth inhibition effect of Enzastaurin in the same panel of MM cell lines using either WST-1 or MTT assay and cell viability assessment by Tripan Blue exclusion. Eighteen cell lines have IC50 value ranging from 1,2 μM to 12,5 μM. To examine molecular mechanisms whereby Enzastaurin induces cytotoxicity, we performed cell cycle profiling using PI and observed a significant increase of the percentage of cells in the sub G0–G1 fraction. To determine whether Enzastaurin-induced cell death is mediated by apoptosis, we studied by ELISA and Western blot caspase 3 and PARP cleavage. We observed induction of caspase 3 and PARP cleavage in a dose and time dependent fashion. Notably, the broad caspase (Z-VAD-FMK) inhibitor reduced Enzastaurin-induced cytotoxicity. We next determined whether Enzastaurin could inhibit AKT phosphorylation in MM cell lines with constitutive phosphorylation of AKT.

Enzastaurin decreased AKT phosphorylation in a dose and time dependent fashion. Phosphorylation of GSK3β, a downstream target protein of AKT, was also markedly inhibited. Phosphorylation of PDK-1, a known upstream activator of AKT, was not affected by Enzastaurin.

In conclusion, our results indicate that Enzastaurin-induced cytotoxicity is mediated via activation of caspase. This effect is associated with significant inhibition of AKT activity and its downstream target GSK3 β. Enzastaurin does not alter the phosphorylation of the upstream AKT activator PDK-1. These data suggest that Enzastaurin inhibit AKT signalling pathway and support its evaluation in a murine model of human MM.