PKC-η mediates glioblastoma cell proliferation through the Akt and mTOR signaling pathways

We previously demonstrated that protein kinase C-η (PKC-η) mediates a phorbol 12-myristate-13-acetate (PMA)-induced proliferative response in human glioblastoma (GBM) cells. In this report, we show that PMA-stimulated activation of PKC-η in U-251 GBM cells resulted in activation of both Akt and the mammalian target of rapamycin (mTOR) signaling pathways and an increase in cell proliferation. Expression of a kinase dead PKC-η (PKC-ηKR) construct reduced the basal and PMA-evoked proliferation of PKC-η-expressing U-251 GBM cells, as well as abrogated the PMA-induced activation of Akt, mTOR, and the mTOR targets 4E-BP1 and STAT-3. Treatment of cells with the PI-3 kinase inhibitor LY294002 (10 μM) or the mTOR inhibitor rapamycin (10 nM) also reduced PMA-induced proliferation and cell-cycle progression. Expression of a constitutively active PKC-η (PKC-ηΔNPS) construct in a GBM cell line with no endogenous PKC-η (U-1242) also provided evidence that PKC-η targets the Akt and mTOR signaling pathways. Moreover, activation of 4E-BP1 and STAT-3 in both PMA-treated U-251 and PKC-ηΔNPS-expressing U-1242 GBM cells was inhibited by rapamycin. However, activation of Akt, but not mTOR was inhibited by the PI-3 kinase inhibitor LY294002. This study identifies Akt and mTOR as downstream targets of PKC-η that are involved in GBM cell proliferation.