1. ¹Department of Pathology and Neurology, University of Virginia, Charlottesville, VA 22908, USA
2. ²Herbert Irving Comprehensive Cancer Center, College of Physicians and Surgeons, Columbia University, New York 10032, USA

Correspondence: SE Aeder, Department of Pathology, University of Virginia, 415 Lane Road, PO Box 800904, Charlottesville, VA 22908, USA. E-mail: sea4s@virginia.edu

Received 28 June 2004; Revised 18 August 2004; Accepted 18 August 2004; Published online 18 October 2004.

Abstract

We previously demonstrated that protein kinase C- (PKC-) 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- 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- (PKC-KR) construct reduced the basal and PMA-evoked proliferation of PKC--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 M) or the mTOR inhibitor rapamycin (10 nM) also reduced PMA-induced proliferation and cell-cycle progression. Expression of a constitutively active PKC- (PKC-NPS) construct in a GBM cell line with no endogenous PKC- (U-1242) also provided evidence that PKC- targets the Akt and mTOR signaling pathways. Moreover, activation of 4E-BP1 and STAT-3 in both PMA-treated U-251 and PKC-NPS-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- that are involved in GBM cell proliferation.