1. ¹Department of Pathology, School of Medicine, University of Virginia, Charlottesville, VA, USA
2. ²Department of Biochemistry and Molecular Genetics, School of Medicine, University of Virginia, Charlottesville, VA, USA

Correspondence: Assistant Professor RM Uht, Department of Pathology, School of Medicine, University of Virginia, MR 5, Rm 3123, 415 Lane Road, Charlottesville, VA 22908, USA. E-mail: ruht@virginia.edu

Received 1 March 2006; Revised 11 September 2006; Accepted 11 September 2006; Published online 4 December 2006.

Abstract

Glioblastoma multiforme (GBM) is the highest grade of astrocytoma. GBM pathogenesis has been linked to receptor tyrosine kinases and kinases further down signal-transduction pathways – in particular, members of the protein kinase C (PKC) family. The expression and activity of various PKC isoforms are increased in malignant astrocytomas, but not in non-neoplastic astrocytes. This suggests that PKC activity contributes to tumor progression. The level of PKC- expressed correlates with the degree of phorbol-12-myristate-13-acetate (PMA)-induced proliferation of two glioblastoma cell lines, U-1242 MG and U-251 MG. Normally, U-1242 cells do not express PKC-, and PMA inhibits their proliferation. Conversely, PMA increases proliferation of U-1242 cells that are stably transfected with PKC- (U-1242-PKC-). PMA treatment also stimulates proliferation of U-251 cells, which express PKC-. Here, we determined that extracellular signal-regulated kinase (ERK) and Elk-1 are downstream targets of PKC-. Elk-1-mediated transcriptional activity correlates with the PKC--mediated mitogenic response. Pretreatment of U-1242-PKC- cells with inhibitors of PKC or MAPK/ERK kinase (MEK) (bisindolyl maleimide (BIM) or U0126, respectively) blocked both PMA-induced Elk-1 transcriptional activity and PMA-stimulated proliferation. An overexpressed dominant-negative PKC- reduced the mitogenic response in U-251 cells, as did reduction of Elk-1 by small interfering RNA. Taken together, these results strongly suggest that PKC--mediated glioblastoma proliferation involves MEK/mitogen-activated protein (MAP) kinase phosphorylation, activation of ERK and subsequently of Elk-1. Elk-1 target genes involved in GBM proliferative responses have yet to be identified.