Glioblastoma multiforme is an aggressive form of brain cancer that responds poorly to chemotherapy and is generally incurable. The basis for the poor response of this cancer to chemotherapy is not well understood. The atypical protein kinases C (PKCi and PKCf) have previously been implicated in leukaemia cell chemoresistance. To assess the role of atypical PKC in glioblastoma cell chemoresistance, RNA interference was used to deplete human glioblastoma cells of PKCi. Transfection of cells with either of two different RNA duplexes specific for PKCi caused a partial sensitisation to cell death induced by the chemotherapy agent cisplatin. To screen for possible mechanisms for PKCi-mediated chemoresistance, microarray analysis of gene expression was performed on RNA from glioblastoma cells that were either untreated or depleted of PKCi. This identified sets of genes that were regulated either positively or negatively by PKCi. Within the set of genes that were negatively regulated by PKCi, the function of the gene coding for GMFb, an enhancer of p38 mitogen-activated protein kinase (MAP kinase) signaling, was investigated further, as the p38 MAP kinase pathway has been previously identified as a key mediator of cisplatin cytotoxicity. The expression of both GMFb mRNA and protein increased upon PKCi depletion, and this was accompanied by an increase in cisplatin-activated p38 MAP kinase signaling. Transient overexpression of GMFb increased cisplatinactivated p38 MAP kinase signaling and also sensitised cells to cisplatin cytotoxicity. The increase in cisplatin cytotoxicity seen with PKCi depletion was blocked by the p38 MAP kinase inhibitor SKF86002. These data show that PKCi can confer partial resistance to cisplatin in glioblastoma cells by suppressing GMFb-mediated enhancement of p38 MAP kinase signaling.

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