The ERK pathway is an important mediator of the oncogenic activity of receptor tyrosine kinases (RTKs) and Ras GTPases (16
), as well as oncogenic alleles of Ras (8
). However, the mechanisms that sustain RTK and Ras-elicited ERK pathway activity in malignant cells have remained elusive. In this work we demonstrate that PME-1 protects ERK pathway activity from PP2A-mediated inactivation in human malignant glioma. Our data show that PME-1 depletion results in increased methylation of PP2Ac leucine 309 and dephosphorylation of MEK, ERK and Elk-1 proteins. PME-1 depletion inhibited ERK pathway activity induced by RasV12, whereas cells expressing constitutively active alleles of either B-Raf or MEK, were found resistant to PME-1 shRNA (, ). Together, these observations suggest that PME-1 supports ERK pathway activity at the level upstream of Raf. As PP2A-mediated dephosphorylation of Raf has been shown to stimulate rather than inhibit Raf kinase activity (17
), the target for PME-1 regulated PP2A activity could be a protein involved in Raf activation. Candidates for such proteins could be scaffolds involved in linking the RTK proteins to Raf/MEK/ERK complex (15
). However, it is clear that future studies are needed to identify the molecular target for PME-1 regulated PP2A activity. Also, based on the data presented here we cannot exclude the possibility that, in addition to its role in supporting ERK activity, PME-1 would in part support malignant cell growth by regulating other PP2A target pathways.
Interestingly, although the role of RTK activation in response to growth factors is well studied, the role of phosphatases in this process remain incompletely understood. Here we show that PME-1 augments the cellular growth factor response ( and ). Based on these results, it is tempting to speculate that in conditions of limited growth factor supply in tumor tissues, cancer cells with high PME-1 expression levels may retain ERK pathway activity, and thereby maintain their proliferative capacity. In addition to results of our cell culture experiments, this notion is clearly supported by in vivo correlation of PME-1 expression, proliferation and ERK pathway activity in human malignant glioblastoma.
Human glioblastoma patients display extremely poor survival, and there are currently no effective treatment regimes for this disease (27
). We have found that PME-1 expression correlates with ERK pathway activity in human malignant glioblastoma () and supports ERK activity in glioblastoma cells (). These results indicate that PME-1 could represent a potential diagnostic marker to identify subgroups of patients that would benefit from treatment with small molecule Raf/MAPK pathway inhibitors, currently in clinical trials for glioblastoma (27
). In addition, as expression of phosphorylated ERK was recently shown to be associated with increased radiation resistance of malignant glioblastoma (29
), inhibition of PME-1 could sensitize glioblastoma cells to radiotherapy. In this regard, identification of small molecule inhibitors of PME-1 methylesterase activity, or PME-1-PP2Ac interaction (6
), would be important to probe potential suitability of PME-1 as a drug target in malignant glioblastoma.
These observations identify PME-1 expression as a hitherto unrecognized mechanism supporting ERK pathway activity in cancer cells. Importantly, our data suggests that in addition to growth factors and their receptors, PP2A functions as an important modulator of the cellular growth factor response. Considering the established role for growth factor-elicited ERK signaling in several human malignancies (7
), we postulate that the importance of these results reaches beyond the demonstrated role of PME-1 in human glioblastomas. Moreover, this first identification of a cellular role for PME-1 in the regulation of cellular signaling further highlights the novelty of the presented results. Together with other recently published data (12
), these findings further emphasizes the relevance of identifying mechanisms that regulate the tumor suppressor function of PP2A in human malignancies. Finally, the results of this work may open novel opportunities for the treatment and diagnosis of human cancers addicted to oncogenic ERK pathway activity (32