Dysregulated inflammation constitutes a major pathogenic factor in cancer development and progression. Prostaglandin E2 is the most abundant proinflammatory prostaglandin produced by the tumor microenvironment. Despite the evidence for a regulatory role of prostaglandin E2 in malignant gliomas, studies into the mechanisms whereby prostaglandin E2 regulates glioma cell growth and survival are very limited. The long-term goal of this R03 proposal is to elucidate signaling networks that can be targeted to interfere with the proliferative and prosurvival signals of prostaglandin E2 to glioma cells. The immediate objective is to elucidate the mechanisms and cellular outcome of extracellular signal-regulated kinase (ERK) inhibition by prostaglandin E2 in glioma cells. In recent studies, we showed that prostaglandin E2 stimulates growth of human glioma cells. Unexpectedly, prostaglandin E2 at concentrations required to stimulate glioma cell growth, inhibited phosphorylation of ERK and Raf-1 at their activating sites. Similar results were detected in other transformed cells, but not in normal cells. Supported by preliminary data, we propose that prostaglandin E2 activates the protein serine-threonine phosphatase PP5, which inactivates Raf-1 and its downstream signaling to ERK. We speculate further that ERK inhibition protects cells from endoplasmic reticulum (ER) stress-induced cell death.
In Aim 1, RNAi-mediated gene silencing, biochemical assays, and confocal imaging analyses will be employed to establish the requirement of PP5 in prostaglandin E2-induced dephosphorylation of Raf-1 at Ser338 and determine the underlying mechanisms. The involvement of prostaglandin E2 receptors will be established.
In Aim 2, morphological and biochemical assays will be employed to monitor ER stress-induced cell death. The consequences of prostaglandin E2 treatment on activation of the unfolded protein response by ER stressors will be determined. Ectopic expression of constitutively active Raf-1 and MEK mutants or dominant negative ERK constructs will be employed to establish the requirement of ERK inhibition for protection against ER stress-induced cell death. The rationale for the proposed work is that it will provide an enhanced understanding into the mechanisms by which glioblastoma cells escape cell death and lead to the identification of unique molecular targets for drug development. The proposed research is innovative because it proposes a previously unrecognized role for prostaglandin E2 in mediating survival responses in tumor cells via inhibition of ERK. In addition, it will identify PP5 as a novel signaling target that can be exploited to limit tumor growth and survival. Results from this work are significant because they will deepen our understanding of the signaling pathways activated by prostaglandin E2 in glioma cells. Moreover, they will have broad implications for the significance of ERK inhibition in other tumor cells. Future translational studies will establish the relevance of the protective role of prostaglandin E2 against ER stress-induced cell death and the efficacy of inhibition of prostaglandin E2- dependent pathways using in vivo glioma models.
The prognosis of patients with glioblastoma multiforme, the most aggressive among malignant gliomas, has not changed significantly in 30 years;patients still die 9-14 months from the initial diagnosis despite therapy. Glioblastoma multiforme inexorably recurs after treatment because of its high propensity to survive even in the most unfavorable microenvironment;therefore, uncovering the mechanisms underlying the survival ability of glioblastoma cells holds potential for identifying and exploiting novel targets to maximize treatment and improve clinical outcome. This research has broad implications for other tumors and premalignant diseases dependent on prostaglandin E2 for growth and survival. Decreasing cancer death is one of the focus area goals of the Department of Health and Human Services outlined in the Healthy People 2010 Report.
