Stress Response Gadd45a Gene Functions as Tumor Suppressor In MYC Expressing Myeloid Cells Modulating Apoptosis and Cytokine Receptor Expression.

Abstract 1556

c-MYC, which can promote cell cycle progression, genomic instability, and block differentiation, is among the most frequently affected genes in human cancers, making this oncoprotein and its down-stream effectors attractive targets for drug discovery. MYC triggered apoptosis provides a built in failsafe program to limit unchecked cell growth when expressed in inappropriate conditions, ensuring that it is restricted to the correct environment. Tumors from cells over-expressing c-MYC often have mutations that disable the apoptotic program. The ramifications with regard to tumor etiology and progression, as well as the response of malignancies to different therapies has been tremendously enhanced by many studies involving the identification of targets involved in the MYC-triggered apoptotic response and on the flip side, identifying which apoptotic regulators are disabled in tumor cells. Many questions still remain to be answered. The biological and genetic setting appears to determine how a cell responds to altered MYC expression. The gadd45 gene family plays pivotal roles as stress sensors that modulate signaling in response to physiological and environmental stressors, also modulating susceptibility of cells for transformation in vitro and tumor development in vivo. Gadd45 behaves as either tumor suppressor or oncogene depending upon the transforming oncogene and the cell type (Tront et al., Cancer Research 66:8448-54, 2006; Tront et al., Cancer Research, in press). To elucidate the role Gadd45a plays in response to the proto-oncogene c-MYC in myeloid cells, bone marrow (BM) cells from wild type (WT) and Gadd45a null mice were retrovirally infected to constitutively express c-MYC. We observed that the response of myeloid cells to deregulated MYC expression depends on the status of the Gadd45 family of stress response genes. We showed that Gadd45a null BM expressing constitutive c-MYC exhibited less apoptosis than its WT counterpart in expansion media (IL-3, IL-6, SCF), demonstrating that Gadd45a is required for optimal MYC mediated apoptosis. We have clarified that there is no apparent enhancement of cell cycle progression; therefore, loss of gadd45a in conjunction with constitutive MYC expression results in decreased apoptosis with no effect on cell proliferation. Next we determined that the stress response Gadd45a gene functions as a tumor suppressor when MYC is deregulated in myeloid cells, via decreased expression of phospho-p38 MAPK, and the concomitant reduction of both activated phospho-PU.1 and the anti-apoptotic protein, MCL-1. MYC infected cells proliferating in GM-CSF differentiating media displayed a similar block/delay in differentiation regardless of gadd45a status. Interestingly, the percent of apoptosis was higher in the Gadd45a null cells expressing constitutive MYC as compared to the WT counterpart, which appears after cells are maintained in culture. Data is consistent with loss of Gadd45a being associated with diminished GM-CSF receptors, which can account for the increased apoptosis. Experiments will be presented to explain how Gadd45a regulates the apoptotic response, depending upon the specific cytokine. Finally, experiments are underway to assess how loss of Gadd45a in vivo can impact on MYC-mediated leukemia using in vivo mouse models and primary human AML BM.