THE ROLE OF THE STRESS RESPONSE GENE GADD45A IN MODULATING MYC MEDIATED APOPTOSIS AND DIFFERENTIATION

Abstract

The Gadd45 family of proteins is known to play a central role as cellular stress sensors that modulate the response of mammalian cells to different stressors, including oncogenic stress. Gadd45a expression is regulated during myeloid cell differentiation, and is also induced in response to acute stimulation with cytokines, myeloablation and inflammation. The proto-oncogene C-myc plays a pivotal role in growth control, differentiation and apoptosis in hematopoietic cells. Deregulated Myc in hematopoietic cells blocks the differentiation program and prevents normal homeostatic cellular apoptosis, which alters the balance of cell populations, often participating in leukemogenesis. The status of Gadd45a expression has been shown to impact on different cancers, including breast cancer and leukemia. How the stress response gene Gadd45a modulates oncogenic stress imparted by deregulated c-Myc in myeloid cells has not been investigated. We hypothesized that Gadd45a and its interacting partner proteins can modulate specific pro-survival or pro-apoptotic signaling pathways, altering the cellular response to oncogenic myc in myeloid cells. Gadd45a may play different roles in proliferating and differentiating cells, and myeloid cells in vivo are at all stages of myeloid development. Therefore, to understand how Gadd45a status impacts on proliferating and differentiating myeloid cells, we decided to study the effect of loss of Gadd45a in myc-expressing cells that are either proliferating or stimulated to undergo differentiation. Therefore, to address this issue we utilized bone marrow from wild-type (wt) and Gadd45a null mice, and retrovirally infected these cells to express constitutive Myc or empty vector control. Using these cells we have shown that bone marrow deficient in Gadd45a and expressing constitutive Myc, display decreased apoptosis under proliferating conditions, yet increased apoptosis in media containing the differentiation inducing cytokine GM-CSF. We show that in expansion media loss of Gadd45a in the presence of Myc elicits its function through the activation of p38, with evidence supporting a role for PU.1 and Mcl-1 expression, which are downstream of p-p38. In contrast, deregulated C-Myc and loss of Gadd45a does not signal through p-38 in GM-CSF, but surprisingly there is a decrease in cytokine receptor expression. This data demonstrates that Gadd45a may be required for optimal cytokine receptor expression in myeloid cells, which can impact on survival of the cells. Although in primary bone marrow Gadd45a status had no effect on differentiation of Myc expressing cells, the loss of Gadd45a in Hoxb8 generated cell lines shifted differentiation towards increased neutrophils. Determining the role of Gadd45a on the biological outcome of myeloid cells in response to deregulated c-Myc will provide vital information in understanding the function of Gadd45a in the development and progression of Myc expressing myeloid leukemia.