Dual Regulation of Glucocorticoid-Induced Leucine Zipper (GILZ) by the Glucocorticoid Receptor and Phosphatidylinositol 3-Kinase/AKT Pathways in Multiple Myeloma

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Multiple Myeloma (MM) is an incurable plasma cell malignancy occurring within the bone marrow. Glucocorticoids (GCs) are widely used in the treatment of MM patients due to potent induction of apoptosis. Though known to occur via the glucocorticoid receptor (GR), the molecular details of GC-induced apoptosis in MM cells remain largely undefined. Glucocorticoid-induced leucine zipper (GILZ) was identified in a micro-array screen as a GC-regulated gene. Compelling data from the literature suggested a potential functional role for GILZ in apoptosis. These experiments were designed to determine the importance of GILZ in GC-induced apoptosis and to gain an understanding of how other signaling molecules in MM cells could regulate GILZ expression and impact GC-induced apoptosis. The up regulation of GILZ by GCs was confirmed in a panel of MM cell lines and clinical patient samples and the GR was shown to be necessary for GILZ induction in these cells. Reducing the levels of GILZ with siRNA constructs decreased the ability of GCs to induce apoptosis of MM cells suggesting that GILZ contributes functionally to MM cell death. Regulation of GILZ by the phosphatidylinositol 3-kinase (PI3-kinase)/AKT pathway was revealed following a RT-PCR screen to identify other regulators of GILZ. Activators of PI3-kinase/AKT (IL-6 and IGF1) down regulate GILZ and block GC-induced up regulation. Inhibitors of PI3-kinase/AKT (LY294002, wortmannin, triciribine, AKT inhibitor VIII) up regulate GILZ at both the mRNA and protein level. This effect was observed in a panel of MM cell lines and clinical patient samples. The combination of GCs and PI3-kinase/AKT inhibitors dramatically increased GILZ levels as measured by real time PCR and enhanced MM cell killing as shown with multiple apoptosis assays. This effect could be inhibited by the addition of exogenous IL-6 or IGF1 or upon co-culturing MM cells with immortalized human bone marrow stromal cell lines. The combination of the synthetic GC, Dexamethasone, and the PI3-kinase inhibitor, LY294002, caused synergistic cell killing of MM cells. This observation suggests cross-talk between the two pathways occurring at the level of GILZ regulation and warrants further investigation into the therapeutic benefit of using GCs and PI3-kinase/AKT inhibitors together to treat MM patients

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  • 08/01/2018
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