MAGE-A Inhibits Apoptosis In Proliferating Multiple Myeloma Cells

Abstract 785

The type I Melanoma Antigen GEne (MAGE) MAGE-A3 is commonly present in primary multiple myeloma cells and its expression is correlated with advanced disease and proliferation. MAGE-A3 belongs to the Cancer-Testis antigen (CTAg) family of tumor-associated proteins, which are present in many cancers, but their normal expression is limited to developing germ cells and placental trophoblast. This unique expression pattern fuels speculation on a role for CTAg in oncogenesis; however, very little is known about their function. In gene expression analyses of primary myeloma cells, CTAg were associated with proliferative gene signatures and poor clinical outcome, suggesting they contribute to the pathogenesis or progression of this disease through effects on survival and/or proliferation of myeloma cells. To investigate this, we examined the impact of MAGE-A on disease progression, proliferation, and apoptosis in primary myeloma specimens and human myeloma cell lines (HMCL). MAGE-A3 protein expression was examined by immunohistochemistry in a new, independent set of myeloma bone marrow specimens from two critical clinical milestones, newly diagnosed, untreated patients and patients who relapsed after chemotherapy. MAGE-A3 was detected in a higher percentage of tumor specimens from relapsed patients (77%) compared to those from newly diagnosed patients (36%, p=0.0003). The percentage of proliferating myeloma cells, as measured by staining for the proliferation marker Ki-67, was significantly higher in relapsed specimens (19.0 ± 3.5%) compared to newly diagnosed (6.9 ± 1.3%, p=0.0002), demonstrating a correlation between MAGE-A3, progression of disease and proliferation. The mechanisms for MAGE-A3 activity were investigated by silencing this gene in primary myeloma cells and HMCL by shRNA interference. Targeted lentiviral shRNA transduction efficiently knocked down MAGE-A3 mRNA and protein in MM.1r (p53+/+) and ARP-1 (p53−/−) HMCL and in primary myeloma cells by 48 hours, and this effect was maintained up to 96 hours. Silencing of MAGE-A did not affect cell cycling, as this intervention did not affect the phosphorylation of the Retinoblastoma gene product (Rb) that is required for progression through the G1 cell cycle checkpoints and entry into S phase. In contrast, MAGE-A was required for survival of proliferating myeloma cells. Silencing of MAGE-A led to a precipitous loss of viable cells within 48–72 hrs compared to controls. This was due to activation of intrinsic apoptosis, as demonstrated by increased annexin V staining, loss of mitochondrial membrane polarization, and cleavage/activation of caspase-9. These effects of MAGE-A knock-down were completely reversed by the pan-caspase inhibitor Quinoline-Val-Asp-CH₂-OPh. Apoptosis after MAGE-A silencing appeared to be mediated by at least two distinct mechanisms; p53-dependent activation of pro-apoptotic Bax and Bak expression and reduced expression of the Inhibitor of Apoptosis Protein survivin through both p53-dependent and independent mechanisms. These results demonstrate that MAGE-A plays a role in the survival of proliferating multiple myeloma cells through the regulation of two critical apoptotic mechanisms.