Differential expression of Bcl-2 in human plasma cell disorders according to proliferation status and malignancy

Multiple myeloma (MM) is a malignancy characterized by a very slow proliferation of malignant plasma cells leading to their accumulation within the bone marrow. This suggests that resistance to apoptosis may play a critical role both in the pathogenesis and resistance to treatment of MM. Bcl-2 is a key protein for the regulation of apoptosis. However, it has been shown that this protein also regulates the state of proliferation. In the current study, we show that malignant plasma cells from both the bone marrow and peripheral blood express high levels of Bcl-2 and are slowly proliferating cells. In contrast, myeloma cells from extramedullary sites (ie pleural effusion, ascitis, mammary and gastric plasmacytoma) express Bcl-2 weakly while being highly proliferative. Normal non-dividing bone marrow plasma cells express high levels of Bcl-2 protein. In contrast, four highly proliferative reactive plasmacytosis express weak levels of Bcl-2. We conclude that there is an inverse correlation between Bcl-2 expression and the proliferation rate of both normal and malignant plasma cells. These data may be explained by the double function of Bcl-2, ie its well known function as an anti-apoptotic molecule and its intriguing function as an inhibitory molecule of cell proliferation.