2-Chlorodeoxyadenosine (2-CDA) and Dexamethasone Induce Apoptosis in Multiple Myeloma (MM) Via Different Mechanisms.

Although 2-CDA has been active on B-lymphocyte derived malignancies, its potential for myeloma growth control has not been fully investigated. In the present study on a pair of MM cell lines, dexamethasone-sensitive (MM1.S) and dexamethasone-resistant (MM1.R), we sought to determine whether 2-CDA can effectively induce apoptosis and growth inhibition in both cell lines and whether there is cross resistance between 2-CDA and dexamethasone in MM1.R cells. Cell proliferation assay (MTS) releaved that 2-CDA significantly inhibited both MM1.S and MM1.R cell growth in a dose dependent manner, with the minimum (10%) and maximum (100%) inhibition concentration of 12.5 nM/L and 500 nM/L for the MM1.S, and 25 nM/L and 500 nM/L for the MM1.R cells, respectively. The IC50 values of 2-CDA in the MM1.S and MM1.R cells were 48 nM/L and 60 nM/L, respectively. No cross resistance was observed between 2-CDA and dexamethasone in the MM1.R cells. On the molecular level, dexamethasone induced PARP and caspase-9 cleavage, and increased the level of p27^kip1 only in the MM1.S cells. 2-CDA treatment in both cell lines resulted in DNA fragmentation as well as strong PARP and caspase-9 cleavage, but no significant changes in the levels of P-Akt, P-MARK, p27^kip1, E2F1, and cyclin D1, indicating that 2-CDA induces growth inhibition and cell death in MM cell lines likely through mitochondria-dependent apoptotic mechanism. Cell cycle analyses by flow cytometry showed that dexamethasone (5μM/L) treatment increased sub-G1 (apoptosis) cells to 8.1% only in the MM1.S cells, while the majority (87%) of cells were arrested in the G1 phase of cell cycle in 24 hours. In contrast, 2-CDA (0.5μM/L for 24 hours) strongly induced apoptosis in both cell lines (sub-G1 population increased to 19.6% and 22.1% for the MM1.S and MM1.R cells, respectively) without changing their cell cycle profiles. These data suggest that dexamethasone and 2-CDA induce apoptosis in myeloma cells via different mechanisms, which would provide a theoretical basis for combination therapy for MM with these two agents. Furthermore, our results also show that 2-CDA alone is capable of inducing apoptosis in the MM1.R cells, suggesting that 2-CDA may have therapeutic potential for MM patients with a dexamethasone-resistant phenotype.