8-Amino-adenosine induces loss of phosphorylation of p38 mitogen-activated protein kinase, extracelluar signal-regulated kinase 1/2, and Akt kinase: Role in induction of apoptosis in multiple myeloma

Multiple myeloma is a slowly proliferating B-cell malignancy that accumulates apoptosis-resistant and replication-quiescent cell populations, posing a challenge for current chemotherapeutics that target rapidly replicating cells. Multiple myeloma remains an incurable disease in need of new therapeutic approaches. The purine nucleoside analogue, 8-amino-adenosine (8-NH₂-Ado), exhibits potent activity in preclinical studies, inducing apoptosis in several multiple myeloma cell lines. This cytotoxic effect requires phosphorylation of 8-NH₂-Ado to its triphosphate form, 8-amino-ATP, and results in a concomitant loss of endogenous ATP levels. Here, we show the novel effect of 8-NH₂-Ado on the phosphorylation status of key cellular signaling molecules. Multiple myeloma cells treated with 8-NH₂-Ado exhibit a dramatic loss of phosphorylation of several important signaling proteins, including extracellular signal-regulated kinase 1/2, p38 mitogen-activated protein kinase, and Akt kinase. Cells depleted of ATP independent of 8-NH₂-Ado do not exhibit the same decrease in phosphorylation of vital cellular proteins. Therefore, the significant shifts in endogenous ATP pools caused by 8-NH₂-Ado treatment cannot account for the changes in phosphorylation levels. Instead, 8-NH₂-Ado may influence the activity of select regulatory protein kinases and/or phosphatases, with preliminary data suggesting that protein phophatase 2A activity is affected by 8-NH₂-Ado. The distinctive effect of 8-NH₂-Ado on the phosphorylation status of cellular proteins is a novel phenomenon for a nucleoside analogue drug and is unique to 8-NH₂-Ado among this class of drugs. The kinetics of 8-NH₂-Ado-mediated changes in phosphorylation levels of critical prosurvival and apoptosis-regulating proteins suggests that the modulation of these proteins by dephosphorylation at early time points may be an important mechanistic step in 8-NH₂-Ado-induced apoptosis.