Signal Regulation By Cd38, Lyn, And Mapk-Associated Molecules During All-Trans Retinoic Acid-Induced Differentiation Of Acute Myelogenous Leukemia Cells

Abstract

All-trans retinoic acid (ATRA) is successful in treating acute promyelocytic leukemia but has shown limited success with other cancers. Identifying signaling pathways that confer ATRA responsiveness and compounds that enhance the effects of the drug could expand its therapeutic efficacy. The leukocyte antigen CD38 is an ectoenzyme and receptor that propels MAPK signaling and ATRA-induced differentiation when overexpressed. CD38 enzymatic activity regulates calcium flux and NAD metabolism while its receptor functions drive phosphorylation of ERK, c-Cbl, and p85 PI3K. We found that crippling enzymatic activity with inhibitors or a point mutation (CD38 E226Q) did not affect ATRA induction, while a cytosolic truncation (CD38 [DELTA]11-20) that prevented CD38 membrane expression crippled ATRA-induced differentiation. This indicated that CD38 receptor function is important for ATRA induction. In contrast, we found CD38 [DELTA]11-20 cells responded to the monocytic inducer 1,25dihydroxyvitamin D3 similar to wild-type cells, suggesting CD38 signaling is important for granulocytic but not monocytic maturation. We also found that CD38 may sustain ERK phosphorylation to maintain a cellular precommitment memory associated with ATRA exposure. Since CD38 receptor functions appeared necessary for ATRA induction we identified CD38 interaction partners. These included SLP-76, Vav1, and Lyn, and we found that this complex was interrupted by inhibiting Lyn. Lyn inhibition also blocked ATRA- and CD38-stimulated phosphorylation of c-Cbl and p85 PI3K, suggesting some aspects of CD38 signaling are dependent on Lyn kinase activity. Finally we found that the Lyn inhibitors PP2 and dasatinib enhanced ATRA-induced differentiation. Co-treatment with ATRA plus either inhibitor upregulated total Lyn and c-RafpS259, and increased associations between Lyn/c-Raf and cRaf/ERK. This was consistent with increased c-Raf C-terminus phosphorylation that may be regulated by ERK-propelled feedback signaling within a KSR1 scaffold. PP2 and dasatinib had different effects on Lyn activity suggesting that Lyn acts as an adaptor/scaffold independent of its kinase function. These results provide valuable insight as to how ATRA-regulated molecules including CD38, Vav1, SLP-76, Lyn, and the Raf/MEK/ERK axis may enhance the differentiation of acute myelogenous leukemia cells. This report also identifies new therapeutic targets and compounds that may be used in combination with ATRA to improve its clinical efficacy.