DISSECTING RECEPTORS AND SIGNALING PATHWAYS DRIVING ALL-TRANS RETINOIC ACID-INDUCED DIFFERENTIATION OF THE ACUTE MYELOID LEUKEMIA CELL LINE, HL-60: IN PURSUIT OF ACTIONABLE TARGETS FOR EXPANSION OF DIFFERENTIATION THERAPY

Abstract

Differentiation therapy based on all-trans retinoic acid (RA) has dramatically improved patient outcomes for those suffering from acute promyelocytic leukemia (APL), a subtype of acute myeloid leukemia (AML), but has been much less effective in treating other types of AML. This may be at least in part due to the heterogeneity of AML. A cell line used to model RA-induced differentiation, HL-60, is a non-APL AML, and our lab has recently shown that it may represent a previously uncharacterized subtype of AML. RA itself is known to regulate transcription of hundreds of genes, but the mechanistic details of the signaling pathways essential for differentiation are unclear. Here, we report interactions between the ectoenzyme receptor CD38 and the cytosolic signaling proteins Lyn, Vav1, SLP-76, and PI3K and show that Lyn also interacts with these cytosolic proteins. Building on prior results showing that Src family kinases enhance RA-induced differentiation, we explore the effects of several Src family kinase inhibitors on the process. We find that their effects on Lyn expression rather than their effects on Lyn activity appear to drive the process and that a clinically relevant inhibitor showing relatively low toxicity, bosutinib, behaves similarly to the more toxic inhibitor dasatinib. We also demonstrate that the ectoenzyme receptor CD38, which had a body of literature implicating it as a key driver of RA-induced differentiation, is not essential for the process. CXCR5, a receptor shown to be essential for RA-induced differentiation, drives migration toward its ligand, CXCL13. While the mechanism by which CXCR5 contributes to the differentiation process remains unknown, we show that it does not appear to function via interaction with a number of candidate binding partners (CD11b, CD38, CXCR4, CXCR5, EBI2, c-Cbl, and Lyn). Finally, as histone deacetylases are known to be aberrantly active in various cancers, we show that a drug targeting the histone deacetylase Sirt1 does not affect differentiation, that two broadly targeting Sirt2 enhance differentiation concurrent with signs of toxicity, and that one specifically targeting Sirt2 is toxic but does not enhance differentiation. These findings contribute to the identification of targets to bring differentiation therapy to AML.