Intrinsic Retinoic Acid Receptor α-Cyclin-Dependent Kinase-Activating Kinase Signaling Involves Coordination of the Restricted Proliferation and Granulocytic Differentiation of Human Hematopoietic Stem Cells

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Abstract
Little is known about the mechanisms by which retinoic acid receptor α (RARα) mediates the effects of retinoic acid (RA) to coordinate granulocytic proliferation/differentiation (P/D) transition. Cyclin-dependent kinase-activating kinase (CAK) complex, whose activity in phosphorylation of RARα is determined by its targeting subunit ménage à trois 1 (MAT1), regulates G1 exit, a cell cycle stage when cells commonly commit to proliferation or to differentiation. We previously found that in myeloid leukemia cells, the lack of RA-induced RARα-CAK dissociation and MAT1 degradation suppresses cell differentiation by inhibiting CAK-dependent G1 exit and sustaining CAK hyperphosphorylation of RARα. This contrasts with our recent findings about the P/D transition in normal primitive hematopoietic cells, where MAT1 degradation proceeds intrinsically together with granulocytic development, in accord with dynamic expression of aldehyde dehydrogenases (ALDHs) 1A1 and 1B1, which catalyze RA synthesis. Blocking ALDH activity inhibits MAT1 degradation and granulocytic differentiation, whereas loss of RARα phosphorylation by CAK induces RA-target gene expression and granulocytic differentiation. These studies suggest that the subversion of RARα-CAK signaling during normal granulopoiesis is crucial to myeloid leukemogenesis and challenges the current paradigm that RA induces cell differentiation solely by transactivating target genes. Disclosure of potential conflicts of interest is found at the end of this article.