Cul4A targets p27 for degradation and regulates proliferation, cell cycle exit, and differentiation during erythropoiesis

As erythroid progenitors differentiate into precursors and finally mature red blood cells, lineage-specific genes are induced, and proliferation declines until cell cycle exit. Cu14A encodes a core subunit of a ubiquitin ligase that targets proteins for ubiquitin-mediated degradation, and Cu14A-haploinsufficient mice display hematopoietic dysregulation with fewer multipotential and erythroid-committed progenitors. In this study, stress induced by 5-fluorouracil or phenylhydrazine revealed a delay in the recovery of erythroid progenitors, early precursors, and normal hematocrits in Cu14A(+/-) mice. Conversely, overexpression of Cu14A in a growth factor-dependent, proerythroblast cell line increased proliferation and the proportion of cells in S phase. When these proerythroblasts were induced to terminally differentiate, endogenous Cu14A protein expression declined 3.6-fold. Its enforced expression interfered with erythrocyte maturation and cell cycle exit and, instead, promoted proliferation. Furthermore, p27 normally accumulates during erythroid terminal differentiation, but Cu14A-enforced expression destabilized p27 and attenuated its accumulation. Cu14A and p27 proteins coimmunoprecipitate, indicating that a Cu14A ubiquitin ligase targets p27 for degradation. These findings indicate that a Cu14A ubiquitin ligase positively regulates proliferation by targeting p27 for degradation and that Cu14A down-regulation during terminal erythroid differentiation allows p27 to accumulate and signal cell cycle exit.