CUL-4A 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, they induce the expression of lineage-specific genes, and their proliferation declines until cell cycle exit. CUL-4A encodes a core subunit of a ubiquitin ligase that targets proteins for ubiquitin-mediated degradation, and CUL-4A haploinsufficient mice display hematopoietic dysregulation with fewer multipotential and erythroid committed progenitors (3-fold and 5-fold, respectively). In this study, stress induced by 5-fluorouracil or phenylhydrazine reveals a delay in the ability of CUL-4A +/− mice to recover erythroid progenitors and precursors and to reestablish normal hematocrits. Conversely, over-expression of CUL-4A in a growth factor-dependent, proerythroblast cell line increases proliferation 34% (tritiated thymidine incorporation) and the proportion of cells in S-phase 5%. When these cells are induced to terminally differentiate, endogenous CUL-4A protein expression declines 3.6-fold. Its enforced expression interferes with erythrocyte maturation (beta-globin induction) and cell cycle exit, and instead promotes proliferation. Furthermore, p27 accumulates during erythroid terminal differentiation, but CUL-4A enforced expression increases p27 protein turnover nearly 7-fold and attenuates its accumulation. CUL-4A and p27 proteins co-immunoprecipitate, indicating that a Cul-4A ubiquitin ligase targets p27 for degradation. These findings suggest that a Cul-4A ubiquitin ligase positively regulates proliferation by targeting p27 for degradation and that CUL-4A down-regulation during erythroid terminal differentiation allows p27 to accumulate and signal cell cycle exit.