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Terminal erythroid differentiation is tightly coordinated with cell-cycle exit, which is regulated by cyclins, cyclin-dependent kinases, and cyclin-dependent kinase inhibitors (CDKI), yet their roles in erythropoiesis remain to be fully defined. We show here that p19INK4d, a member of CDKI family, is abundantly expressed in erythroblasts and that p19INK4dknockdown delayed erythroid differentiation, inhibited cell growth, and led to increased apoptosis and generation of abnormally nucleated late-stage erythroblasts. Unexpectedly, p19INK4dknockdown did not affect cell cycle. Rather, it led to decreased expression of GATA1 protein. Importantly, the differentiation and nuclear defects were rescued by ectopic expression of GATA1. Because the GATA1 protein is protected by nuclear heat shock protein family (HSP) member HSP70, we examined the effects of p19INK4dknockdown on HSP70 and found that p19INK4dknockdown led to decreased expression of HSP70 and its nuclear localization. The reduced levels of HSP70 are the result of reduced extracellular signal-regulated kinase (ERK) activation. Further biochemical analysis revealed that p19INK4ddirectly binds to Raf kinase inhibitor PEBP1 and that p19INK4dknockdown increased the expression of PEBP1, which in turn led to reduced ERK activation. Thus we have identified an unexpected role for p19INK4dvia a novel PEBP1-p-ERK-HSP70-GATA1 pathway. These findings are likely to have implications for improved understanding of disordered erythropoiesis.

Erythropoiesis is a process during which hematopoietic stem cell (HSCs) are first committed to erythroid progenitors, which subsequently undergo terminal erythroid differentiation to produce mature red blood cells. During terminal erythroid differentiation, proerythroblasts undergo 4-5 mitoses to sequentially generate basophilic erythroblasts, polychromatic erythroblasts and orthochromatic erythroblasts that expel their nuclei to produce enucleated reticulocytes. Terminal erythropoiesis is a tightly regulated process. The most well studied regulatory mechanisms include EPO/EPOR mediated signal transduction and transcription factors among which GATA1 plays critical role. Terminal erythroid differentiation is also tightly coordinated with cell cycle exit, which is regulated by cyclins, cyclin-dependent kinases and cyclin-dependent kinase inhibitors (CDKI), yet their roles in erythropoiesis remain largely undefined. Our RNA-seq of human terminal erythroid differentiation shows that of seven CDKI members, only three of them, p18INK4c, p19INK4dand p27KIP1, are abundantly expressed in erythroid cells and their expressions are significantly upregulated in late stage erythroblasts, which were further confirmed by western blotting analysis. In contrast to demonstrated roles of p18INK4cand p27KIP1in terminal erythroid differentiation, the function of p19INK4dthis process has not been studied. To explore the role of p19INK4dduring human erythropoiesis, we employed a shRNA-mediated knockdown approach in CD34+cells and found that p19INK4dknockdown delayed erythroid differentiation, inhibited cell growth, led to increased apoptosis and generation of abnormally nucleated late stage erythroblasts. Unexpectedly, p19INK4dknockdown did not affect cell cycle. Rather it led to decreased GATA1 protein levels. Importantly, the differentiation and nucleus defects were rescued by ectopic expression of GATA1. As GATA1 protein is protected by nuclear HSP70, to explore the mechanism for the decreased GATA1 protein levels, we examined the effects of p19INK4dknockdown on HSP70 and found p19INK4dknockdown led to decreased nuclear localization of HSP70 due to reduced ERK activation. Further biochemical analysis revealed that p19INK4ddirectly binds to Ras kinase inhibitor PEBP1 and that p19INK4dknockdown increased the expression of PEBP1 which in turn led to reduced ERK activation. These results demonstrate that p19INK4dmaintains GATA1 protein levels through PEBP1-pERK-HSP70-GATA1 pathway. Our findings identify previously unknown and unexpected roles for p19INK4din human terminal erythroid differentiation via a novel pathway.