A Novel Erythroid Regulator MLF1IP Promotes Proliferation in Mice By Regulating Cell Cycle

Purpose: MLF1IP is a novel constitutive kinetochore component essential in physiological progress of cell cycle. Mouse homologue appears to be lineage-specifically expressed, but little is known about how it functions in hematopoiesis and whether it is involved in hematopoietic disorders. The goal of the study was to confirm biological function of MLF1IP in human K562 erythroleukemic cells, analyzed hematological abnormalities in MLF1IP transgenic mice and potential mechanisms underlying these abnormalities, and evaluated MLF1IP expression in erythroid colonies from PV and MDS.

Methods: MLF1IP was down-regulated by lentivirus in K562 cell line. The proliferation of K562 cells was analyzed by cell colony formation assay. VP16 was used to induce apoptosis of K562 cells. The entire coding region of human MLF1IP was cloned into the HS21/45-vav plasmid to generate transgenic mice, which were crossed with wild-type C57BL/6J mice, negative siblings of transgenic mice were used as controls. Genotypes of the transgenic mice were identified by PCR of tail genomic DNA with primers specific for transgenic MLF1IP sequence. Relative quantitative real-time PCR was used to detect mRNA level in BM cells. Western blot were carried out to check the expression of cell cycle associated proteins. Flow cytometry analyses were used to assess cell cycles and proportions of different stages of erythroblast populations. Absolute quantitative real-time PCR were used to examine MLF1IP expression in colonies from healthy donors and subjects.

Results: MLF1IP knock-down K562 cells are arrested in G2/M and have reduced colony-forming activity. MLF1IP knockdown had no detectable effect on apoptosis in K562 cells, while significantly enhanced etoposide-induced apoptosis was seen in MLF1IP-knockdown K562 cells at 48 h after treatment compared with that in same treated control cells. MLF1IP transgenic mice show increased hemoglobin, with normal levels of platelets and neutrophils. Erythroid progenitors and early-stage erythroblasts in bone marrow are markedly increased. These mice have increased cyclin D2, reduced p27 and p21 and increased cycling, suggesting that enhanced MLF1IP expression leads to erythroid expansion via accelerating cell cycle progression in bone marrow cells. Interestingly, MLF1IP expression in colony forming unit-erythropoiesis (CFU-E) is increased in polycythemia vera (PV) and decreased in low-risk myelodysplastic syndrome (MDS), consistent with erythropoiesis characteristics of these disorders.

Conclusion: Our data demonstrate that MLF1IP expression promotes erythroid proliferation via regulating cell cycle status of erythroid cells, implicating abnormal MLF1IP levels in erythropoiesis disorders. Correction of MLF1IP expression could be a novel therapy target in diverse dysfunctional erythropoiesis conditions.