Mortalin-2 (mthsp70/PBP74/Grp75) Mediates the Erythropoietin Signal To Stimulate Growth of Purified Human Erythroid Progenitor Cells.

Erythropoietin (EPO) stimulates erythroid growth by enhancing the proliferation, maturation and survival of late stage erythroid progenitor cells. Intracellular signaling molecules such as Janus kinase 2 (JAK2) and phosphoinositide-3 kinase (PI-3K)/Akt are considered mediators of the EPO signal; however, the entire process of EPO stimulation remains undetermined. Previously, we used siRNA to show that mortalin-2, which is a member of the heat shock protein 70 family of chaperones, mediates EPO signaling to stimulate the growth of human erythroid colony forming cells (ECFCs). In the present study, we examined the relationship between cell growth and mortalin-2 overexpressed in ECFCs, and analyzed the gene expression pattern of ECFCs treated with mortalin-2 siRNA using a DNA microarray, to further clarify the intracellular mechanism by which EPO and mortalin-2 interact. In the presence of different concentrations of EPO (0–1.0 U/ml), the effect of mortalin-2 overexpression on ECFC growth was determined by MTT assay. There was tendency to better viability of ECFCs treated with mortalin-2 expression vectors than control cells, especially in the EPO 0 U/ml group (p=0.08). BrdU ELISA, used to investigate the effects of mortalin-2 on the DNA synthesis of ECFCs, revealed that when ECFCs were treated with mortalin-2 expression vectors, the cells showed an increase in the amount of BrdU incorporation into DNA without EPO. Next we analyzed the gene expression pattern using mRNA obtained from ECFCs cultured with or without EPO after treatment with mortalin-2 siRNA or control siRNA. When ECFCs were cultured with EPO after treatment with mortalin-2 siRNA, the expression of 19 genes was suppressed to less than 0.6 fold, and these genes included those involved in cell growth, apoptosis or transport, such as interleukin 6 receptor, ATP-binding cassette, Mdm2, BCL2 interacting protein and interleukin 10 receptor alpha. Furthermore, the expression of 8 genes was upregulated to over 1.5 fold, and these genes included transcription or signal transmission related genes, such as ubiquitin A-52 residue ribosomal protein fusion product 1 and serum/glucocorticoid regulated kinase 2. There were also some genes whose expressions overlapped with genes obtained from ECFCs cultured without EPO after treatment with control siRNA. Our data suggests that mortalin-2 expression depends on various pathways, and that one of these pathways mediates EPO signaling to stimulate mortalin-2 expression, which is related to the growth of erythroid progenitor cells.