Evidence for a protective role of the STAT5 transcription factor against oxidative stress in human leukemic pre-B cells.

STAT5 transcription factors are involved in normal B lymphocyte development and in leukemogenesis. We show that the inhibition of STAT5A expression or activity in the NALM6, 697 and Reh leukemic pre-B cell lines, results in a higher spontaneous apoptosis and an increased FAS-induced cell death. However, the molecular mechanisms underlying the altered pre-B cell survival are unclear. We used a proteomic approach to identify proteins that are differentially regulated in cells expressing (NALM6Δ5A) or not a dominant negative form of STAT5A. Among the 14 proteins identified, six were involved in the control of the oxidative stress like glutathione (GSH) synthetase and DJ-1. Accordingly, we showed increased levels of reactive oxygen species (ROS) in NALM6Δ5A cells and suppression of the increased sensitivity to Fas-mediated apoptosis by the GSH tripeptide. Similar results were observed when NALM6 cells were treated with TAT-STAT5Δ5A fusion proteins or STAT5A shRNA. In addition, the 697 and Reh pre-B cells were found to share number of molecular changes observed in NALM6Δ5A cells including ROS generation, following inhibition of STAT5 expression or function. Our results point out to a hitherto undescribed link between STAT5 and oxidative stress and provide new insights into STAT5 functions and their roles in leukemogenesis.

Involvement of the p38 mitogen-activated protein kinase pathway in tissue inhibitor of metalloproteinases-1-induced erythroid differentiation.

We examined the role of the mitogen-activated protein (MAP) kinase pathway in tissue inhibitor of metalloproteinases-1 (TIMP-1)-mediated cellular effects in a human erythroleukemic cell line UT-7. We show that TIMP-1 induced both UT-7 cell erythroid differentiation and proliferation and tyrosine phosphorylation of many intracellular proteins. Using a panel of phosphospecific antibodies, we also demonstrate that phosphorylation of the p38 and c-Jun N-terminal kinases is increased by TIMP-1 whereas phosphorylation of extracellular signal-regulated kinase 1/2 is not induced. Moreover, inhibition of the p38 activity by SB203580 significantly reduces erythroid differentiation induced by TIMP-1, suggesting that the p38 MAP kinase pathway is involved in TIMP-1-induced erythroid differentiation.

Erythropoietin induction of tissue inhibitors of metalloproteinase-1 expression and secretion is mediated by mitogen-activated protein kinase and phosphatidylinositol 3-kinase pathways.

In the present study, we demonstrate that erythropoietin (Epo) induces the expression and the release of tissue inhibitors of metalloproteinase-1 (TIMP-1) in a time- and dose-dependent manner in Epo-dependent cell line UT-7 cells and in normal human erythroid progenitor cells from cord blood (CD36+) and required de novo protein synthesis. TIMP-1 was not expressed in the absence of Epo. Inhibition of the mitogen-activated protein kinase pathway by the specific inhibitors PD98059 and U0126 and of phosphatidylinositol 3-kinase by LY294002, strongly inhibited Epo-induced TIMP-1 expression and secretion. In the absence of Epo, both latent and active forms of matrix metalloproteinase-9 (MMP-9) were secreted into media. Upon Epo stimulation, MMP-9 and pro-MMP-9 secretion was inhibited in a dose-dependent manner parallel to TIMP-1 induction. The addition of PD98059, U0126, and LY294002 in the presence of Epo restored MMP-9 production in UT-7 and CD36+ cells. Our findings strongly suggest an inversely coordinated regulation of the TIMP-1 gene and MMP-9 production by Epo via mitogen-activated protein kinase and phosphatidylinositol 3-kinase pathways.

Erythropoietin induces glycosylphosphatidylinositol hydrolysis. Possible involvement of phospholipase c-gamma(2).

We showed that erythropoietin induced rapid glycosylphosphatidylinositol (GPI) hydrolysis and tyrosine phosphorylation of phospholipase C (PLC)-gamma(2) in FDC-P1 cells transfected with the wild-type erythropoietin-receptor. Erythropoietin-induced tyrosine phosphorylation of PLC-gamma(2) was time- and dose-dependent. By using FDC-P1 cells transfected with an erythropoietin receptor devoid of tyrosine residues, we showed that both effects required the tyrosine residues of intracellular domain on the erythropoietin receptor. Erythropoietin-activated PLC-gamma(2) hydrolyzed purified [(3)H]GPI indicating that GPI hydrolysis and PLC-gamma(2) activation under erythropoietin stimulation were correlated. Results obtained on FDC-P1 cells transfected with erythropoietin receptor mutated on tyrosine residues suggest that tyrosines 343, 401, 464, and/or 479 are involved in erythropoietin-induced GPI hydrolysis and tyrosine phosphorylation of PLC-gamma(2), whereas tyrosines 429 and/or 431 seem to be involved in an inhibition of both effects. Thus, our results suggest that erythropoietin regulates GPI hydrolysis via tyrosine phosphorylation of its receptor and PLC-gamma(2) activation.