Protein Kinase C Mediates the Mitogenic Action of Thrombopoietin in c-Mpl–Expressing UT-7 Cells

GATA-dependent regulation of TPO-induced c-mpl gene expression during megakaryopoiesis

Thrombopoietin (TPO) and its receptor, c-Mpl, play the crucial role during megakaryocytopoiesis. Previously, we have shown that the promoter activity of c-mpl induced by TPO is modulated by transcription through a PKC-dependent pathway and that GATA(-77) is involved as a positive regulatory element in TPO-induced c-mpl gene expression in the megakaryoblastic CMK cells. In this research, to examine participating possibility of GATA promoter element in TPO- induced c-mpl gene expression through a PKC-independent pathway, the promoter activity of site-directed mutagenesis and the effect of potein kinase C modulator were measured by a transient transfection assay system. Together with our previous results on the TPO-induced c-mpl promoter, this study indicates destruction of -77GATA in c-mpl promoter decreased the activity by 47.3% under existence of GF109203. These results suggest that GATA promoter element plays significant role in TPO-induced c-mpl gene expression through a PKC-independent pathway.

Role of promoter element in c-mpl gene expression induced by TPO

Thrombopoietin (TPO) and its receptor, c-Mpl, play the crucial role for the development of megakaryocyte and considered to regulate megakaryocytopoiesis. Previously we reported that TPO increased the c-mpl promoter activity determined by a transient expression system using a vector containing the luciferase gene as a reporter and the expression of the c-mpl gene is modulated by transcription through a protein kinase C (PKC)-dependent pathway in the megakaryoblastic cells. In this research, to elucidate the required elements in c-mpl promoter, the promoter activity of the deletion constructs and site-directed mutagenesis were measured by a transient transfection assay system. Destruction of -77GATA in c-mpl promoter decreased the activity by 22.8%. Our study elucidated that -77GATA involved in TPO-induced c-mpl gene expression in a human megakaryoblastic cell line, CMK.

Modulation mechanism of c-Mpl promoter activity in megakaryoblastic cells

Thrombopoietin receptor (c-Mpl) is considered to regulate megakaryocytopoiesis. In this study, we investigated an effect of activation of a protein kinase C (PKC) on c-mpl promoter activity to elucidate the underlying mechanisms of c-mpl gene expression in megakaryoblastic cells. PKC is a member of a family of serine/threonine protein kinases in the cytosol involved in cell growth and differentiation. Phorbol 12-myristate 13-acetate (PMA) is known as PKC activator, significantly enhanced the c-mpl promoter activity and PKC inhibitors (H7, GF109203) suppressed the up-regulation of PMA-induced promoter activity and reduced the steady level of its activity. These results strongly suggest that PKC plays the essential role in the modulation of c-mpl promoter activity of megakaryoblastic cells.