INVOLVEMENT OF MAP KINASE PHOSPHATASE-1 IN MESANGIAL CELL-GROWTH REGULATION

Mitogen-activated protein (MAP) kinase phosphatase-1 (MKP-1) is encode d by the mitogen-inducible gene 3CH134. MKP-1 has recently been shown to be a dual specificity (serine/threonine, tyrosine) protein phosphat ase, which dephosphorylates and inactivates MAP kinases in vitro and i n vivo. To seek the role of MKP-1 in growth regulation of mesangial ce lls, expression of MKP-1 mRNA in cultured mesangial cells and in glome ruli isolated from anti-Thy 1.1 mesangial proliferative glomerulonephr itis rats was studied. The effect of inhibition of endogenous MKP-1 by use of antisense-DNA technology on the regulation of MAP kinase activ ity and the growth regulation of rat cultured mesangial cells was also studied. By northern blot analysis, it was demonstrated that in mesan gial cells, MKP-1 mRNA expression was rapidly induced after the stimul ation by serum, growth factors and vasoactive peptides. Maximal signal s were found in 30-60 min in all growth factors tested. Fetal calf ser um (FCS) was the most potent stimulus of MKP-1 mRNA expression, follow ed by platelet-derived growth factor (PDGF)-B and arginine vasopressin . To elucidate a possible involvement of MKP-1 in disease development of mesangial proliferative glomerulonephritis, MKP-1 mRNA expression w as examined in rat anti-Thy 1.1 glomerulonephritis model. A marked inc rease in MKP-1 mRNA level in isolated glomeruli was observed at day 3 after disease induction (4.3-fold over control). In situ hybridization of MKP-1 mRNA in Thy 1.1 glomerulonephritis rats confirmed the enhanc ed glomerular expression of MKP-1. To study the role of MKP-1 in mesan gial cell growth regulation, phosphorothioate oligodeoxynucleotide (OD N) were used to modulate MKP-1 expression. An antisense ODN targeting the translation initiation site of MKP-1 mRNA inhibited stimulated (by FCS, or PDGF-B) DNA synthesis and FCS- or PDGF-induced mitogenesis in mesangial cells. Sense ODN or mismatched ODN had no effect on the DNA synthesis or mitogenic response of mesangial cells. These results sug gest that MKP-1 is an immediately early gene in rat mesangial cells an d it plays a critical role in growth regulation of mesangial cells in vivo.