Inhibitor of Grow 4 (ING4) is Upregulated by low K intake and Inhibits ROMK channels by stimulation of Mitogen‐Activated Protein Kinase (MAPK)

Abstract

We have used Western blot to examine the effect of low K intake on the expression of ING4, a 29 kDa protein, in the kidney and employed two-electrode voltage-clamp (TEVC) to study the effect of ING4 on ROMK1 current in Xenopus oocytes. K-restriction significantly increased the expression of ING4 in the renal cortex and outer medulla while it had no effect on ING4 expression in the heart, skeletal muscle and liver. The role of superoxide in mediating the effect of low K intake on ING4 expression is suggested by the finding that treatment of M1 cells or HEK cells with hydrogen peroxide significantly increased ING4 expression. Moreover, treatment of rats with tempol not only decreases the superoxide levels but also the expression of ING4. This suggests that increases in superoxide levels induced by low K intake were responsible for the induction of ING4 expression. Transfection of HEK cells with ING4 or injection of Xenopus oocytes with ING4 significantly increased the phosphorylation of P38 and ERK MAPK. To study the effect of ING4 on ROMK channel activity we examined the K current in oocytes injected with ROMK1 or ROMK1+ING4. Injection of ING4 reduced K current by 40% in oocytes injected with ROMK1. In contrast, injection of ING4 had no effect on Na current in oocytes injected with ENaC subunits. Moreover, inhibition of P38 and ERK MAPK restored the K currents in oocytes injected with ROMK1 and ING4. We conclude that ING4 is involved in mediating the effect of low K intake on ROMK channel activity and that the effect of ING4 on K channel activity is the result of stimulation of phosphorylation of P38 and ERK MAPK.