Abstract

Parathyroid hormone–related protein (PTHrP) increases the content and mRNA level of insulin in a mouse β-cell line, MIN6, and primary-cultured mouse islets. We examined the mechanism of PTHrP-induced insulin expression. The PTHrP effect was markedly augmented by SB203580, a mitogen-activated protein (MAP) kinase inhibitor, and SB203580 itself increased insulin expression extensively, even without PTHrP. Because SB203580 inhibits both p38 and c-jun NH₂-terminal kinases (JNKs), we investigated the JNK-specific inhibitor SP600125. SP600125 also increased insulin content and its mRNA level. PTHrP induced dephosphorylation of JNK1/2, and PTHrP-induced insulin expression was blocked by a dominant-negative type JNK-APF. We suspected that dual specificity MAP kinase phosphatases (MKPs) may be involved in the PTHrP-induced insulin expression by inactivating JNK1/2. MIN6 cells contained at least five MKPs, among which only MKP-1 was inducible by PTHrP. PTHrP-induced insulin expression was blocked by the MKP-1 expression inhibitor Ro-31-8220, indicating that the PTHrP effect is mediated by MKP-1. Indeed, adenoviral MKP-1 expression increased insulin expression by decreasing a phosphorylation form of JNKs and a resulting phosphorylated form of c-jun in MIN6 cells. The phosphorylated form of c-jun is known to repress cAMP-dependent insulin gene promoter activity. Thus, MKP-1 controls the insulin expression by downregulating a JNK/c-jun pathway.