MEK1-RSK2 contributes to Hedgehog signaling by stabilizing GLI2 transcription factor and inhibiting ubiquitination.

The transcription factor GLI2 has an important role in the transduction of Hedgehog signaling and thereby regulates tumorigenesis in a wide variety of human tumors. However, the mechanisms controlling GLI2 protein expression and stabilization are incompletely understood. In this study, we show that the mitogen-activated protein kinase MEK1 modulates GLI2 both at the mRNA and protein level. Constitutively activated MEK1 prolonged the half-life of GLI2 and increased its nuclear translocation, accompanied by attenuated ubiquitination of GLI2 protein. RSK2, a protein kinase lying downstream of MEK-ERK cascade, mimicked the effect of MEK on GLI2 stabilization. MEK1 and RSK2 failed to augment the half-life of GLI2 lacking GSK-3β phosphorylation sites, indicating that MEK-RSK stabilizes GLI2 by controlling targeting GSK-3β-mediated phosphorylation and ubiquitination of GLI2. The significance of MEK-RSK stabilization was demonstrated in experiments showing that activation of MEK-RSK paralleled higher protein level of GLI2 in several multiple myelomas (MM) cells relative to normal B cells. Moreover, combined treatment with RSK and GLI inhibitors led to an enhanced apoptosis of MM cells. Thus, our results indicate that MEK-RSK cascade positively regulates GLI2 stabilization and represses its degradation via inhibiting GSK-3β-dependent phosphorylation and ubiquitination of GLI2.