Serum/glucocorticoid-regulated kinase 1 as a novel transcriptional target of bone morphogenetic protein-ALK1 receptor signaling in vascular endothelial cells

Bone morphogenetic protein 9 (BMP9)/BMP10-ALK1 receptor signaling is essential for endothelial differentiation and vascular morphogenesis. Mutations in ALK1/ACVRL1 and other signal-related genes are implicated in human vascular diseases, and the Alk1/Acvrl1 deletion in mice causes severe impairment of vascular formation and embryonic lethality. In the microarray screen to search for novel downstream genes of ALK1 signaling, we found that the mRNA and protein expression of serum/glucocorticoid-regulated kinase 1 (SGK1) was rapidly up-regulated by the BMP9 stimulation of cultured human endothelial cells. The increase in SGK1 mRNA was completely blocked by the transcriptional inhibitor actinomycin D and significantly suppressed by the siRNA treatment against the co-SMAD transcription factor SMAD4. Upon the BMP9 treatment of endothelial cells, phosphorylated SMAD1/5/9 bound to a consensus site upstream of the SGK1 gene, which was necessary for BMP9-dependent increment of the luciferase reporter activity driven by the SGK1 proximal enhancer. The Sgk1 mRNA expression in mouse embryos was enriched in vascular endothelial cells at embryonic day 9.0–9.5, at which Sgk1 null mice showed embryonic lethality due to abnormal vascular formation, and its mRNA as well as protein expression was clearly reduced in Alk1/Acvrl1 null embryos. These results indicate that SGK1 is a novel target gene of BMP9/BMP10-ALK1 signaling in endothelial cells and further suggest a possibility that down-regulation of the Sgk1 expression may be involved in the mechanisms of vascular defects by the ALK1 signaling deficiency.