Collagen represses canonical Notch signaling and binds to Notch ectodomain

The Notch signaling system features a growing number of modulators that include extracellular proteins that bind to the Notch ectodomain. Collagens are a complex, heterogeneous family of secreted proteins that serve both structural and signaling functions, most prominently through binding to integrins and DDR. The shared widespread tissue distribution of Notch and collagen prompted us to investigate the effects of collagen on Notch signaling. In a cell co-culture signaling assay, we found that type IV collagen inhibited Notch signaling in H460 and A7R5 cell lines. Moreover, Notch-stimulated expression of mature smooth muscle genes SMA, MHC, SM22, and calponin, which define the physiologic phenotype of normal vascular smooth muscle, was inhibited by type IV collagen in A7R5 cells. Cloned promoters of three of these genes were also inhibited by exposure to collagen. Collagen-dependent repression of Notch signaling required an RBP-jK site within the SM22 promoter. Moreover, repression by collagen required extracellular stimulation of the Notch signaling pathway. Type IV collagen bound to both Notch3 and Jagged1 proteins in purified protein binding assays. In addition, type I collagen also inhibited Notch signaling and bound to Notch and Jagged. We conclude that type IV and type I collagen repress canonical Notch signaling to alter expression of Notch target genes.