Elongin C is a mediator of Notch4 activity in human renal tubule cells

Notch proteins (Notch 1-4) are a family of trans-membrane cell surface receptors that are converted into transcriptional regulators when activated by interactions with cell surface ligands on adjacent cells. Ligand-binding stimulates proteolytic cleavage of the trans-membrane domain, releasing an active intracellular domain (ICD) that translocates to the nucleus and impacts transcription. In transit, the ICD may interact with regulatory proteins that modulate the expression and transcriptional activity. We have found that Notch4^ICD expression is enhanced in the tubule cells of fibrotic kidneys from diabetic mice and humans and identified Notch4^ICD interacting proteins that could be pertinent to normal and pathological functions. Using proteomic techniques, several components of the Elongin C complex were identified as candidate Notch4^ICD interactors. Elongin C complexes can function as ubiquitin ligases capable of regulating proteasomal degradation of specific protein substrates. Our studies indicate that ectopic Elongin C expression stimulates Notch4^ICD degradation and inhibits its transcriptional activity in human kidney tubule HK11 cells. Blocking Elongin C mediated degradation by MG132 indicates the potential for ubiquitin-mediated Elongin C regulation of Notch4^ICD. Functional interaction of Notch4^ICD and Elongin C provides novel insight into regulation of Notch signaling in epithelial cell biology and disease.