KITENIN functions as a fine regulator of ErbB4 expression level in colorectal cancer via protection of ErbB4 from E3-ligase Nrdp1-mediated degradation.

Understanding the complex biological functions of E3-ubiquitin ligases may facilitate the development of mechanism-based anti-cancer drugs. We recently identified that the KITENIN/ErbB4-Dvl2-c-Jun axis works as a novel unconventional downstream signal of epidermal growth factor (EGF) in colorectal cancer (CRC) tissues. Here we addressed whether E3-ubiquitin ligases are required for operation of this axis. We found that Nrdp1, an E3-ligase for ErbB3/ErbB4, interacted with KITENIN (KAI1 C-terminal interacting tetraspanin) to form a functional KITENIN/ErbB4/Nrdp1 complex and is responsible for down-regulating Dvl2 within this complex. Interestingly, ErbB4 was resistant to degradation by Nrdp1 in KITENIN/Nrdp1-co-transfected CRC cells, and KITENIN bound to the C-terminal coiled-coil domain of Nrdp1. Chemical blockade of ErbB kinase did not block the action of EGF to increase in total/phospho-ErbB4 and phospho-ERK in KITENIN/ErbB4-cotransfected cells, whereas it blocked the action of EGF in ErbB4 alone-transfected CRC cells. In human CRC tissues, higher expressions of ErbB4 and KITENIN and lower expression of Dvl2 was observed in stage IV samples than in stage I, but a low level of Nrdp1 was expressed in both stages and it did not differ significantly by stage. These results indicated that Nrdp1 is necessary for the reduction in Dvl2 to generate c-Jun in the EGF-KITENIN/ErbB4-c-Jun axis, but more importantly, elevated KITENIN protects KITENIN-bound ErbB4 from Nrdp1-mediated degradation via physical collaboration between the KITENIN/ErbB4 complex and Nrdp1, but not via modulation of ErbB kinase activity. Thus, KITENIN functions in the maintenance of a higher expression level of ErbB4 in advanced CRC tissues, independent of ubiquitin-mediated degradation via Nrdp1.