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. © 2016 Wiley Periodicals, Inc.

Prolyl isomerase PIN1 negatively regulates SGK1 stability to mediate tamoxifen resistance in breast cancer cells.

BACKGROUND/AIM: Endocrine therapies that inhibit oestrogen receptor (ER)-α signaling are the most common and effective treatment for ER-α-positive breast cancer. The present study aimed to elucidate the mechanisms by which down-regulation of serum- and glucocorticoid-inducible protein kinase-1 (SGK1) expression confers tamoxifen resistance in breast cancer. MATERIALS AND METHODS: SGK1 expression and the cytotoxic effects of combinatorial 4-hydroxy-tamoxifen (4-OHT) treatment with SGK1 overexpression were investigated by immunoblotting, bromodeoxyuridine incorporation, and soft agar assay. RESULTS: We showed that PIN1 down-regulates SGK1 expression through interaction with and ubiquitination of SGK1. PIN1 silencing in MCF7 cells increased SGK1 expression. In tamoxifen-resistant human breast cancer, immunohistochemical staining analysis showed an inverse correlation between SGK1 expression and severity of tamoxifen resistance. Importantly, 4-OHT in combination with overexpression of SGK1 increased cleavage of poly-(ADP-ribose) polymerase and DNA fragmentation to inhibit clonogenic growth of tamoxifen-resistant MCF7 (TAMR-MCF7) cells. CONCLUSION: We suggest that PIN1-mediated SGK1 ubiquitination is a major regulator of tamoxifen-resistant breast cancer cell growth and survival.