The Role Of Ubiquitin Conjugating Enzyme UBE2C In Lung Tumorigenesis

Ubiquitin-proteasome system（UPS）is vital for eukaryotic cell to control protein quality and protein turnover,which plays a critical role in many basic cellular processes,including cell cycle progression,differentiation,DNA repair,transcriptional regulation,and tumorigenesis among many others.Given these fundamental roles,it is not surprising that aberrations in the UPS are associated with many human diseases,particularly cancer.UBE2C,also known as Ubc H10,is an E2 ubiquitin-conjugating enzyme that couples with APC/C E3 ligase to promote ubiquitylation of substrates,such as Cyclin B1,Securin,and PLK1,via the K11 linkage.Previous studies have showed that as compared with their corresponding normal tissues,UBE2C is overexpressed in a variety of primary human cancers derived from the lung,stomach,uterus,and bladder,suggesting its possible role in promoting tumorigenesis.In this study,we have focused on the role of UBE2C in lung cancer.Cancer database analysis revealed that UBE2C is overexpressed in lung cancer tissues,and importantly,UBE2C overexpression is associated with poor survival of lung cancer patients.However,this association study did not reveal whether UBE2C overexpression is the cause or consequence of lung tumorigenesis.To this end,we generated Ube2C^fl/flmouse model in combination with LSL-Kras ^G12Dmice to determine whether UBE2C plays a causal role in lung tumorigenesis,triggered by Kras ^G12D,which is totally unknown previously.We found that Ube2c deletion inhibits Kras ^G12D-induced lung tumorigenesis by reducing the tumor formation,leading to extended mouse life-span.Thus,Ube2C is a Kras-cooperative gene,required for Kras-induced lung tumorigenesis.In cell culture models,we used lung cancer cell lines harboring Kras mutation,and found that UBE2C knockdown significantly inhibited growth and survival of cancer cells,mainly by inducing G2/M arrest and apoptosis.While UBE2C knockdown did not affect the levels of the APC/C substrates,such as Cyclin B1、PLK1 and securin,it caused accumulation of DEPTOR,a naturally occurring inhibitor of both m TORC1/2,leading to m TORC1 inactivation,as reflected by reduced phosphorylation of 4E-BP1.Importantly,growth suppression induced by UBE2C knockdown was partially rescued by simultaneous DEPTOR knockdown,indicating a causal,at least in part,role of DEPTOR.DEPTOR could be a novel substrate of APC/C E3.Indeed,knockdown of either CDH1,substrate recognizing subunit of APC/C E3,extended the protein half-life of DEPTOR.Taken together,our study demonstrated that UBE2C is an oncogenic protein,required for Kras-induced lung tumorigenesis,and growth/survival of lung cancer cells.This tumor-promoting effect appears to be achieved,at least in part,by reducing the levels of DEPTOR,leading to activation of the m TORC1 pathway.