Abstract
Non-melanoma skin cancer (NMSC) is the most common form of skin cancer. The primary risk factor of NMSC is prolonged exposure to ultraviolet (UV) radiation. UV radiation acts as both the tumor initiator and tumor promoter by causing molecular and cellular stress and DNA damage in cells within the skin, leading to mutations that promote the development of skin cancer. BubR1 is an essential mitotic checkpoint protein that plays a critical role in regulating chromosome segregation and maintaining genomic stability. BubR1 dysregulation can lead to an increase in chromosomal instability and aneuploidy, a common phenomenon associated with most cancers. Preliminary work from our lab have identified β-TRCP, an F-box containing protein, as an interacting factor for BubR1. β-TRCP is the substrate recognition subunit for the SCF (Skp, Cullin, F-box) complex, belonging to the Cullin RING E3 ubiquitin ligase (CRL) family of E3 ubiquitin ligases that regulates the stability of a wide range of cellular proteins by recognizing and binding specific substrates through a conserved degron motif and targeting them for ubiquitination and subsequent proteasome degradation. BubR1 contains a putative β-TRCP degron motif, indicating the possibility that β-TRCP may play a role in regulating BubR1 stability under various stimuli. Therefore, we hypothesized that UV radiation downregulates BubR1 protein levels through β-TRCP-dependent proteasome-mediated degradation. To test this hypothesis, short-term dose- and time-dependent UV experiments were carried out on human skin cell lines to assess the effect of UVC on BubR1 protein abundance. UV radiation led to the decrease of BubR1 protein and mRNA levels. In addition, β-TRCP protein abundance also showed a trend towards downregulation following UVC treatment, and a significant downregulation of both β-TRCP1 and β-TRCP2 transcripts was observed. Global inhibition of the 26S proteasome, or specific inhibition of Cullin RING E3 ubiquitin ligases, blocked BubR1 downregulation in response to UVC treatment, indicating that UV-induced degradation of BubR1 was caused by both downregulation of BubR1 gene expression and protein degradation mediated by CRL-mediated ubiquitination. More specifically, we found that depletion of β-TRCP1 rescued BubR1 protein abundance and enhanced cell viability following UV exposure. Taken together, these results demonstrate that UV-induced BubR1 downregulation occurs in part through proteasome-dependent degradation regulated by β-TRCP which may function to promote cell death.