FBXW7 and USP7 regulate CCDC6 turnover during the cell cycle and affect cancer drugs susceptibility in NSCLC

Abstract

CCDC6 gene product is a pro-apoptotic protein substrate of ATM, whose loss or inactivation enhances tumour progression. In primary tumours, the impaired function of CCDC6 protein has been ascribed to CCDC6 rearrangements and to somatic mutations in several neoplasia. Recently, low levels of CCDC6 protein, in NSCLC, have been correlated with tumor prognosis. However, the mechanisms responsible for the variable levels of CCDC6 in primary tumors have not been described yet.We show that CCDC6 turnover is regulated in a cell cycle dependent manner. CCDC6 undergoes a cyclic variation in the phosphorylated status and in protein levels that peak at G2 and decrease in mitosis. The reduced stability of CCDC6 in the M phase is dependent on mitotic kinases and on degron motifs that are present in CCDC6 and direct the recruitment of CCDC6 to the FBXW7 E3 Ubl. The de-ubiquitinase enzyme USP7 appears responsible of the fine tuning of the CCDC6 stability, affecting cells behaviour and drug response.Thus, we propose that the amount of CCDC6 protein in primary tumors, as reported in lung, may depend on the impairment of the CCDC6 turnover due to altered protein-protein interaction and post-translational modifications and may be critical in optimizing personalized therapy.

USP7 inhibitors, downregulating CCDC6, sensitize lung neuroendocrine cancer cells to PARP-inhibitor drugs

“…This result is in agreement with our previous observations in non-neuroendocrine lung tumors [19]. Nevertheless, the remaining 20 samples analysed displayed a positive immunostaining for CCDC6 and USP7 proteins.…”

“…In particular, besides stabilizing p53, the knockdown of USP7 enzyme alters PTEN, p21 turnover and also the CCDC6 half life by affecting their stability [19,27].…”

“…Altered levels of CCDC6 in a wide group of non-NE lung tumors, analised by TMA immunostaining, revealed that low CCDC6 expression, observed in about 30% of these tumors, negatively correlated with disease free survival (DFS) and overall survival (OS). Alterations of CCDC6 protein levels depend on the activity of CCDC6 modifiers (Fbxw7 E3 ubiquitin ligase or USP7) that affect the CCDC6 protein turnover [19].…”

Expression of USP7 and MARCH7 Is Correlated with Poor Prognosis in Epithelial Ovarian Cancer

“…damage response, such as USP1, USP7 and USP28 (Nijman et al 2005;Zhang et al 2006;Morra et al 2015). The USP7 can de-ubiquitinate several E3 ligases (MDM2/ MDMX, viral proteins ICP0) and tumor suppressors (p53, FOXO, PTEN, and claspin), therefore regulates important cellular process correlated with the tumorigenesis (Everett et al 1997;Li et al 2004;Song et al 2008;Hussain et al 2009).…”

CCDC6 (coiled-coil domain containing 6)

“…The loss of the CCDC6 region recognised by ATM kinase or the full protein deficiency determines an increase in cell survival, allows for DNA synthesis and permits cell to progress into mitosis, following the exposure to genotoxic stress (Merolla et al, 2007). CCDC6 gene product undergoes multiple posttranslational modifications such as sumoylation (Luise et al, 2012), ubiquitination (Povlsen et al, 2012;Morra et al, 2015), and phosphorylation (Celetti et al, 2004;Beausoleil et al, 2004;Brill et al, 2004;Morra et al, 2015), suggesting that CCDC6 protein activity is highly regulated ( Figure 3). …”