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The p53 tumor suppressor is controlled by an interactive network of factors that stimulate or inhibit its transcriptional activity. Within that network, Mdm2 functions as the major antagonist of p53 by promoting its ubiquitylation and degradation. Conversely, Tip60 activates p53 through direct association on target promoters as well as acetylation of p53 at lysine 120 (K120). This study examines the functional relationship between Mdm2 and Tip60 with a novel p53 regulator, NIAM (nuclear interactor of ARF and Mdm2). Previous work showed NIAM can suppress proliferation and activate p53 independently of ARF, indicating that other factors mediate those activities. Here, we demonstrate that NIAM is a chromatin-associated protein that binds Tip60. NIAM can promote p53 K120 acetylation, although that modification is not required for NIAM to inhibit proliferation or induce p53 transactivation of the p21 promoter. Notably, Tip60 silencing showed it contributes to but is not sufficient for NIAM-mediated p53 activation, suggesting other mechanisms are involved. Indeed, growth-inhibitory forms of NIAM also bind to Mdm2, and increased NIAM expression levels disrupt p53–Mdm2 association, inhibit p53 polyubiquitylation, and prevent Mdm2-mediated inhibition of p53 transcriptional activity. Importantly, loss of NIAM significantly impairs p53 activation. Together, these results show that NIAM activates p53 through multiple mechanisms involving Tip60 association and Mdm2 inhibition. Thus, NIAM regulates 2 critical pathways that control p53 function and are altered in human cancers, implying an important role for NIAM in tumorigenesis.

Pancreatic ductal adenocarcinoma (PDAC) is characterized by early recurrence following pancreatectomy, rapid progression, and chemoresistance. Novel prognostic and predictive biomarkers are urgently needed to both stratify patients for clinical trials and select patients for adjuvant therapy regimens. This study sought to determine the biological significance of RABL6A (RAB, member RAS oncogene family-like protein 6 isoform A), a novel pancreatic protein, in PDAC. Analyses of RABL6A protein expression in PDAC specimens from 73 patients who underwent pancreatic resection showed that RABL6A levels are altered in 74% of tumors relative to adjacent benign ductal epithelium. Undetectable RABL6A expression, found in 7% (5/73) of patients, correlated with improved overall survival (range 41 to 118 months with 3/5 patients still living), while patients with RABL6A expression had a worse outcome (range 3.3 to 100 months, median survival 20.3 months) (P= 0.0134). In agreement with those findings, RABL6A expression was increased in pancreatic cancer cell lines compared to normal pancreatic epithelial cells, and its knockdown inhibited pancreatic cancer cell proliferation and induced apoptosis. Moreover, RABL6A depletion selectively sensitized cells to oxaliplatin-induced arrest and death. This work reveals that RABL6A promotes the proliferation, survival, and oxaliplatin resistance of PDAC cells, whereas its loss is associated with extended survival in patients with resected PDAC. Such data suggest RABL6A is a novel biomarker of PDAC and potential target for anticancer therapy.

The ARF tumor suppressor protects us against cancer through protein-protein interactionsin partially defined p53-dependent and p53-independent pathways. We performed a two-hybridscreen using ARF as bait and present the identification of several new ARF partners that mayregulate its growth inhibitory signaling. The potential physiological roles of these novel ARFbinding proteins in regulating ARF signaling are discussed.