NIPA Deficiency Leads To Acceleration Of The Lymphoma Development In EµMyc Mice

Timely degradation of proteins that control cell proliferation and apoptosis is an essential mechanism in keeping normal growth from turning into runaway malignancy. We previously reported the cloning of NIPA (Nuclear-Interaction-Partner-of-ALK) and characterized it as a F-Box-protein that defines an oscillating E3-ubiquitin-ligase. Using a conditional-knockout strategy we inactivated Nipa and found Nipa^-/- animals to be viable, but sterile due to a block of spermatogenesis. Our studies demonstrate that loss of Nipa has no substantive effect on physiological cell cycle progression of primary MEFs indicating that this cell cycle checkpoint is inactive under optimal proliferation conditions. Interestingly, Nipa checkpoint control can be unmasked by oncogenic c-Myc-transformation. Here we show significant differences in c-Myc-induced transformation: Focus formation ability of c-Myc-infected Nipa^-/- MEFs was greatly reduced. Moreover, Nipa-deficiency leads to premature senescence in cultured primary MEFs. Ectopic reexpression of Nipa resulted vice versa in delayed senescence of knockout MEFs. Next, we sought to know, whether increased apoptosis in Nipa^-/- c-Myc-transduced MEFs is dependent on a functional p53-Axis. Interestingly, the effect of Nipa deficiency on c-Myc-mediated transformation was totally abolished by p53-knockdown. We observed no differences in focus formation ability or growth behaviour in Nipa^-/- MEFs with inactivated p53, suggesting the importance of p53 in Nipa-induced cell death. Looking in more detail on the c-myc-p53 axis we detected a substantial increase in Arf-p19 levels in Nipa^-/- cells. Moreover, Nipa-knockdown in Zn-inducible-Arf-NIH/3T3 cells lead to stabilization of Arf p19. To test the impact of these findings in a relevant in-vivo model we intercrossed Nipa^-/- animals with a transgene EµMyc-Strain. Nipa^-/-EµMyc^TG/wt animals developed lymphomas within a significantly shorter latency than Nipa^+/+EµMyc^TG/wt animals. Furthermore, lymphomas of knockout animals were more aggressive. FACS- and biochemical-analyses showed no gross differences between Nipa^-/- and wt lymphomas except highly elevated Arf-p19 levels in Nipa^-/- lymphomas, pointing to an important role of Nipa in Myc-p19-signalling. Taken together our results highlight the functional importance of the Nipa-p53-axis in cell cycle regulation and suggest that deregulation of the protein provides a substantial contribution during the process of tumorigenesis.