Cellular functions of 14-3-3ζ in apoptosis and cell adhesion emphasize its oncogenic character

14-3-3 proteins are relevant to cancer biology as they are key regulators of major cellular processes such as proliferation, differentiation, senescence and apoptosis. So far, the sigma isoform (14-3-3 sigma) has most directly been implicated in carcinogenesis and was recognized as a tumour-suppressor gene. The other six members of the mammalian 14-3-3 gene family likely behave as oncogenes, although direct evidence supporting this view is largely circumstantial. In this report, we show that knockdown of 14-3-3 zeta induces at least two isoform-specific phenotypes that are consistent with a potential oncogenic activity during tumorigenesis. Firstly, down-regulation of 14-3-3 zeta sensitized cells to stress-induced apoptosis and JNK/p38 signalling and secondly, it enforced cell-cell contacts and expression of adhesion proteins. Apparently, the zeta isoform restrains both cell adhesion and the cellular propensity for apoptosis, two activities that are also restrained during carcinogenesis. The assumption that 14-3-3 zeta has oncogenic properties was substantiated with a web-based meta-analysis (Oncomine), revealing that 14-3-3 zeta is overexpressed in various types of carcinomas. As the highly conserved human 14-3-3 gene family encodes proteins with either tumour-promoting or tumour-suppressing activities, we infer that the cellular balance between the various 14-3-3 isoforms is crucial for the proper functioning of cells.