When expressed alone at high levels, the human adenovirus E4orf4 protein exhibits tumor cell-specific p53-independent toxicity. A major E4orf4 target is the B55 class of PP2A regulatory subunits and we have shown recently that binding of E4orf4 inhibits PP2AB55 phosphatase activity in a dose-dependent fashion by preventing access of substrates. While interaction with B55 subunits is essential for toxicity, E4orf4 mutants exist that, despite binding B55 at high levels, are defective in cell killing, suggesting that other essential targets may exist. In an attempt to identify additional targets we undertook a proteomics approach to characterize E4orf4-interacting proteins. Our findings indicated that, in addition to PP2AB55 subunits, ASPP-PP1 complex subunits were found among the major E4orf4-binding species. Both the PP2A and ASPP-PP1 phosphatases are known to positively regulate effectors of the Hippo signaling pathway that controls expression of cell growth/survival genes by dephosphorylating the YAP transcriptional co-activator. We find here that expression of E4orf4 results in hyperphosphorylation of YAP, suggesting that Hippo signaling may be affected by E4orf4 interactions with PP2AB55 and/or ASPP-PP1 phosphatases. Furthermore, knockdown YAP1 expression was seen to enhance E4orf4 killing, again consistent with a link between E4orf4 toxicity and inhibition of the Hippo pathway. This effect may in fact contribute to the cancer cell specificity of E4orf4 toxicity as many human cancer cells rely heavily on the Hippo pathway for their enhanced proliferation.
IMPORTANCE:
The human adenovirus E4orf4 protein has been known for some time to induce tumor cell-specific death when expressed at high levels, and thus knowledge of its mode of action could be of importance for development of new cancer therapies. Although the B55 form of the phosphatase PP2A has long been known as an essential E4orf4 target, genetic analyses indicated that others must exist. To identify additional E4orf4 targets we performed, for the first time, a large scale affinity purification/mass spectrometry analysis of E4orf4 binding partners. Several additional candidates were detected, including key regulators of the Hippo signaling pathway that enhances cell viability in many cancers, and results of preliminary studies suggested a possible link between inhibition of Hippo signaling and E4orf4 toxicity.