Complex regulation of TAZ by the abundance of YAP in mammalian cells

[Truncated] Aberrant expression or activation of YAP and TAZ, the two effectors of the Hippo signalling pathway, plays an oncogenic role in several human cancers. Liver progenitor cells (LPCs) are a putative cancer stem cell in primary liver cancers, and Hippo pathway dysregulation is implicated in LPC-driven cancers. Investigation of the role of YAP in LPCs using shRNA revealed a transient reduction in cell proliferation in YAP-depleted cells that was accompanied by a concomitant increase in TAZ abundance. Conversely, increased YAP expression induced TAZ degradation. These experiments revealed a novel mechanism whereby YAP inversely regulates TAZ abundance posttranslationally. Using a range of chemical inhibitors, it was found that proteasomes, and to a lesser extent proteases, mediate YAP-induced TAZ degradation. In contrast, TAZ expression did not affect YAP abundance. Slower turnover and increased stability of YAP relative to TAZ is proposed to account for this.

Structure/function analyses suggest that YAP-induced TAZ degradation requires transcription by YAP. The TEAD-binding, PDZ-binding and transactivation domains of YAP were all necessary to induce TAZ degradation. It was found that YAP, but not TAZ, could induce Lats kinase abundance and it was hypothesised that the increased Lats expression might mediate TAZ degradation. However, Lats knockdown did not block YAP-induced TAZ degradation, thereby eliminating this possibility. Further investigation of possible upstream regulators of TAZ degradation revealed that neither PI3 kinase signalling nor CK-1 is involved in YAP-induced TAZ degradation. However, treatment with a HSP90 or GSK-3 inhibitor increased TAZ abundance in growing cells and importantly prevented YAP-induced TAZ degradation. This indicates both HSP90 and GSK-3 are integral for intrinsic TAZ turnover as well as being necessary to mediate YAP-induced TAZ degradation. The YAP-transactivated target gene/s that increase GSK-3 activity, possibly via HSP90, remain to be identified and this is an area for future study.