The TLK1>Nek1 Axis Promotes Nuclear Retention and Activation of YAP with Implications for CRPC

Despite some advances in controlling progression of Prostate Cancer (PCa) that is refractory to the use of ADT/ARSI, most patients eventually succumb to the disease, and there is a pressing need to understand the mechanisms that lead to the development of CRPC. A common mechanism is the ability to integrate AR signals from vanishing levels of testosterone, with the frequent participation of YAP as a coactivator, and pointing to deregulation of the Hippo pathway as a major determinant. We have recently shown that YAP is post-transcriptionally activated via the TLK1NEK1 axis by a stabilizing phosphorylation at Y407. We are now solidifying this work by showing that: 1) The phosphorylation of Y407 is critical for YAP retention/partition in the nuclei, and that J54 (TLK1i) reverses this along with YAP407 dephosphorylation. 2) That the enhanced degradation of (cytoplasmic) YAP is increased by J54 counteracting its Enzalutamide-induced accumulation. 3) That the basis for all these effects, including YAP nuclear retention, can be explained by the stronger association of pYAP-Y407 with its transcriptional co-activators, AR and TEAD1. 4) We demonstrate that ChIP for GFP-YAP-wt, but hardly for the GFP-YAP-Y407F mutant, at promoters of typical ARE- and TEAD1-driven genes is readily detected but becomes displaced after treatment with J54. 5) While xenografts of LNCaP cells show rapid regression following treatment with ARSI+J54, in the VCaP model, driven by the TMPRSS2-ERG oncogenic translocation, tumors initially respond well to the combination but subsequently recur, despite the continuous suppression of pNek1-T141 and pYAP-Y407. This suggests an alternative parallel pathway for CRPC progression for VCaP tumors long-term, that may be separate from the observed ENZ-driven YAP deregulation, although clearly some YAP gene targets like PD-L1, that are found accumulate following prolonged ENZ treatment, are still suppressed by concomitant addition of J54.