Assembly of nuclear dimers of the PI3K regulatory subunits underpins the proliferative activity of Activated Cdc42-associated Kinase, ACK

ACK is an oncogenic non-receptor tyrosine kinase associated with poor prognosis in human cancers. ACK promotes proliferation, in part, by contributing to the activation of Akt, the major PI3K effector. The work presented here demonstrates that ACK also regulates proliferation via interactions with other key components of PI3K signalling, the PI3K regulatory subunits. ACK directly phosphorylates 4 out of the 5 isoforms (p85α, p55α, p50α, p85β) on a conserved tyrosine located on the iSH2 region (Tyr607 for p85α). Phosphorylation of p85β at this site was found to promote cell proliferation, yet counterintuitively, ACK does not stimulate PI3K catalytic activity. This suggested that ACK mediated phosphorylation of the PI3K regulatory subunits drives proliferation via a PI3K-independent mechanism. The PI3K heterodimer is composed of a regulatory subunit and a catalytic subunit. The regulatory subunits are in excess over the catalytic subunits, and undertake critical functions independent of PI3K. The regulatory subunits are already reported to dimerise via interactions between their N-terminal protein domains and the work presented here identifies a novel mode of dimerisation mediated by C-terminal protein domain interactions. The pTyr607 residue of one monomer binds to the nSH2 domain of another via canonical pTyr-SH2 binding. These regulatory subunit phosphodimers form exclusively in nuclear-enriched cell fractions where pTyr607 p85 resides. It is inferred that ACK targets p110-independent p85 and the regulatory subunit dimers that form undertake pro-proliferative nuclear functions that contribute to Cdc42/ACK driven oncogenesis. These dimers represent a previously undescribed mode of regulation for the Class IA PI3K regulatory subunits and potentially reveal additional avenues for therapeutic intervention. The work presented here demonstrates that ACK interacts with the PI3K pathway at multiple nodes, including Akt, the PI3K regulatory subunits and via PTEN, the terminator phosphatase. With a growing body of evidence implicating ACK in the progression of prostate cancer, CRISPR/Cas9 knock out of ACK in a prostate cancer cell line panel was used to assess the validity of targeting ACK therapeutically. Overall, this work contributes to the growing evidence implicating ACK as a key player in carcinogenesis.