The class I PI3K/Akt pathway is critical for cancer cell survival in dogs and offers an opportunity for therapeutic intervention

We demonstrate that growth and survival of the cell lines tested are predominantly dependent on class I PI3K/Akt signaling rather than mTORC1 signaling. In addition, the newly developed inhibitors ZSTK474 and KP372-1 which selectively target pan-class I PI3K and Akt, respectively, and Rapamycin which has been well-established as highly specific mTOR inhibitor, decrease viability of canine cancer cell lines. All inhibitors demonstrated inhibition of phosphorylation of pathway members. Annexin V staining demonstrated that KP372-1 is a potent inducer of apoptosis whereas ZSTK474 and Rapamycin are weaker inducers of apoptosis. Simultaneous inhibition of class I PI3K and mTORC1 by ZSTK474 combined with Rapamycin additively or synergistically reduced cell viability whereas responses to the PI3K pathway inhibitors in combination with conventional drug Doxorubicin were cell line-dependent.This study highlighted the importance of class I PI3K/Akt axis signaling in canine tumour cells and identifies it as a promising therapeutic target.The class I phosphatidylinositol 3-kinase (PI3K) signaling pathway comprises a series of serine/threonine kinase cascades that regulate a variety of cellular processes including cell cycle progression, cell survival and migration, and protein synthesis. Recent evidence supports the hypothesis that the dysregulation of class I PI3K signaling promotes tumourigenesis and angiogenesis in various cancer types [1-3].Class I PI3K is predominantly activated by receptor tyrosine kinases (RTKs) upon receiving growth factor stimulation. The activated RTKs undergo either autophosphorylation of tyrosine (Y) residues at the intracellular domains or phosphorylation of their substrates such as IRS-1, IRS-2 and Gab on Y residues. The phosphorylated Y residues are soon recognized by SH2 domains in p85 regulatory subunit of class I PI3K, recruiting class I PI3K to plasma membrane, triggering activation of PI3K downstream pathways (reviewed in ref. [4,5]). Alterna