The first PI3K gene to be cloned was S. cerevisiae Vps34, which is required for vacuolar protein sorting in yeast and is the only PI3K gene in that organism. Mammals express four class I catalytic isoforms (p110α, β, γ, and δ encoded by PIK3CA, PIK3CB, PIK3CG, and PIK3CD) that catalyze the phosphorylation of PtdIns-4,5-P2 to generate PtdIns-3,4,5-P3 Human cells express three classes of PI3K enzymes, there are three class II PI3Ks (PI3K-C2α, β, γ) and a single class III PI3K (hVPS34). PI3Ks are capable of being activated by receptor-coupled tyrosine kinase activities, small Ras-related GTPases, and heterotrimeric G proteins. Each class I isoform has a domain that interacts with members of the Ras GTPase superfamily PI3K signaling is evolutionarily conserved among multicellular organisms as a mechanism to respond to external growth cues. 
In mammals PI3K signaling is activated downstream of a myriad of growth factor receptors, including PDGF receptor (PDGFR) and epidermal growth factor receptor (EGFR), which drive proliferation and migration, insulin-like growth factor receptor (IGFR) which stimulates growth and survival, and insulin receptor (INSR) which regulates metabolic homeostasis. To coordinate responses to extracellular queues, the effectors of PI3K need to alter multiple facets of the cell, e.g. signaling that drives cell cycle progression also generates increased demand for metabolic programs to produce the energy and macromolecular synthesis to support cell growth and mitotic cell division. The recognition that PI3K signaling was aberrantly activated in the majority of human cancers, together with the presence of actionable target proteins in the PI3K/mTOR network, spurred expectations that PI3K/mTOR pathway inhibitors would spawn a major paradigm shift in cancer therapy.


1.Fruman DA, et al. Cell. 2017;170(4):605–635.