RAS proteins signal through direct interaction with a number of effector enzymes, including type I phosphatidylinositol 3-kinases (PI3Ks).
Mice with mutations in the RAS binding domain (RBD) of the Pik3ca gene encoding the PI3K catalytic p110α isoform are highly resistant to endogenous KRAS oncogene induced lung tumourigenesis and HRAS oncogene induced skin carcinogenesis (Gupta et al., 2007; Cell. 129(5): 957-68).
The interaction of RAS with p110α is thus required in vivo for RAS-driven tumour formation. We have also generated mice with inducible expression of the inactivating mutation in the RAS binding domain of p110α so that the requirement of this interaction for maintenance of established tumours can be assessed.
Blocking the RAS/p110α interaction causes partial regression and stasis of tumours, although more complete regression requires coordinate inhibition of the MEK pathway (Castellano et al., Cancer Cell 2013). There is both a tumour cell autonomous and a tumour microenvironment component to this effect (Murillo et al., J Clinical Investigation 2014). In addition, RAS signalling through PI3-Kinase p110α controls cell migration and metastatic spread via modulation of Reelin expression (Castellano et al., Nature Communications 2016).
While combined inhibition of the RAS effector pathways MEK and PI3K can cause impressive tumour regression, this combination has high toxicity that may be problematic in the clinic. We have sought to find less toxic ways of inhibiting PI3K in KRAS mutant lung cancer cells using a drug library screen and have found that inhibition of IGF1 receptor allows this. It appears that the activation of PI3K by mutant KRAS in lung cancer cells is dependent on basal signalling by IGF1 receptor. The combination of MEK and IGF1 receptor inhibition shows potential in preclinical models of KRAS mutant lung cancer (Molina-Arcas et al., 2013;Cancer Discovery. 3(5): 548-63).
We have also created a mouse with inactivating mutations in the RAS binding domain of p110β the other ubiquitously expressed PI 3-kinase catalytic subunit isoform, and are testing the effects of this mutation on tumour initiation and maintenance, especially in the context of PTEN deletion, where p110β is thought to be particularly important.
Our investigations with p110β have led us to the surprising observation that this isoform is not controlled by direct interaction with RAS, unlike p110α, γ and δ, but rather that the RBD of p110β interacts directly with a number of other small GTPases with distinct biological function - the RAC and CDC42 proteins. This has led us to a significantly revised model of how extracellular stimuli, especially those signalling through G protein coupled receptors, activate the PI 3-kinase activity of p110β, and the importance of this mechanism in cancer metastasis and also fibrosis (Fritsch et al., 2013; Cell. 153(5): 1050-63).
Figure 1: Differential interaction of RAS and RHO subfamily GTPases with different type I PI3K isoforms. RAS subfamily proteins, including the oncoproteins KRAS, HRAS and NRAS, interact with PI3K p110α, γ and δ isoforms, whereas the RHO subfamily proteins RAC and CDC42 interact with PI3K p110β isoform. In all cases, the interaction is dependent on the GTPase being in the activated GTP-bound conformation, acts through the conserved RBD region of p110 and results in activation of the PI3K enzymatic activity (Fritsch et al., Cell 2013).