In contrast to our evaluation here, Ebi et al didn’t see a unfavorable effect of removal of KRAS by RNAi knockdown on PI3K activity in KRAS mutant cells. The basis for this distinction is unclear. One possibility is that it reflects the differing tissue varieties of origin on the cells, the frequency of coincident mutation of KRAS and PIK3CA in colon but not lung cancer suggests that there could possibly be important differences within the interplay involving these signaling systems within the two tissues. A quantitative model of RAS signaling to PI3K concludes that the relative contributions of RAS and RTKs to PI3K activation rely strongly around the quantities and binding affinities in the interacting proteins, which are likely to vary greatly across various cell forms and stimuli. Alternatively, this might possibly reflect variations within the efficiency of KRAS knockdown involving the shRNA and siRNA approaches made use of.
It is feasible that RAS protein expression has to be reduced beneath numerous thresholds to possess an influence on RAF and on PI3K signaling. The tendency of MEK and mTOR inhibition to cause PI3K activation due to relief of damaging feedback onto IRS1 can also obscure the direct impact of loss of RAS expression on PI3K activity, which could be revealed selleck chemicals when mTOR activity is artificially inhibited by rapamycin, as shown in Fig. five. The use of a post translationally activatible form of oncogenic RAS allows extra precise probing from the part of RAS in PI3K regulation, which includes in a time frame that should be minimally impacted by RAS pathway induced changes in gene expression. From this, it is actually clear that short term RAS activation can lead to stimulation of PI3K, but that this is dependent on input from the IGF1R tyrosine kinase.
It is therefore most likely that RAS calls for relief from the inhibitory impact on the unliganded p85 regulatory subunit of PI3K in an effort to be capable of effectively activate its lipid kinase activity via direct RAS p110 selleck interaction, and that, in KRAS mutant lung cancer, this signaling input into p85 is offered by basal IGF1R signaling. This impact was observed in untransformed immortalized breast epithelial cells and also in two different cultures of typical immortalized lung epithelial cells with post translationally inducible RAS activity. We also tested this in an NSCLC line lacking KRAS mutation. When this showed dependence of RAS induced PI3K pathway activation on IGF1R function, there was also a component of EGFR dependence. It can be probably that this reflects the mixed IGF1R and EGFR dependence of your parental KRAS wild variety SK MES 1 cell line, although the KRAS mutant NSCLC lines seem to become a lot more dependent on IGF1R rather then EGFR signaling. We speculate that within this inducible system, acutely activated RAS will utilize input from whatever basally active RTK is present inside the cells to relieve p85 mediated auto inhibition of PI3K activity, in KRAS mutant NSCLC this is predominantly IGF1R, while in KRAS wild sort NSCLC each IGF1R and EGFR contribute.