Abstract A37: PI3-kinase signaling is rate limiting for KRASG12D-driven lung tumorigenesis but is not required for tumor maintenance

In genetically engineered mice (GEM), KRASG12D-driven lung tumorigenesis is dependent on signaling through both the RAF>MEK>ERK and the PI39K>AKT signaling pathways. However, there is an extended period of latency prior to the onset of KRASG12D-initiated lung tumorigenesis. Moreover, the level of KRASG12D-activated signaling (pERK & pAKT) is only weakly detectable in early-stage initiated tumor cells. To test the hypothesis that PI39K signaling is rate-limiting for KRASG12D-driven tumorigenesis we generated GEM models that express both KRASG12D and a constitutively active form of PIK3CA (PIK3CAH1047R) in the lung epithelium. As shown previously, expression of PIK3CAH1047R alone has no effect on the adult mouse lung. However, when PIK3CAH1047R was co-expressed with KRASG12D we noted a dramatic increase in overall lung tumor burden and a dramatic decrease in tumor latency. In addition, a larger fraction of KRASG12D/PIK3CAH1047R expressing lung tumors progressed to non-small cell lung cancer than in the KRASG12D alone controls. Hence, unlike oncogenic KRASG12D or BRAFV600E that serve as initiators of lung tumorigenesis, PIK3CAH1047R does not appear to be an initiator but rather a promoter of lung cancer progression. Next we tested the importance of PI39-kinase signaling for lung tumor maintenance. Perhaps surprisingly, pharmacological inhibition of class 1 PI39-kinases or the protein kinases AKT1-3 did not lead to substantial regression of established KRASG12D/PIK3CAH1047R expressing lung cancers. At best, these agents elicited cytostatic effects that led to slower tumor growth but with no statistically significant effect on overall mouse survival. These results stand in contrast to the effects of a MEK inhibitor that elicited frank regression of KRASG12D-induced lung tumors. Using mouse or human lung tumor derived cell lines expressing mutationally activated KRAS and PIK3CA we observed that pharmacological inhibition of either the ERK1/2 MAP kinase or the PI39-kinase pathways resulted in a G1 cell cycle arrest that was enhanced when both pathways were inhibited simultaneously. However, only limited apoptosis was observed in cells treated with combined pathway inhibitors. Immunoblot analysis of downstream effectors indicated the ERK1/2 MAP kinase and the PI39-kinase pathways coordinately regulate expression of c-MYC, p27KIP1, Cyclins D1 and A and the phosphorylation of the Retinoblastoma protein pRB thereby explaining the strong cooperative effects we see in cell cycle progression when both pathways are activated. Ongoing work aims to further analyze nodes of dual regulation and the factors that render KRASG12D/PIK3CAH1047R expressing lung cancers resistant to dual pathway inhibition. Identifying the signals generated by KRAS and PIK3CA to promote lung tumorigenesis will provide a better understanding of how best to target such cancers in the clinic and perhaps how best to combine pathway-targeted inhibitors to enhance apoptosis and thereby elicit tumor regression. Citation Format: Shon Green, Martin McMahon. PI3-kinase signaling is rate limiting for KRASG12D-driven lung tumorigenesis but is not required for tumor maintenance. [abstract]. In: Proceedings of the AACR Special Conference: Targeting the PI3K-mTOR Network in Cancer; Sep 14-17, 2014; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(7 Suppl):Abstract nr A37.