Targeted cancer therapies have exploited specific mutations that drive survival signals in subsets of tumors, leading to successful genotype-directed clinical applications of small molecule inhibitors (reviewed in (Haber et al., 2011
). However, KRAS
activating mutations, which are common in multiple human cancers, remain a critical therapeutic challenge. KRAS
mutations are generally associated with treatment-refractory tumors (Downward, 2003
). For instance, KRAS
mutant lung cancers are generally refractory to EGFR-targeted small molecule inhibitors and in colon cancer, KRAS
mutations predict failure of response to antibodies targeting overexpressed wild-type EGFR (Normanno et al., 2009
). Thus, patients with KRAS
-mutant colon cancers are excluded from targeted EGFR therapies and are faced with limited therapeutic options.
To date, pharmacologic targeting of activated KRAS has not been successful. Mutationally-activated KRAS proteins are not susceptible to agents that target GTPase function and drug therapies that disrupt KRAS post-translational modifications have not been clinically efficacious (Whyte et al., 1997
). More recently, several genome-wide shRNA screens revealed synthetic lethal interactions in which knockdown of a candidate gene appeared to suppress proliferation of KRAS
mutant cancer cell lines (Barbie et al., 2009
; Luo et al., 2009a
; Scholl et al., 2009
; Singh et al., 2009
). To date, these findings have not led to the development of effective inhibitors to treat KRAS
-mutant cancers, possibly due in part to the contextual complexity of KRAS
mutations and the difficulty in generalizing synthetic lethal interactions across a broad range of tumor-specific backgrounds.
In analyzing large panels of KRAS
-mutant tumor-derived cell lines, we noted that approximately half of these cell lines underwent apoptosis following shRNA-mediated knockdown of KRAS (so-called KRAS-dependent), whereas the other half maintained viability (KRAS-independent) (Singh et al., 2009
). These differences were independent of the particular KRAS
mutation, although KRAS-dependent cells generally exhibited higher KRAS protein expression levels. Cellular context appeared to play a significant role in the dependency of KRAS
mutant cells on continued KRAS signaling for their survival. In both lung and pancreatic cancer cells harboring KRAS
mutations, the presence of epithelial markers was highly correlated with KRAS dependency, whereas epithelial to mesenchymal transformation (EMT) was associated with KRAS independence, despite the presence of a KRAS
mutation. Reversion of mesenchymal cells to an epithelial phenotype was associated with restored dependence on KRAS, suggesting that mesenchymal-associated signals may provide alternative survival pathways when KRAS activity is disrupted (Singh et al., 2009
). Interestingly, the association of KRAS dependency with expression of epithelial markers was not evident in KRAS
-mutant colon cancers prompting our interest in identifying lineage-specific determinants of KRAS-dependency in colon cancers.
The cellular context of KRAS
mutations in colon cancer is complex. APC
loss-of-function mutations that arise in early adenomas lead to inappropriate activation of canonical Wnt signaling, and β-catenin-mediated transcriptional changes (Behrens et al., 1996
mutations are most common during the progression from adenoma to carcinoma, occurring in approximately 50% of cases, and subsequently followed by frequent deletions of SMAD4, targeting the transforming growth factor beta(TGF-β) signaling pathway. (Vogelstein et al., 1988
). The contribution of KRAS
mutations to colon carcinogenesis is thus uniquely linked to altered Wnt and TGF-β signaling.
Here, we compared KRAS-dependent and -independent colon cancer cells using a combination of gene expression and shRNA knockdown studies, which led to the identification of MAP3K7
, encoding the TGF-β activated kinase (TAK1), as a driver of cell survival in KRAS-dependent, APC
-deficient cells. Together with the recently reported enhancement of Wnt signaling by KRAS (Janssen et al., 2006
; Phelps et al., 2009
), these observations point to an unappreciated yet critical signaling node in a subset of colon cancers. We demonstrate that, in KRAS-dependent, but not in KRAS-independent cells, KRAS activates Bone Morphogenetic Protein-7 (BMP-7) signaling, leading to TAK1 activation, β-catenin nuclear localization, and transcriptional upregulation of Wnt target genes. This is also accompanied by KRAS- and TAK1-regulated activation of the NF-κB pathway. Reconstitution studies confirm that activation of this KRAS-dependent signaling network underlies exquisite sensitivity to TAK1 inhibition. Together, these observations point to a potential therapeutic strategy, based on targeting a vulnerable node in an identifiable subset of APC/KRAS
mutant colon cancers.