Combined Inactivation of MYC and K-Ras Oncogenes Reverses Tumorigenesis in Lung Adenocarcinomas and Lymphomas

Conditional transgenic models have established that tumors require sustained oncogene activation for tumor maintenance, exhibiting the phenomenon known as "oncogene-addiction." However, most cancers are caused by multiple genetic events making it difficult to determine which oncogenes or combination of oncogenes will be the most effective targets for their treatment.To examine how the MYC and K-ras(G12D) oncogenes cooperate for the initiation and maintenance of tumorigenesis, we generated double conditional transgenic tumor models of lung adenocarcinoma and lymphoma. The ability of MYC and K-ras(G12D) to cooperate for tumorigenesis and the ability of the inactivation of these oncogenes to result in tumor regression depended upon the specific tissue context. MYC-, K-ras(G12D)- or MYC/K-ras(G12D)-induced lymphomas exhibited sustained regression upon the inactivation of either or both oncogenes. However, in marked contrast, MYC-induced lung tumors failed to regress completely upon oncogene inactivation; whereas K-ras(G12D)-induced lung tumors regressed completely. Importantly, the combined inactivation of both MYC and K-ras(G12D) resulted more frequently in complete lung tumor regression. To account for the different roles of MYC and K-ras(G12D) in maintenance of lung tumors, we found that the down-stream mediators of K-ras(G12D) signaling, Stat3 and Stat5, are dephosphorylated following conditional K-ras(G12D) but not MYC inactivation. In contrast, Stat3 becomes dephosphorylated in lymphoma cells upon inactivation of MYC and/or K-ras(G12D). Interestingly, MYC-induced lung tumors that failed to regress upon MYC inactivation were found to have persistent Stat3 and Stat5 phosphorylation.Taken together, our findings point to the importance of the K-Ras and associated down-stream Stat effector pathways in the initiation and maintenance of lymphomas and lung tumors. We suggest that combined targeting of oncogenic pathways is more likely to be effective in the treatment of lung cancers and lymphomas

Abstract 5036: NF-κB pathway inhibition delays the onset of MYC-induced liver cancer

Abstract Liver cancer is one of the most lethal forms of cancer worldwide. Hence, it is critical to identify pathways that may be essential for the genesis of liver cancer and can thus be considered as targets for therapeutic intervention. Previously, our laboratory developed a conditional transgenic murine model of MYC-induced liver cancer that is dependent upon the expression of this oncogene and regresses following its inactivation. Here, we present evidence of an important role for the nuclear factor-kappa B (NF-κB) pathway in MYC-induced hepatocarcinogenesis. In order to examine whether NF-κB activation is necessary for MYC-induced liver cancer, we established a transgenic system in which MYC can be activated in hepatocytes concomitantly with a mutant form of IκB, thereby inhibiting the NF-κB pathway. Our approach has revealed that suppression of NF-κB activation can significantly inhibit MYC-induced hepatocellular carcinoma (HCC) in adult hosts. Indeed, animals with hepatocyte-inhibited NF-κB remain tumor free up to 8 months after oncogene activation compared to mortality in 100% within 6 months in animals expressing MYC alone. Moreover, inhibition of NF-κB reduces disease penetrance from 100% to 75%. In hosts that do develop liver tumors, there is evidence for activation of the NF-κB pathway, suggesting a necessity for NF-κB activation in MYC-induced HCC. Importantly, suppression of NF-κB activation in neonatal hosts is not only able to similarly suppress MYC-induced tumor formation, but animals that develop liver cancer in this context demonstrate a strikingly altered tumor phenotype. In addition, MYC inactivation results in a concomitant decrease in expression of NF-κB target genes, suggesting a direct role of MYC in the activation of NF-κB in hepatocytes. The inhibition of NF-κB did not result in increased apoptosis upon MYC activation. Rather, NF-κB activation may be important for MYC to evade oncogene-induced senescence, thereby facilitating tumorigenesis. Our data suggests that the inhibition of the NF-κB pathway may be sufficient to abrogate MYC's ability to induce tumorigenesis in the liver and thereby establish NF-κB activation as an attractive therapeutic target for liver cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5036.</jats:p

Altered localization of p120 catenin during epithelial to mesenchymal transition of colon carcinoma is prognostic for aggressive disease

We examined the expression and localization of p120 catenin (p120ctn) as a consequence of the epithelial to mesenchymal transition (EMT) of highly differentiated colon carcinoma cells (LIM1863 cells). This unique line grows in suspension as spheroids and undergoes an EMT within 24 hours following stimulation with transforming growth factor-beta and tumor necrosis factor-alpha. Although p120ctn expression remains stable during the EMT, its localization shifts from cell-cell junctions to the cytoplasm. Interestingly, a marked decrease in RhoA activation coincident with E-cadherin loss occurs during the EMT and correlates with the formation of a p120ctn/RhoA complex. Use of RNA interference showed that p120ctn reduction results in increased RhoA activity and a significant decrease in the motility of post-EMT cells. To determine the relevance of these findings to colorectal cancer progression, we assessed p120ctn expression by immunohistochemistry in 557 primary tumors. Of note, we observed that 53% of tumors presented cytoplasmic staining for p120ctn, and statistical analysis revealed that this localization is predictive of poor patient outcome. Cytoplasmic p120ctn correlated with later-stage tumors, significantly reduced 5- and 10-year survival times and a greater propensity for metastasis to lymph nodes compared with junctional p120ctn. We also confirmed that altered localization of p120ctn corresponded with loss or cytoplasmic localization of E-cadherin. These alterations in E-cadherin are also associated with a significant reduction in patient survival time and an increase in tumor stage and lymph node metastasis. These data provide a compelling argument for the importance of both p120ctn and the EMT itself in the progression of colorectal carcinoma

Hepatotoxin-induced changes in the adult murine liver promote MYC-induced tumorigenesis.

Overexpression of the human c-MYC (MYC) oncogene is one of the most frequently implicated events in the pathogenesis of hepatocellular carcinoma (HCC). Previously, we have shown in a conditional transgenic mouse model that MYC overexpression is restrained from inducing mitotic cellular division and tumorigenesis in the adult liver; whereas, in marked contrast, MYC induces robust proliferation associated with the very rapid onset of tumorigenesis in embryonic and neonatal mice.Here, we show that non-genotoxic hepatotoxins induce changes in the liver cellular context associated with increased cellular proliferation and enhanced tumorigenesis. Both 5-diethoxycarbonyl-1,4-dihydrocollidine (DDC) and carbon tetrachloride (CCl(4)) cooperate with MYC to greatly accelerate the onset of liver cancer in an adult host to less than 7 days versus a mean latency of onset of over 35 weeks for MYC alone. These hepatotoxin-enhanced liver tumors grossly and histologically resemble embryonic and neonatal liver tumors. Importantly, we found that MYC overexpression is only capable of inducing expression of the mitotic Cyclin B1 in embryonic/neonatal hosts or adult hosts that were treated with either carcinogen.Our results suggest a model whereby oncogenes can remain latently activated, but exposure of the adult liver to hepatotoxins that promote hepatocyte proliferation can rapidly uncover their malignant potential