Sequential adaptive changes in a c-Myc-driven model of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a common cancer that frequently overexpresses the c-Myc (Myc) oncoprotein. Using a mouse model of Myc-induced HCC, we studied the metabolic, biochemical, and molecular changes accompanying HCC progression, regression, and recurrence. These involved altered rates of pyruvate and fatty acid β-oxidation and the likely re-directing of glutamine into biosynthetic rather than energy-generating pathways. Initial tumors also showed reduced mitochondrial mass and differential contributions of electron transport chain complexes I and II to respiration. The uncoupling of complex II's electron transport function from its succinate dehydrogenase activity also suggested a mechanism by which Myc generates reactive oxygen species. RNA sequence studies revealed an orderly progression of transcriptional changes involving pathways pertinent to DNA damage repair, cell cycle progression, insulin-like growth factor signaling, innate immunity, and further metabolic re-programming. Only a subset of functions deregulated in initial tumors was similarly deregulated in recurrent tumors thereby indicating that the latter can “normalize” some behaviors to suit their needs. An interactive and freely available software tool was developed to allow continued analyses of these and other transcriptional profiles. Collectively, these studies define the metabolic, biochemical, and molecular events accompanyingHCCevolution, regression, and recurrence in the absence of any potentially confounding therapies. Liver tissues were collected from a genetically engineered mouse model of hepatocellular carcinoma (FVB/N-Tg(tetO-MYC)36aBop/J × LAP-tTA) under seven experimental conditions. Mice were maintained on doxycycline (100 μg/ml) to suppress Myc expression. Tumor initiation was triggered by doxycycline withdrawal, and tissues were collected at day 3 (D3) and day 7 (D7) post-withdrawal, prior to visible tumor formation. Additional mice were maintained off doxycycline for 3–4 weeks until aggressive HCCs developed, after which tumors were sampled (Tumor group). Doxycycline was then reintroduced to induce regression, and tissues were collected after 3 days (R3) and 7 days (R7). A final group underwent long-term regression (2–3 months) followed by doxycycline withdrawal to induce tumor recurrence (Recurrence group). RNA-seq was performed on liver samples from each condition to profile gene expression during tumor progression, regression, and recurrence.