Inducible transgenics. New lessons on events governing the induction and commitment in mammary tumorigenesis

Breast cancer arises from multiple genetic events that together contribute to the established, irreversible malignant phenotype. The development of inducible tissue-specific transgenics has allowed a careful dissection of the events required for induction and subsequent maintenance of tumorigenesis. Mammary gland targeted expression of oncogenic Ras or c-Myc is sufficient for the induction of mammary gland tumorigenesis in the rodent, and when overexpressed together the rate of tumor onset is substantially enhanced. In an exciting recent finding, D'Cruz et al discovered tetracycline-regulated c-Myc overexpression in the mammary gland induced invasive mammary tumors that regressed upon withdrawal of c-Myc expression. Almost one-half of the c-Myc-induced tumors harbored K-ras or N-ras gene point mutations, correlating with tumor persistence on withdrawal of c-Myc transgene expression. These findings suggest maintenance of tumorigenesis may involve a second mutation within the Ras pathway

Mitochondria as new therapeutic targets for eradicating cancer stem cells: Quantitative proteomics and functional validation via MCT1/2 inhibition.

Here, we used quantitative proteomics analysis to identify novel therapeutic targets in cancer stem cells and/or progenitor cells. For this purpose, mammospheres from two ER-positive breast cancer cell lines (MCF7 and T47D) were grown in suspension using low-attachment plates and directly compared to attached monolayer cells grown in parallel. This allowed us to identify a subset of proteins that were selectively over-expressed in mammospheres, relative to epithelial monolayers. We focused on mitochondrial proteins, as they appeared to be highly upregulated in both MCF7 and T47D mammospheres. Key mitochondrial-related enzymes involved in beta-oxidation and ketone metabolism were significantly upregulated in mammospheres, as well as proteins involved in mitochondrial biogenesis, and specific protein inhibitors of autophagy/mitophagy. Overall, we identified >40 “metabolic targets” that were commonly upregulated in both MCF7 and T47D mammospheres. Most of these “metabolic targets” were also transcriptionally upregulated in human breast cancer cells in vivo, validating their clinical relevance. Based on this analysis, we propose that increased mitochondrial biogenesis and decreased mitochondrial degradation could provide a novel mechanism for the accumulation of mitochondrial mass in cancer stem cells. To functionally validate our observations, we utilized a specific MCT1/2 inhibitor (AR-C155858), which blocks the cellular uptake of two types of mitochondrial fuels, namely ketone bodies and L-lactate. Our results indicate that inhibition of MCT1/2 function effectively reduces mammosphere formation, with an IC-50 of ~1 μM, in both ER-positive and ER-negative breast cancer cell lines. Very similar results were obtained with oligomycin A, an inhibitor of the mitochondrial ATP synthase. Thus, the proliferative clonal expansion of cancer stem cells appears to require oxidative mitochondrial metabolism, related to the re-use of monocarboxylic acids, such as ketones or L-lactate. Our findings have important clinical implications for exploiting mitochondrial metabolism to eradicate cancer stem cells and to prevent recurrence, metastasis and drug resistance in cancer patients. Importantly, a related MCT1/2 inhibitor (AZD3965) is currently in phase I clinical trials in patients with advanced cancers: http://clinicaltrials.gov/show/NCT01791595

Roles of cell fusion, hybridization and polyploid cell formation in cancer metastasis

Cell-cell fusion is a normal biological process playing essential roles in organ formation and tissue differentiation, repair and regeneration. Through cell fusion somatic cells undergo rapid nuclear reprogramming and epigenetic modifications to form hybrid cells with new genetic and phenotypic properties at a rate exceeding that achievable by random mutations. Factors that stimulate cell fusion are inflammation and hypoxia. Fusion of cancer cells with non-neoplastic cells facilitates several malignancy-related cell phenotypes, e.g., reprogramming of somatic cell into induced pluripotent stem cells and epithelial to mesenchymal transition. There is now considerable in vitro, in vivo and clinical evidence that fusion of cancer cells with motile leucocytes such as macrophages plays a major role in cancer metastasis. Of the many changes in cancer cells after hybridizing with leucocytes, it is notable that hybrids acquire resistance to chemo- and radiation therapy. One phenomenon that has been largely overlooked yet plays a role in these processes is polyploidization. Regardless of the mechanism of polyploid cell formation, it happens in response to genotoxic stresses and enhances a cancer cells ability to survive. Here we summarize the recent progress in research of cell fusion and with a focus on an important role for polyploid cells in cancer metastasis. In addition, we discuss the clinical evidence and the importance of cell fusion and polyploidization in solid tumors

Ethanol exposure induces the cancer-associated fibroblast phenotype and lethal tumor metabolism: implications for breast cancer prevention.

Little is known about how alcohol consumption promotes the onset of human breast cancer(s). One hypothesis is that ethanol induces metabolic changes in the tumor microenvironment, which then enhances epithelial tumor growth. To experimentally test this hypothesis, we used a co-culture system consisting of human breast cancer cells (MCF7) and hTERT-immortalized fibroblasts. Here, we show that ethanol treatment (100 mM) promotes ROS production and oxidative stress in cancer-associated fibroblasts, which is sufficient to induce myofibroblastic differentiation. Oxidative stress in stromal fibroblasts also results in the onset of autophagy/mitophagy, driving the induction of ketone body production in the tumor microenvironment. Interestingly, ethanol has just the opposite effect in epithelial cancer cells, where it confers autophagy resistance, elevates mitochondrial biogenesis and induces key enzymes associated with ketone re-utilization (ACAT1/OXCT1). During co-culture, ethanol treatment also converts MCF7 cells from an ER(+) to an ER(-) status, which is thought to be associated with “stemness,” more aggressive behavior and a worse prognosis. Thus, ethanol treatment induces ketone production in cancer-associated fibroblasts and ketone re-utilization in epithelial cancer cells, fueling tumor cell growth via oxidative mitochondrial metabolism (OXPHOS). This “two-compartment” metabolic model is consistent with previous historical observations that ethanol is first converted to acetaldehyde (which induces oxidative stress) and then ultimately to acetyl-CoA (a high-energy mitochondrial fuel), or can be used to synthesize ketone bodies. As such, our results provide a novel mechanism by which alcohol consumption could metabolically convert “low-risk” breast cancer patients to “high-risk” status, explaining tumor recurrence or disease progression. Hence, our findings have clear implications for both breast cancer prevention and therapy. Remarkably, our results also show that antioxidants [such as N-acetyl cysteine (NAC)] can effectively reverse or prevent ethanol-induced oxidative stress in cancer-associated fibroblasts, suggesting a novel strategy for cancer prevention. We also show that caveolin-1 and MCt4 protein expression can be effectively used as new biomarkers to monitor oxidative stress induced by ethanol