Normalizing Tumor Cell Metabolism In Breast Cancer Metastasis: A Novel Therapeutic Approach

Despite advances in clinical therapy, metastasis is still the leading cause of death in breast cancer patients. A better understanding of mechanisms that drive metastasis is a prerequisite for new approaches to effectively prevent and inhibit this most dangerous advancement of the disease. While alterations in the nuclear genome are pivotal in oncogenesis, a role of mitochondria in cancer progression has remained largely unexplored. Mutations in mitochondrial DNA are found in breast tumors and other cancers, however their involvement in driving the disease is unclear. Our study identifies mitochondrial complex I as critical for defining an aggressive phenotype in breast cancer cells. Complex I is the gate-keeper of the respiratory chain and catalyzes the first step of NADH oxidation. It elevates the cellular NAD+/NADH ratio and translocates protons across the inner mitochondrial membrane, which ultimately leads to energy production. We used a unique approach to define contributions of mitochondrial complex I activity to breast cancer progression, based on expression of yeast NADH dehydrogenase Ndi1. Ndi1 encodes a single protein that translocates to mitochondrial, faces the inner mitochondrial matrix and oxidizes NADH from the Krebs cycle. Specific enhancement of mitochondrial complex I activity by Ndi1 expression inhibited tumor growth and metastasis through regulation of the tumor cell NAD+/NADH redox balance, mTORC1 activity, and autophagy. Conversely, non-lethal reduction of NAD+ levels by interfering with nicotinamide phosphoribosyltransferase expression to disturb the NAD+ synthesis and recycling pathway, rendered tumor cells more aggressive and increased metastasis. Thus, the results indicate a cause-and-effect relationship between reduced NAD+/NADH ratios and metastatic activity. Having established that enhancement of NAD+/NADH levels by augmenting breast cancer cell complex I activity inhibits tumorigenicity and metastasis, we used this new concept therapeutically and hypothesized that supplementing tumor cell nutrients with NAD+ precursors, such as nicotinic acid (NIC) or nicotinamide (NAM), could interfere with breast cancer progression. We demonstrate that enhancing NAD+ levels through NAD+ precursor treatment effectively inhibits experimental metastasis of human breast cancer cells in xenograft models. Importantly, this treatment also inhibited spontaneous metastasis, and increased animal survival when the therapy was started after surgical removal of primary tumors. Furthermore, NAD+ precursor treatment strongly interferes with oncogene driven breast cancer development and progression in transgenic MMTV-PyMT mice. Thus, aberration in mitochondrial complex I NADH dehydrogenase activity can profoundly enhance the aggressiveness of human breast cancer cells while therapeutic normalization of the NAD+/NADH balance can inhibit metastasis and prevent disease progression. Our study demonstrates that mitochondrial complex I regulation of tumor cell NAD+/NADH levels impacts breast cancer growth and metastasis, and translates into a new therapeutic approach for preventing breast cancer progression. This is highly relevant as current standard of care for cancer patients relies primarily on chemo- and radiation therapies aimed at killing the tumor cells. Evolutionary models predict that selective pressure imposed by these approaches causes survival of resistant clones that eventually re-activate the disease. Based on the central involvement of metabolic tumor cell alterations in cancer, therapeutic normalization of tumor cell metabolism might interfere with the expansion of residual and break-through clones. Thus, a combination of standard therapy with NAD+ precursor treatment may halt breast cancer progression and prevent relapse. Citation Format: Antonio F. Santidrian, Akemi Matsuno-Yagi, Melissa Ritland, Byoung B. Seo 1,2 , Sarah E. LeBoeuf, Laurie J. Gay, Takao Yagi, Brunhilde Felding-Habermann. Normalizing tumor cell metabolism in breast cancer metastasis: A novel therapeutic approach. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Invasion and Metastasis; Jan 20-23, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;73(3 Suppl):Abstract nr IA3.