Nicotinamide adenine dinucleotide (NAD) is a key regulator of several metabolic and signaling pathways that are relevant in cancer cell survival. Cancer cells have an increased energy demand associated with an increased NAD turnover. Nicotinamide phosphoribosyltransferase (NAMPT), a key enzyme of the NAD salvage pathway, plays a crucial role in maintaining the intracellular NAD levels and in regulating the activity of NAD-dependent enzymes, such as sirtuins (SIRTs). The inhibition of NAMPT activity and the use of phytochemicals, such as resveratrol, represent novel therapeutic approaches in cancer therapy. Based on these facts, this thesis aimed to investigate (1) the chemotherapeutic potential and molecular mechanisms of FK866, a specific NAMPT inhibitor, and resveratrol on hepatocarcinoma cells and to find out whether there are differences compared to primary human hepatocytes; (2) to address the impact of NAMPT inhibition on the energy metabolism in cancer cells; and (3) to investigate the roles of NAMPT and SIRT1 in resveratrol´s mode of action and chemotherapeutic effects. This work demonstrates that FK866 and resveratrol possess potent chemotherapeutic effects in hepatocarcinoma cells which were absent in human hepatocytes. Hepatocarcinoma cells display a dysregulation in the AMP-activated kinase (AMPK)/mammalian target of rapamycin (mTOR) signaling as well as in the NAMPT-mediated NAD salvage pathway compared to human hepatocytes. FK866-induced NAMPT inhibition induces ATP depletion associated with AMPK activation and mTOR inhibition whereas resveratrol induces caspase3-mediated apoptosis that is not dependent on NAMPT and SIRT1 function. NAMPT and SIRT1 are differentially regulated by resveratrol in hepatocarcinoma cells and human hepatocytes. This work also reveals that resveratrol activates p53-induced cell cycle arrest in hepatocarcinoma cells which is partly mediated by SIRT1 inhibition. In summary, this thesis provides new insight into the role of the NAMPT-mediated NAD salvage pathway in energy metabolism and characterized FK866 and resveratrol as promising potential chemotherapeutic agents for treatment of hepatocellular carcinoma.

Nicotinamide adenine dinucleotide (NAD) is a key regulator of several metabolic and signaling pathways that are relevant in cancer cell survival. Cancer cells have an increased energy demand associated with an increased NAD turnover. Nicotinamide phosphoribosyltransferase (NAMPT), a key enzyme of the NAD salvage pathway, plays a crucial role in maintaining the intracellular NAD levels and in regulating the activity of NAD-dependent enzymes, such as sirtuins (SIRTs). The inhibition of NAMPT activity and the use of phytochemicals, such as resveratrol, represent novel therapeutic approaches in cancer therapy. Based on these facts, this thesis aimed to investigate (1) the chemotherapeutic potential and molecular mechanisms of FK866, a specific NAMPT inhibitor, and resveratrol on hepatocarcinoma cells and to find out whether there are differences compared to primary human hepatocytes; (2) to address the impact of NAMPT inhibition on the energy metabolism in cancer cells; and (3) to investigate the roles of NAMPT and SIRT1 in resveratrol´s mode of action and chemotherapeutic effects. This work demonstrates that FK866 and resveratrol possess potent chemotherapeutic effects in hepatocarcinoma cells which were absent in human hepatocytes. Hepatocarcinoma cells display a dysregulation in the AMP-activated kinase (AMPK)/mammalian target of rapamycin (mTOR) signaling as well as in the NAMPT-mediated NAD salvage pathway compared to human hepatocytes. FK866-induced NAMPT inhibition induces ATP depletion associated with AMPK activation and mTOR inhibition whereas resveratrol induces caspase3-mediated apoptosis that is not dependent on NAMPT and SIRT1 function. NAMPT and SIRT1 are differentially regulated by resveratrol in hepatocarcinoma cells and human hepatocytes. This work also reveals that resveratrol activates p53-induced cell cycle arrest in hepatocarcinoma cells which is partly mediated by SIRT1 inhibition. In summary, this thesis provides new insight into the role of the NAMPT-mediated NAD salvage pathway in energy metabolism and characterized FK866 and resveratrol as promising potential chemotherapeutic agents for treatment of hepatocellular carcinoma.

Background: Nicotinamide phosphoribosyltransferase (NAMPT) is the key enzyme of the NAD salvage pathway starting from nicotinamide. Cancer cells have an increased demand for NAD due to their high proliferation and DNA repair rate. Consequently, NAMPT is considered as a putative target for anti-cancer therapies. There is evidence that AMP-activated protein kinase (AMPK) and mammalian target of rapamycin (mTOR) become dysregulated during the development of hepatocellular carcinoma (HCC). Here, we investigated the effects of NAMPT inhibition by its specific inhibitor FK866 on the viability of hepatocarcinoma cells and analyzed the effects of FK866 on the nutrient sensor AMPK and mTOR complex1 (mTORC1) signaling. Results: FK866 markedly decreased NAMPT activity and NAD content in hepatocarcinoma cells (Huh7 cells, Hep3B cells) and led to delayed ATP reduction which was associated with increased cell death. These effects could be abrogated by administration of nicotinamide mononucleotide (NMN), the enzyme product of NAMPT. Our results demonstrated a dysregulation of the AMPK/mTOR pathway in hep- atocarcinoma cells compared to non-cancerous hepatocytes with a higher expression of mTOR and a lower AMPKa activation in hepatocarcinoma cells. We found that NAMPT inhibition by FK866 signifi- cantly activated AMPKa and inhibited the activation of mTOR and its downstream targets p70S6 kinase and 4E-BP1 in hepatocarcinoma cells. Non-cancerous hepatocytes were less sensitive to FK866 and did not show changes in AMPK/mTOR signaling after FK866 treatment. Conclusion: Taken together, these findings reveal an important role of the NAMPT-mediated NAD salvage pathway in the energy homeostasis of hepatocarcinoma cells and suggest NAMPT inhibition as a po- tential treatment option for HCC.

NAMPT (Nicotinamide phosphoribosyltransferase) catalyses the rate-limiting step in the NAD biosynthesis from nicotinamide and thereby regulates the activity of NAD-dependent enzymes. Cancer cells are highly dependent on NAD for energy and DNA repair processes and are assumed to be more susceptible to an inhibition of NAD synthesis than non-transformed cells. We aimed to investigate whether or not inhibition of NAMPT with its specific inhibitor FK866 can sensitize leukemia cells for chemotherapeutic agents. NAMPT protein abundance, enzymatic activity and NAD concentrations were significantly higher in Jurkat and Molt-4 leukemia cell lines compared to normal peripheral blood mononuclear cells. Combination of etoposide and FK866 caused increased cell death in leukemia cell lines compared to etoposide alone. Etoposide decreased protein abundance of NAD-dependent deacetylases SIRTUIN1. After combining etoposide and FK866 treatment SIRTUIN2 was further decreased and accumulation and acetylation of the downstream target p53 was further enhanced in MOLT4 cells. Concomitantly, protein abundance of p21 and cleaved BAX was increased. Targeting NAMPT could be a novel therapeutic strategy to enhance the efficacy of chemotherapeutic agents such as etoposide against leukemia.