Biologic Effects And Mechanisms Of The Antitumor Action Of Natural Product Neoalbaconol

Massive of secondary metabolites with delicate structures and unique biologic activities are produced by plants and microbes. These natural products have shown promising results in cancer therapy and prevention for the last several decades. More than half of all low molecular weight drugs developed in the last several decades are natural compounds or their synthetic analogs, and this ratio is even higher in the field of antibiotics and anticancer drugs. There are estimated to be more than140,000species of higher fungi in the world, only14,000of them have been discovered and many fewer lucubrated. The task of identifying new, fungal drugs with potential benefits to human health has just begun.The fungus Albatrellus confluens of the family Polyporaceae is distributed mainly in southeast China. Several compounds with anticancer potential have been isolated from this fungus such as grifolin, neogrifolin, albaconol and neoalbaconol. Grifolin, neogrifolin and albaconol are known compounds, while neoalbaconol is a new, small molecular compound recently obtained from the fruiting body of this higher fungus.The primary efficacy of anticancer drugs in cancer therapy is the growth inhibition and death induction of cancer cells; several cell death mechanisms have been described, including apoptosis, necrosis, autophagic cell death and, most recently, a process termed "necroptosis". Necroptosis is characterized by necrotic cell death morphology and activation of autophagy; cells undergoing necroptosis normally sufferd from inhibited energy metabolism; this mechanism of cell death can be specifically inhibited by a small molecule necrostatin-1. Autophagy can be activated by signaling pathways that also control apoptosis, and it can occur with or without apoptosis. The role of autophagy in cell death remains poorly defined and somewhat controversial so far.The phosphoinositide3-kinase-Akt (PI3K/Akt) signaling pathway has a dramatic effect on reprogramming metabolic networks for the enhancement of glycolysis through multiple targets, such as tuberin, FOXO, GSK3and especially the hexokinase family, which directly couple intramitochondrial ATP synthesis to glucose metabolism. PI3K/Akt pathway is perhaps the most commonly activated signaling pathway in human cancer. It is a critical signal transduction system for cellular energy metabolism, and a variety of small molecules which block PI3K/Akt signaling also block many aspects of the tumor-cell phenotype. As downstream effectors in the Akt pathway, hexokinases are known to couple intramitochondrial ATP synthesis directly to glucose metabolism. Akt increases mitochondrial hexokinase activity, and the anti-apoptotic activity of Akt is exerted only at the first committed step of glucose metabolism catalyzed by hexokinase.To evaluate the tumor suppression activity of NA, we first observed its effects on the growth of tumor cell lines. NA selectively inhibited growth in a wide variety of tumor cell lines without significant affect on the cell cycle progress. In the cell lines tested, NA induced cell death in a dose-and time-dependent manner, resulting in the activation of the intrinsic and extrinsic pathway to induce apoptosis; however, this cell death was only weakly rescued by pan caspase inhibitor zVAD-fmk, while the necroptosis inhibitor Nec-1greatly increased the cell viability. To confirm the induction of necroptosis, and considering the fact that cancer cells exploit autophagy as an energy source, we chose to observe autophagy and energy metabolism in NA-treated cells.Microtubule-associated protein1light chain3(LC3), a homologue of Apg8p essential for autophagy in yeast, is associated to the autophagosome membranes after processing; it has been widely used as an autophagosomal marker in mammary cells. The protein level of LC3was elevated after NA treatment; this was associated with JNK activation and was independent of apoptosis. Glucose consumption and ATP synthesis were decreased, indicating an energy crisis induced by NA treatment. Inhibition of autophagy with3-MA or JNK inhibitor SP600125increased cell death; however, NAD+, which also inhibited autophagy, increased cell viability under NA treatment.The phosphoinositide3-kinase-Akt (PI3K/Akt) signaling pathway has a dramatic effect on reprogramming metabolic networks for the enhancement of glycolysis through multiple targets, such as tuberin, FOXO, GSK3and especially the hexokinase family, which directly couple intramitochondrial ATP synthesis to glucose metabolism16. In vitro results showed that the PI3K/Akt signaling pathway was suppressed by NA. Factors critical in energy metabolism, such as Hexokinase2(HK2), were regulated by NA in an Akt-dependent manner, which resulted in a decrease in glucose metabolism and ATP synthesis. Using the nude mice NPC model, we further proved that NA decreased xenograft tumor growth by way of PI3K/Akt pathway suppression.These findings suggested that NA induces energy depletion of cells assoiated with apoptosis, autophagy and necroptosis by inhibition of PI3K/Akt signaling pathway in vitro and in vivo; NA induced cell death is mainly caused by necroptosis, a most recently defined non-apoptotic programmed cell death mechanism. NA then may have potentials in chemotherapy and chemoprevention as a lead-compound.