Tylophora atrofolliculata is a centuries used
Tylophora atrofolliculata is a centuries-used folk medicine for the treatment of rheumatism in China (Jiangsu New Medical College, 1977). One group of its bioactive components is phenanthroindolizidine alkaloids, which have been demonstrated as lead compounds of anti-tumor agents in our previous studies (Chen et al., 2016a, b). Our further phytochemical exploration led to the isolation of a phenanthroindolizidine glycoside, 6-O-β-D-glucopyranosyl-tylophorinidine (1). Herein, we report the structure elucidation of compound 1 together with its capabilities in inhibition of HIF-1 activation and cancer nos inhibitor growth.
Conclusion This investigation led to the isolation of a new phenanthroindolizidine glycoside, named 6-O-β-D-glucopyranosyl-tylophorinidine (1), with glycosylation at C-6 of the aglycone part. Biological activity evaluation suggested that 1 demonstrated more potent HIF-1 inhibitory effect than digoxin. SAR analyses indicated that glycosylation at C-6 of 1 reduced its cytotoxicity against normal cells, but enhance cancer cell inhibitory selectivity in vitro. Above all, this is the first glucoside of phenanthroindolizidine type alkaloid from natural resource. Moreover, our results suggested that 6-O-β-D-glucopyranosyl-tylophorinidine (1) could be a promising lead compound in the development of anti-cancer drugs.
Material and methods
Acknowledgement The authors would like to thank the Macao Science and Technology Development Fund for the financial support (015/2017/AFJ to Z.H.J. and 023/2016/AFJ to J.R.W.).
Introduction Hypoxia-inducible factor-1 (HIF-1) senses cellular oxygen (O2) levels and induces an adaptive response to hypoxia by upregulating an array of genes associated with angiogenesis, cell growth, and glycolytic metabolism [1,2]. HIF-1 is composed of O2-regulated HIF-1α and its constitutive subunit HIF-1β . In the presence of normal O2 levels, the stability of the HIF-1α protein is regulated by prolyl hydroxylases domain protein (PHD)-mediated hydroxylation at the Pro402 and Pro564 residues and the Von Hippel-Lindau protein (VHL)-mediated proteasomal degradation [2,4,5]. In this reaction, O2 is used as a substrate of PHDs and, therefore, the hypoxic condition causes inhibition of the PHD-mediated hydroxylation and VHL-mediated degradation of HIF-1α. Accumulated HIF-1α binds to hypoxia-responsive elements (HREs) on the promoter of its target genes, which are involved in aggressive behaviors of cancers, including blood vessel formation [6,7], metabolic reprogramming [8,9] and metastasis/invasion [10,11]. The modulation of metabolic pathways is critical for cancer cell survival and growth in response to environmental changes. Upon facing hypoxia, cancer cells switch their metabolism to glycolytic pathways through HIF-1α signaling . First, HIF-1α accumulation directly upregulates glucose transporter-1 (GLUT1), a transporter for cellular uptake of glucose, and hexokinase-2 (HK2), an enzyme initiating glucose metabolism by phosphorylating glucose to produce glucose-6-phosphate (G6P) . HIF-1α additionally inhibits mitochondrial oxidative phosphorylation and tricarboxylic cycle (TCA cycle) through the induction of pyruvate dehydrogenase kinase-1 (PDK1), an enzyme that inhibits pyruvate dehydrogenase (PDH) through phosphorylation . This, in turn, represses the conversion of glycolytic pyruvate into mitochondrial acetyl-coenzyme A and thereby impairs mitochondrial oxidative phosphorylation. This leads to accumulation of pyruvate and further conversion to lactate through the enzymatic action of lactate dehydrogenase A (LDHA), which is also regulated by HIF-1α . Additionally, HIF-1α elevation has been shown to benefit cancer cells by promoting anabolic metabolism, including the pentose phosphate pathway (PPP), which can provide hypoxic cancer cells with nucleotides as building blocks . Autophagy is important for maintaining energy in times of nutrient deprivation . The process of autophagy involves the formation of double-membrane structured autophagosomes, which fuse with lysosomes to form autolysosomes where cellular components are degraded by lysosomal enzymes [18,19]. Autophagy participates in the maintenance of basal cellular homeostasis by clearing abnormal proteins and removing injured intracellular organelles such as mitochondria [19,20]. Although autophagy is considered to be a normal cellular process maintaining cellular homeostasis, it is generally thought to act as a protective mechanism against various stress conditions such as nutrient depletion, protein aggregations, and genotoxic agents [21,22].