Not all secondary metabolites or natural products can be ful

Not all secondary metabolites or natural products can be fully synthesized due to their very complex structures that are too difficult and expensive on industrial scale. Hence, there is an urgent need to search for alternative remedies as naturally occurring biologically active secondary metabolites from plant origin. At present, there are a large number of such bioactive compounds isolated from crude extracts and their chemical structure were elucidated (Bajpai, Majumder, & Park, 2016). Although natural extract are known to contain high concentrations of polyphenols, it abscisic acid is not clear which of these polyphenols are the actual contributing components to the known biological activities (Si et al., 2006). For these reasons, a preferred approach would be the bioassay-guided fractionation and purification technique that could directly link the activity of Lippia citriodora to its components. In this way, isolation by semi-preparative and preparative liquid chromatography (LC) with C18 reversed phase (RP) offers high versatility to separate a wide range of nitrogenous and non-nitrogenous bioactive compounds (Cádiz-Gurrea et al., 2014, Jiménez-Sánchez et al., 2017). Thus, the aims of this study were to: (1) characterize the phenolic profile of a commercial Lippia citriodora extract by HPLC-ESI-TOF-MS, (2) fractionate this extract by semi-preparative HPLC and characterize the obtained fractions, and (3) evaluate the capacity to modulate AMPK activity using the well-established 3T3-L1 adipocyte model. This will provide a better understanding of the relationship between AMPK modulation activity and chemical structure of isolated polyphenols from Lippia citriodora in order to be included as a supplement or potential ingredient in functional foods.
Material and methods
Results and discussion
Conclusions In an attempt to identify those compounds contributing to such active AMPK, the bioassay-guided fractionation of Lippia citriodora extract has been achieved by HPLC semi-preparative. Fraction containing pure verbascoside (F7) showed the highest activating capacity. Other fractions enriched in a combination of iridoids and phenylpropanoids (F1, F3) or fractions containing terpenes, phenylpropanoids and flavones (F6, F10) also showed a significant activating capacity. Fractions containing a combination of phenylpropanoids and flavones (F8, F9 and F11) activated AMPK but showed a cytotoxic effect when the concentration was increased, probably due to the presence of some flavones.
Main Text Mammalian 5′-AMP-activated protein kinase (AMPK) is usually regarded as a sensor of adenine nucleotides that is activated in states of low cellular energy, and acts to restore energy balance under those circumstances by switching on alternative catabolic pathways that generate ATP while switching off anabolic pathways and other processes consuming ATP. This view of the system, which was proposed in the 1990s (Hardie and Carling, 1997), has prevailed for the last 20 years. However, recent findings (Zhang et al., 2013, 2014, 2017) show that mammalian AMPK can sense glucose in the absence of any changes in cellular energy state, and that this takes place on the lysosome where an AMPK-activation complex acts in opposition to another key sensor of cellular nutrient availability, the mechanistic target-of-rapamycin complex 1 (mTORC1). A major aim of this review is to discuss these new concepts and to place them in context with the canonical energy-sensing role of AMPK. We also speculate about the evolutionary origins of AMPK and its ability to sense cellular glucose availability and energy status. We focus particularly on pathways upstream of AMPK rather than downstream events, since these have been reviewed extensively elsewhere (Day et al., 2017; Garcia and Shaw, 2017; Hardie et al., 2016).
Acknowledgments Recent studies in the D.G.H. laboratory have been funded by a program grant from Cancer Research UK (C37030/A15101) and an investigator award from the Wellcome Trust (097726/Z/11/Z). The S.-C.L. laboratory is funded by the National Foundation of Sciences and Ministry of Science and Technology of China. S.-C.L. was supported by grants from the National Key R&D Program of China (2016YFA0502001) and the National Natural Science Foundation of China (31430094 and 31601152). We thank all members of the D.G.H. and S.-C.L. laboratories for experimental work and valuable discussions.