Asperuloside enhances taste perception, decreases inflammation and prevents weight gain in high‑fat fed mice

Obesity rates are increasing in the developed and developing world, and its consequences represent a major public health concern. Obesity produces a low-grade systemic inflammation that is associated with many adverse health conditions, including cardiovascular diseases, type 2 diabetes, multiple types of cancers and, as we recently learned, with complications of Covid-19.
In the pharmacological treatment of obesity, the short-term use of weight-loss medications has been proven to be unsuccessful when the patients need to maintain the reduced weight indefinitely. On the other hand, long-term pharmacological approaches appear to be the most effective in helping patients adhere to the dietary requirements to maintain reduced body weight. However, subsequent to the withdrawal from the market of several anti-obesity medications for safety reasons, there is a need for a better and safer pharmacological strategy.
Over the last decade, anti-obesity drug discovery programmes have increasingly aimed to identify active compounds from plant sources and traditional medicines to respond to an obesogenic environment that promotes weight gain. Among several emerging therapeutic compounds, the iridoid glycoside asperuloside (ASP) has produced promising anti-obesity results in animal models.
Three months of ASP administration reduced food intake and body weight in rats consuming a high-fat diet (HFD) ad libitum. This compound reduced plasma triglyceride, total cholesterol, free fatty acid levels, and glucose and insulin circulating levels. Gene expression studies also indicated that ASP regulates the mRNA levels of enzymes involved in lipid metabolism. In a mouse model of acute lung injury (ALI), asp also showed anti-inflammatory proprieties downregulating inflammatory markers such as tumour necrosis factor-alpha (TNF-α), interleukin (IL)-1β, and IL-6 levels. Similarly, in lipopolysaccharide (LPS)-induced inflammation in Raw 264.7 cells, pre-treatment with asperuloside significantly reduced the phosphorylation of inhibitor of nuclear factor kappa-B (IκBα), extracellular signal-related kinases 1 and 2 (ERK1/2), c-Jun. N terminal kinase (JNK) and p38 mitogen-activated protein kinase (p38MAPK) in a dose-dependent manner.
However, the mechanisms by which ASP reduces food intake, body weight as well as its safety profile were not elucidated. For these reasons, this thesis is divided into two studies.
However, the mechanisms by which ASP reduces food intake, body weight as well as its safety profile were not elucidated. For these reasons, this thesis is divided into two studies.
Specifically, in Study 1, I identified mechanisms that could be responsible for the anti-obesity properties of ASP while confirming previously reported anti-obesity properties. Over the 12 weeks of the experimental period, oral ASP administration (3mg/day) induced a significant reduction in food intake and body weight in only mice consuming HFD compared to their control group. Weight loss reached significance after seven weeks of treatment and was maintained until the end of the experiment, with a final body weight difference of 10.5% between treated and untreated animals. In the HFD group, ASP also induced a 12.8% reduction in daily energy intake compared to the control group. In contrast, in mice eating a standard chow diet, body weight and daily or cumulative energy intake were not affected by ASP. The compound promoted a significant reduction of visceral adipose mass (epididymal and retroperitoneal fat pads) and an increase in interscapular brown adipose tissue (IBAT) only in mice consuming HFD. ASP significantly reduced circulating blood glucose levels and also lowered insulin levels in mice consuming HFD.
In our study, ASP did not affect body weight, adipose mass, daily or cumulative energy intake, and mice's hypothalamic and lingual mRNA levels on a standard chow diet. In addition to previous studies investigating the anti-obesity properties of ASP, I aimed to describe a possible mechanism of action for the natural compound. My results suggest that ASP might exert its therapeutic effect by altering fat and sweet receptors in the oral cavity, which are known to affect appetite, satiety and metabolism through afferent signalling to the hypothalamus, a brain area that regulates homeostatic energy responses to nutrient utilisation.
In Study 2, I addressed the safety profile of ASP in HepG2 cells via a battery of in vitro experiments, including Adenosine triphosphate (ATP), Water-soluble tetrazolium salt (WST-1) and Colony formation assay (CFA). HepG2 cells are widely used to study the toxic effect of novel potential compounds. Based on viability assays, ASP did not exhibit any cytotoxicity in ATP and WST-1 assay up to 800uM compared to positive control rotenone. However, in the CFA, a long-term cytotoxic assay, ASP’s only significant cytotoxic effect was observed at an extremely high concentration of 1000uM. In addition to in-vitro studies, I investigated the ant ASP's anti-inflammatory and anti-obesity properties hypothalamus and liver of obese mice. HFD consumption significantly increased the mRNA level of hypothalamic and hepatic pro-inflammatory cytokines IL-1β, IL-6, and TNF-α and oral ASP administration (3mg/day) for 12 weeks significantly reduced those levels.
In summary, the present thesis attempts to explain how ASP reduces weight gain when exposed to HFD consumption. While I confirmed the previous findings regarding the anti-obesity properties of ASP, my results suggest that the therapeutical effect of ASP most likely occurs via alterations in nutrient-sensing mechanisms of the taste buds, which are known to be involved in the homeostatic and hedonic regulation of food intake through afferent signalling to the hypothalamus. My results also indicated that ASP shows a safe profile in vitro investigations and that it can reduce murine hypothalamic and hepatic pro-inflammatory cytokines increased by HFD consumption.
Previous studies have failed to deliver significant clinical results in targeting signalling pathways in metabolic tissues such as the liver, adipocytes, and skeletal muscles. The present pre-clinical studies revealed a promising and effective new anti-obesity strategy via targeting the gustatory signalling pathways. This unique and novel mechanism of action that targets peripheral sensory pathways of the gustatory system should be exploited in future drug discovery programmes to identify new drug candidates that can provide clinically relevant anti-obesity agents.