A natural compound Evo(kes) signaling for fat regulation

Regulation of body fat is critical to prevent progression of metabolic syndrome and extend lifespan. Adipose tissue is largely responsible for fat metabolism, affecting glucose metabolism and insulin sensitivity under the control of various hormones and cytokines. Indeed, obesity, characterized by excess fat deposition mainly in white adipose tissue (WAT), is a serious health risk in industrialized societies. Since obesity is the result of an energy imbalance when intake exceeds expenditure, intervention to reduce caloric intake through hormonal regulation and/or to increase energy expenditure by increasing thermogenic functions such as uncoupling protein 1 (UCP1) activity in brown adipose tissue (BAT) could be reasonable ways of preventing obesity. The recent rediscovery of functional BAT in humans, despite its reduction with age, has accelerated basic and clinical studies on the stimulation of BAT formation and activity as a potential therapeutic target in the battle against obesity and insulin resistance; however, an alternative strategy independent of UCP1 thermogenesis is needed for BAT-negative individuals.

Evodiamine (Evo), an alkaloid extracted from the dried unripe fruit of Evodia rutaecarpa Bentham (Rutaceae), has long been used in traditional Chinese herbal medicine to treat pain, vomiting and pyresis. Similar to capsaicin, Evo has wide-ranging bioactivity with anti-nociceptive, anti-tumor and anti-obesity effects. In earlier studies, the anti-obesity effect of Evo was attributed to BAT activation, because this compound may stimulate the sympathetic nervous system by acting as a capsaicin receptor agonist, inducing UCP1 thermogenesis. However, we found that Evo unexpectedly decreased diet-induced obesity and glucose intolerance in a UCP1-independent manner.¹ In that report, we revealed that Evo, but not capsaicin, increased phosphorylation of extracellular signal-regulated kinase (ERK) and reduced expression of peroxisome proliferator-activated receptor-γ (PPARγ) in preadipocytes, strongly inhibiting their differentiation into mature adipocytes. Evo also inhibited adipogenesis by activating the EGFR-PKCα-ERK signaling pathway.² These findings, based on Evo signaling in preadipocytes, suggest that this compound may offer a useful treatment for obesity by preventing the expansion of WAT.

In a recent paper, we further reported an unidentified mechanism whereby Evo improves insulin resistance by signaling through mammalian target of rapamycin (mTOR) and ribosomal S6 protein kinase (S6K) in mature adipocytes.³ Recent advances in understanding of mTOR signaling unveiled the crucial role of this pathway in nutrient metabolism, cell growth and aging.⁴ We first found significant decreases in mTOR and Akt phosphorylation in WAT, but not BAT, of obese/diabetic KK-Ay mice treated with Evo, in which various obesity phenotypes including glucose tolerance were improved without any change in food intake. Evo treatment also reduced mTOR (not Akt) phosphorylation in liver, whereas its effect on mTOR-S6K signaling appeared more potent in WAT. In particular, down-regulation of insulin receptor substrate 1 (IRS1) serine phosphorylation (an indicator of insulin resistance) was remarkable, while its tyrosine phosphorylation (indicating insulin sensitivity) was intact in Evo-treated WAT, suggesting suppression of the negative feedback loop from S6K to IRS1, as reported by Um et al.⁵ These results led us to examine the effect of Evo on nutrient signaling in mature adipocytes, which are central to the physiological and pathological function of WAT, although our previous studies revealed anti-adipogenic ERK signaling by Evo in preadipocytes. In cultures of 3T3-L1 adipocytes, we confirmed that Evo inhibited the insulin-stimulated phosphorylation of mTOR and S6K, leading to downregulation of IRS1 serine phosphorylation. Evo alone did not affect ERK and Akt phosphorylation, contradicting the results observed in preadipocytes. In addition, this compound was found to stimulate phosphorylation of AMP-activated protein kinase (AMPK), a cellular energy sensor, in adipocyte cultures and in Evo-treated WAT. The anti-diabetic compound rosiglitazone showed similar effects to those of Evo on AMPK, mTOR and IRS1 phosphorylation in adipocytes, although rosiglitazone stimulates adipogenesis. We also detected activation of tuberous sclerosis complex 2, a regulator of mTOR activity, through AMPK activation in Evo-treated adipocytes (our unpublished data). Taken together, these findings suggest that Evo might be a unique compound able to combat obesity and insulin resistance through activation of AMPK and inhibition of mTOR-S6K signaling in white adipocytes, thus contributing to preventing progression of metabolic syndrome and extending the healthy lifespan (Fig. 1).

Figure 1.

Proposed mechanisms of evodiamine signaling in white adipocytes. Excess caloric intake stimulates insulin and mTOR signaling, increases adipogenesis, and causes obesity and insulin resistance. Evodiamine stimulates EGFR-ERK signaling in preadipocytes to inhibit adipogenesis (blue pathway) but also activates AMPK and inhibits insulin-stimulated mTOR-S6K signaling (red pathway) in mature adipocytes, leading to improvements of obesity and insulin resistance.

In conclusion, we demonstrate that evodiamine evokes several signaling pathways controlling fat regulation in WAT. Because modulation of the mTOR pathway is intimately linked with age-related diseases and longevity,⁶ the Evo effect on longevity is of great interest and a long-term feeding study involving Evo is under investigation in mice. The impact of rapamycin intervention on lifespan extension emphasizes the importance of the mTOR signaling pathway in the regulation of longevity in small animals. It is also interesting that Evo stimulates AMPK phosphorylation in mice without affecting food intake, even though its phosphorylation is generally increased under conditions of caloric restriction or starvation when the cellular AMP:ATP ratio increases. Because mTOR and AMPK are involved in autophagy signaling, Evo could induce so-called lipophagy to reduce excess fat deposition in adipocytes. This mechanism may also explain the Evo effect, which significantly improved hepatic steatosis in our mouse models; however this mechanism remains to be clarified as does as the question of how Evo activates AMPK.