Numerous studies have demonstrated that the postnatal nutritional environment during the suckling period could affect body weight and energy homeostasis into adulthood[32, 33]. In the present study, the rat model of postnatal overfeeding induced by SL rearing led to several metabolic alterations such as higher body weight and WAT mass, increased serum lipids and insulin resistance at weaning and adulthood. These findings were in accordance with previous studies[6, 34].
Rats fed a post-weaning diet supplemented with CUR exhibited lower body weight, less fat mass, higher energy expenditure and improved glucose and lipid metabolism in adulthood compared to a standard diet in SL-reared rats. Furthermore, UCP1-positive brown fat-like cells emerged in the SAT of these rats after the CUR intervention. These data suggest that a dietary CUR supplement could stimulate the development of WAT browning and might be a strategy to increase energy expenditure for preventing obesity induced by postnatal overfeeding.
CUR’s capacity as an anti-obesity nutraceutical that increases weight loss and lowers fat mass has been verified previously in models of obesity at adulthood[20, 35]. The present study is the first to show this effect in overfed rats in the SL-rearing model. Recent studies have identified several windows of opportunity from preconception to childhood during which interventions could have long-lasting effects that could halt the transgenerational cycle of obesity and type 2 diabetes. In this study, we found that dietary CUR supplementation after weaning (SL1% CUR and SL2% CUR rats) was effective to reduce body weight, adipose tissue mass, and adipocyte volume in rats. Many metabolic diseases are increasing in parallel with the prevalence of obesity and overweight in youth, including type 2 diabetes, insulin resistance and hyperlipidemia[37, 38]. Interestingly, this study initially found that dietary CUR supplementation was also useful in improving glucose intolerance and hyperlipidemia induced by postnatal overfeeding.
The balance of energy intake and energy expenditure is the basis for maintaining a healthy body weight. The rationales of treatments for weight loss usually include reducing total energy uptake and increasing energy expenditure. In the present study, dietary administration of CUR (1% or 2% diet) increased energy expenditure but did not affect food intake in SL rats, which suggests that CUR could enhance energy metabolism rather than inhibit energy intake. Evidence from in vitro and in vivo studies has shown that CUR can increase the basal metabolic rate, thereby contributing to increased energy expenditure. Unsurprisingly, we observed an increase in the energy expenditure of SL rats fed a diet containing 2% CUR.
Adipose tissue is a critical regulator of systemic energy homeostasis by acting as a caloric reservoir. Induction of WAT browning could increase energy consumption and help to alleviate metabolic disorders[9–11]. Upregulation of UCP1 expression is closely related to increased adaptive thermogenic and energy expenditure and is widely used as a marker during BAT activity and WAT browning. In rodents, WAT depots have different propensities to form beige adipocytes. Induction of WAT browning occurs easily in subcutaneous depots compared with visceral mesenteric or epididymal depots[43, 44]. In this study, we mainly observed the browning feature of CUR in SAT and found that 2% dietary CUR supplement could increase both UCP1 mRNA and protein expression levels in the SAT of SL rats. The transcription of UCP1 requires coactivators, including PGC1α, PRDM16 and PPARγ[<link rid="bib9">9</link>], which returned to normal levels in SL2% CUR rats. Puigserver et al. have shown that ectopic expression of PGC1α in WAT depots is required to commit the cells to thermogenesis.
PGC1α is also a regulator of mitochondrial biogenesis, which is another characteristic manifestation of browning. Moreover, transmembrane protein 26 (TMEM26), a specific beige-selective gene that can distinguish beige adipocytes from brown or white adipocytes in adipose tissues, was upregulated in the SAT of SL rats with a diet containing 2% CUR. Taken together, these findings indicate that CUR may act as a thermogenic activator to promote the browning of SAT. This function could explain, at least in part, the positive effects of the dietary CUR intervention on metabolic disorders in obese rats induced by postnatal overfeeding.
Furthermore, we observed the serum NE and β3-ARmRNA expression levels in the SAT of rats. The release of NE from the adrenal medulla and sympathetic terminals in WAT is mandatory for the immediate activation of existing beige adipocytes and the differentiation of beige adipocytes from their precursors. NE acts on the β3-AR expressed in brown/beige adipocytes and then activates c-AMP pathway-dependent mitochondrial UCP1, which is the major mediator of adaptive thermogenesis in brown and beige adipose tissue. Many factors activate the β3-AR signaling pathway in WAT; the most effective is chronic cold exposure, which could induce the browning process.
In this study, we found that the SL rats experienced a decrease in serum NE and β3-AR mRNA expression levels in their SAT at both W3 and W13, but this could be reversed by feeding them a 2% CUR dietary supplement post-weaning. In vitro, we found that the expression of browning marker genes was significantly increased following treatment of CUR in preadipocytes, and the increase was suppressed by β3-AR antagonist. Taken together, our findings support that CUR-induced SAT browning may be associated with sympathetic stimulation through the norepinephrine-β3-AR pathway.
The effects of functional foods are ordinarily influenced by the dose administered. Generally, orally ingested CUR is metabolized in the liver and small intestine, and liver enzymes (AST and ALT) are usually used to detect oral toxicity. There was no significant difference in the serum AST and ALT levels in the SL1% CUR or SL2%CUR rats compared to the NL rats, which suggests that neither dose of CUR used in the present study resulted in liver injury.
In previous studies, the major intervention for CUR in obese animal models was oral administration, including gavage and dietary supplementation. The concentrations used in the latter range widely, from 0.1–3%. In this study, we provided the SL rats with two different doses of CUR (95% standardized CUR extract) to understand the effects of dose difference. The doses set in the current study are 1% and 2%, referring to the concentration of dietary CUR (95% standardized CUR extract) used in previous studies on obese mice models induced by high-fat diet and Western diet, which were 1% and 3%, respectively.
In the present study, the changes of body weight, serum chemical parameters, adipocyte surface area, and energy expenditure as well as expressions of browning-related genes in the SL2%CUR rats were slightly better than those of the SL1%CUR rats, but there were no statistical differences in these obesity parameters and browning genes between the SL1% CUR and SL2%CUR rats. Therefore, dietary 2% CUR supplementation seemed to be an efficient dose for an anti-obesity effect, but we cannot exclude that the effects may be enhanced by a higher dose of CUR.