Aims/hypothesis Pre-adipocytes and adipocytes are responsive to the acute phase protein serum amyloid A (SAA). The combined effects triggered by SAA encompass an increase in preadipocyte proliferation, an induction of TNF-α and IL-6 release and a decrease in glucose uptake in mature adipocytes, strongly supporting a role for SAA in obesity and related comorbidities. This study addressed whether SAA depletion modulates weight gain and insulin resistance induced by a high-fat diet (HFD). Methods Male Swiss Webster mice were fed an HFD for 10 weeks under an SAA-targeted antisense oligonucleotide (ASO SAA ) treatment in order to evaluate the role of SAA in weight gain. Results With ASO SAA treatment, mice receiving an HFD did not differ in energy intake when compared with their controls, but were prevented from gaining weight and developing insulin resistance. The phenotype was characterised by a lack of adipose tissue expansion, with low accumulation of epididymal, retroperitoneal and subcutaneous fat content and decreased inflammatory markers, such as SAA3 and toll-like receptor (TLR)-4 expression, as well as macrophage infiltration into the adipose tissue. Furthermore, a metabolic status similar to chow-fed mice counterparts could be observed, with equivalent levels of leptin, adiponectin, IGF-I, SAA, fasting glucose and insulin, and remarkable improvement in glucose and insulin tolerance test profiles. Surprisingly, the expected HFD-induced metabolic endotoxaemia was also prevented by the ASO SAA treatment. Conclusions/interpretationThis study provides further evidence of the role of SAA in weight gain and insulin resistance. Moreover, we also suggest that beyond its proliferative and inflammatory effects, SAA is part of the lipopolysaccharide signalling pathway that links inflammation to obesity and insulin resistance.

In the course of infection and intense endotoxemia processes, induction of a catabolic state leading to weight loss is observed in mice and humans. However, the late effects of acute inflammation on energy homeostasis, regulation of body weight and glucose metabolism are yet to be elucidated. Here, we addressed whether serial intense endotoxemia, characterized by an acute phase response and weight loss, could be an aggravating or predisposing factor to weight gain and associated metabolic complications. Male Swiss Webster mice were submitted to 8 consecutive doses of lipopolysaccharide (10 mg/kg LPS), followed by 10 weeks on a high-fat diet (HFD). LPS-treated mice did not show changes in weight when fed standard chow. However, when challenged by a high-fat diet, LPS-treated mice showed greater weight gain, with larger fat depot areas, increased serum leptin and insulin levels and impaired insulin sensitivity when compared to mice on HFD only. Acute endotoxemia caused a long-lasting increase in mRNA expression of inflammatory markers such as TLR-4, CD14 and serum amyloid A (SAA) in the adipose tissue, which may represent the key factors connecting inflammation to increased susceptibility to weight gain and impaired glucose homeostasis. In an independent experimental model, and using publicly available microarray data from adipose tissue from mice infected with Gram-negative bacteria, we performed gene set enrichment analysis and confirmed upregulation of a set of genes responsible for cell proliferation and inflammation, including TLR-4 and SAA. Together, we showed that conditions leading to intense and recurring endotoxemia, such as common childhood bacterial infections, may resound for a long time and aggravate the effects of a western diet. If confirmed in humans, infections should be considered an additional factor contributing to obesity and type 2 diabetes epidemics.

AimIntense endotoxaemia and infection are able to reduce appetite and induce a catabolic state, therefore leading to weight loss. However, it is underexplored its late effects on energy homeostasis, regulation of body weight and glucose metabolism. Here we addressed whether serial intense endotoxaemia, characterized by an acute phase response and weight loss, could be an aggravating or predisposing factor to diet-induced obesity (DIO) and associated metabolic impairments. MethodsMale Swiss Webster mice were submitted to 8 consecutive doses of lipopolysaccharide (LPS -10 mg/kg), followed by 10 weeks in high-fat diet (HFD). ResultsAfter the end of the acute endotoxaemia period, mice under chow diet recovered their weight rapidly, within one-week recovery period, which remained similar to its control counterparts. However, acute endotoxaemia caused a long-lasting adipose tissue expression of the inflammatory markers TLR-4, CD14 and serum amyloid A (SAA) and, when challenged by a HFD, LPS-treated mice gained more weight, showed increased fat depots, leptin and insulin levels, and also impaired insulin sensitivity. ConclusionsLPS-treated mice showed a higher susceptibility to the harmful effects of a subsequent HFD. Conditions leading to intense and recurrent endotoxaemia, such as common childhood bacterial infections, may resound for a long time and aggravate the effects of a western diet. If confirmed in humans, infections should be considered an additional factor contributing to obesity and type 2 diabetes epidemics and additionally impose more rigorous dietary recommendations for patients in post-infection recovery.