Important role of 5‐hydroxytryptamine in glucocorticoid‐induced insulin resistance in liver and intra‐abdominal adipose tissue of rats

Abstract

Aim/IntroductionBoth glucocorticoids and 5‐hydroxytryptamine (5‐HT) have been shown to induce insulin resistance (IR) in hepatocytes and adipocytes. Here, we explore whether there is a correlation between them.Materials and MethodsExcept for the control group, male rats were exposed to dexamethasone treated with or without para‐chlorophenylalanine (pCPA), or carbidopa for 20 days. Except for the control group, buffalo rat liver 3A (BRL‐3A) cells were exposed to dexamethasone for 24 h, treated with or without pCPA, carbidopa, or clorgiline for 48 h, or exposed to 5‐HT treated with or without fluoxetine for 48 h. Whole‐body IR was determined by both glucose tolerance test and measurement of fasting blood glucose and insulin, whereas hepatocytes or adipocytes IR was determined by examining either hepatic gluconeogenesis, steatosis and glucose transporter 2 expression or lipolysis.ResultsDexamethasone‐induced whole‐body IR, liver and intraabdominal adipose IR were accompanied by upregulated expressions of tryptophan hydroxylase‐1 and aromatic amino acid decarboxylase with increased 5‐HT level in both tissues, which were attenuated significantly by pCPA, inhibiting tryptophan hydroxylase‐1, or carbidopa, inhibiting aromatic amino acid decarboxylase. [Correction added on 22 September 2015, after first online publication: ‘inhibiting aromatic amino acid decarboxylase’ was duplicated and has been replaced by ‘tryptophan hydroxylase‐1’.] In the BRL‐3A cells, dexamethasone‐induced IR was also accompanied by upregulated 5‐HT synthesis in dose‐ and time‐dependent manners, and was attenuated by pCPA or carbidopa, but exacerbated by clorgiline, inhibiting monoamine oxidase‐A to further increase 5‐HT level. Dexamethasone also enhanced 5‐HT 2A and 2B receptor expressions in both tissues and BRL‐3A cells. Additionally, blocking 5‐HT transporter with fluoxetine significantly suppressed 5‐HT‐induced IR in BRL‐3A cells.ConclusionEnhancement of 5‐HT synthesis in liver and intra‐abdominal adipose is an important reason for glucocorticoids‐induced IR.

Effective treatment with combination of peripheral 5‐hydroxytryptamine synthetic inhibitor and 5‐hydroxytryptamine 2 receptor antagonist on glucocorticoid‐induced whole‐body insulin resistance with hyperglycemia

“…To examine whether 5‐HT is synthesized and is influenced by Dex in the liver, visceral adipose and skeletal muscle tissue of rats, we assessed Tph1 and AADC expression by western blot, and 5‐HT levels in these three tissues and serum. In agreement with a previous study, the expressions of Tph1 and AADC (Figure a,b) were detected in the liver and visceral adipose tissues, but were not detected in the skeletal muscle (data not shown), whereas rats' chronic exposure to Dex showed markedly upregulated Tph1 and AADC expression in both tissues. Accordingly, 5‐HT levels in the serum and three tissues (Figure d) were increased significantly by Dex, which were inhibited by CDP in a dose‐dependent manner in the CDP or Sar and CDP co‐treatment rats, whereas Sar alone did not obviously change 5‐HT levels in the serum or in the three tissues.…”

“…Dex-induced production of 5-HT in the entero-chromaffin cells has been observed in the small intestine, especially in the duodenum of rats 51 . However, both our previous study 22 and present study found that 5-HT is also synthesized in the liver and visceral adipose tissues, which were upregulated by Dex with upregulated expressions of both Tph1 and AADC, and that increased 5-HT levels by Dex in both tissues came from tissue itself rather than the blood. 5-HT by acting on 5-HT 2A R mediates hepatic steatosis and IR has been shown in several studies 24,52 , whereas activation of the mTOR-S6K pathway has been shown to be the mechanism of 5-HT action in the liver 53 , and in the adipocytes and C2C12 myotubes by inhibition of insulin-stimulated activation of the IRS-1-AKT signaling pathway with glucose uptake 54 .…”

“…In a previous study, we found that 5‐HT is synthesized in both the liver and visceral adipose tissues, which are enhanced by chronic GC exposure, and is important for GC‐induced IR in both organs and whole body; GC also upregulates expressions of 5‐HT 2A R and 5‐HT 2B R in both organs, which might be another reason for GC‐induced IR. To examine the hypothesis that GC‐induced IR can be effectively treated by inhibition of both peripheral tissue's 5‐HT synthesis and 5‐HT 2 R, insulin‐resistant rats induced by long‐term exposure to dexamethasone (Dex) were treated with an AADC inhibitor, carbidopa (CDP), or/and a 5‐HT 2 R antagonist, sarpogrelate (Sar).…”

Kynurenines: Tryptophan’s metabolites in exercise, inflammation, and mental health

“…Serotonin binds to mechanistic target of rapamycin (mTOR), increasing liver lipogenesis and impairing insulin signaling in adipocytes. Accordingly, inhibition of serotonin degradation by monoamine oxidase A (MAOA) exacerbates the effects (110). Compounding the problem, serotonin has well-documented effects on brain-mediated control of appetite.…”

Selection of Suitable Reference Genes for Quantitative Real-Time PCR Normalization in Three Types of Rat Adipose Tissue

“…Therefore, the stability of frequently-used reference genes is relative. From a survey of different studies of adipose tissue, qRT-PCR data were normalized to different reference genes, such as Glyceraldehyde-3-phosphate dehydrogenase ( GAPDH ) [13], β-actin [14], 18S [15] and acidic ribosomal phosphoprotein P0 ( 36B4 ) [16]. However, no research has studied the stability of frequently-used reference genes in different types of rat adipose tissue.…”