Intestinal lipid handling: evidence and implication of insulin signaling abnormalities in human obese subjects

Animal models have evidenced the role of intestinal triglyceride-rich lipoprotein overproduction in dyslipidemia. However, few studies have confronted this issue in humans and disclosed the intrinsic mechanisms. This work aimed to establish whether intestinal insulin resistance modifies lipid and lipoprotein homeostasis in the intestine of obese subjects.

Duodenal specimens obtained from 20 obese subjects undergoing bariatric surgery were paired for age, sex, and body mass index with or without insulin resistance, as defined by the homeostasis model assessment of insulin resistance. Insulin signaling, biomarkers of inflammation and oxidative stress, and lipoprotein assembly were assessed. The intestine of insulin-resistant subjects showed defects in insulin signaling as demonstrated by reduced protein kinase B phosphorylation and increased p38 mitogen-activated protein kinase phosphorylation, likely as the result of high oxidative stress (evidenced by malondialdehyde and conjugated dienes) and inflammation (highlighted by nuclear factor-κB, tumor necrosis factor-α, interleukin-6, intercellular adhesion molecule-1, and cyclooxygenase-2). Enhanced de novo lipogenesis rate and apolipoprotein B-48 biogenesis along with exaggerated triglyceride-rich lipoprotein production were observed, concomitantly with the high expression levels of liver and intestinal fatty acid-binding proteins and microsomal transfer protein. The presence of an aberrant intracellular cholesterol transport/metabolism was also suggested by the reduced expression of ATP-binding cassette A1 transporter and proprotein convertase subtilisin/kexin type 9.

According to the present data, the small intestine may be classified as an insulin-sensitive tissue. Dysregulation of intestinal insulin signaling, possibly triggered by oxidative stress and inflammation, was associated with exaggerated lipogenesis and lipoprotein synthesis, which may represent a key mechanism for atherogenic dyslipidemia in patients with metabolic syndrome.