Hepatic insulin resistance, metabolic syndrome and cardiovascular disease.

BACKGROUND: The metabolic syndrome is a constellation of common metabolic disorders that is associated with cardiovascular disease. Insulin resistance has a central role in the pathophysiology of metabolic syndrome. RECENT ADVANCES: It is now commonly accepted that chronic inflammation associated with visceral obesity induces insulin resistance in the liver. Chronic inflammation is characterized by the production of abnormal adipokines and cytokines such as TNF-alpha, FFA, IL-1, IL-6, leptin and resistin. These factors inhibit insulin signalling in hepatocytes by activating SOCS proteins, several kinases such as JNK, IKK-beta and PKC and protein tyrosine phosphatases such as PTP1B and PTEN, that in turn impair insulin signalling at insulin receptor and insulin receptor substrate (IRS) level. Hepatic insulin resistance in turn causes impaired suppression of glucose production by insulin in hepatocytes leading to hyperglycemia. An important and early complication of hepatic insulin resistance is the induction of hepatic VLDL production, via changes in the rate of apoB synthesis and degradation and de novo lipogenesis, or increased FFA flux from adipose tissue into the liver. Insulin resistance also stimulates the production of CRP and PAI-1, both markers of an inflammatory state. All metabolic abnormalities related to hepatic insulin resistance have been shown to directly or indirectly promote atherosclerosis. Hyperglycemia induces a series of alterations including endothelial dysfunction, cellular proliferation, changes in extracellular matrix conformation and impairment of LDL receptor-mediated uptake decreasing the in vivo clearance of LDL. Small dense LDLs associated with high circulating VLDL have higher affinity to the intimal proteoglycans leading to the penetration of more LDL particles into the arterial wall. CRP can also accelerate atherosclerosis by increasing the expression of PAI-1 and adhesion molecules in endothelial cells, inhibition of nitric oxide formation and increasing LDL uptake into macrophages.
CONCLUSIONS: Overall, growing evidence suggests that hepatic insulin resistance is sufficient to induce several components of the metabolic syndrome and promote progression to cardiovascular disease. Many unresolved questions remain however on the molecular and cellular mechanisms that trigger hepatic insulin resistance and promote the development of clinical metabolic syndrome.