Impact of Kupffer cells on high fat induced insulin resistance and liver fetuin-A expression.

Backgroun and aims: Hepatokines (liver secreted proteins with possible distant action) are emerging potential players in insulin resistance in type 2 diabetic patients. Here, we explore the effect of a high fat diet on the expression of fetuin-A, one of those candidate liver proteins, and its relation with liver macrophage (Kupffer cell) activation. Methods: Male mice of 5 weeks of age were fed a normal diet (ND) or a high fat diet (HFD) for 3 days, known to initiate steatosis and insulin resistance. A preventive Kupffer cell (KC) depletion was obtained by intravenous injection of clodronate loaded liposomes and compared with PBS liposomes. The mRNA and protein expression of fetuin-A was evaluated by RT-PCR, Western-blot and immunofluorescence (IF) on different insulin-sensitive tissues (liver, adipose tissue and muscle). Results: Short term HFD induced steatosis, KC activation and insulin resistance together with a significant increased expression of liver fetuin-A mRNA (1.5 fold, p<0.01). However, liver fetuin-A protein expression remained unchanged under short term HFD. This increase in fetuin-A under high fat diet was not evidenced in the peripheral insulin sensitive tissues (skeletal muscle and adipose tissue) whether at the mRNA or at the protein level. Kupffer cell depletion in this setting did not reduce hepatic steatosis but significantly ameliorated insulin sensitivity proved by clamp studies. This amelioration in insulin sensitivity in KC-depleted mice was associated with a significant decrease in fetuin-A mRNA expression (0.7 fold, p<0.01) compared to animals with KC. On immunofluorescence, fetuin-A was mostly expressed in centrilobular hepatocytes. Interestingly, while selectively depleting liver macrophages without affecting adipose tissue macrophage infiltration, intravenous clodronate injection was associated with a significant reduction in epididymal adipose tissue expansion compared to PBS injection (1.1% of body weight versus 1.6% of body weight, p