Liver-secreted RBP4 does not impair glucose homeostasis in mice

Ronja Fedders,Matthias Muenzner,Pamela Weber,Manuela Sommerfeld,Miriam Knauer,Sarah Kedziora,Naomi Kast,Steffi Heidenreich,Jens Raila,Stefan Weger,Andrea Henze,Michael Schupp

doi:10.1074/jbc.ra118.004294

Abstract

Retinol-binding protein 4 (RBP4) is the major transport protein for retinol in blood. Recent evidence from genetic mouse models shows that circulating RBP4 derives exclusively from hepatocytes. Because RBP4 is elevated in obesity and associates with the development of glucose intolerance and insulin resistance, we tested whether a liver-specific overexpression of RBP4 in mice impairs glucose homeostasis. We used adeno-associated viruses (AAV) that contain a highly liver-specific promoter to drive expression of murine RBP4 in livers of adult mice. The resulting increase in serum RBP4 levels in these mice was comparable with elevated levels that were reported in obesity. Surprisingly, we found that increasing circulating RBP4 had no effect on glucose homeostasis. Also during a high-fat diet challenge, elevated levels of RBP4 in the circulation failed to aggravate the worsening of systemic parameters of glucose and energy homeostasis. These findings show that liver-secreted RBP4 does not impair glucose homeostasis. We conclude that a modest increase of its circulating levels in mice, as observed in the obese, insulin-resistant state, is unlikely to be a causative factor for impaired glucose homeostasis.

Highlights

Retinol-binding protein 4 (RBP4) is the major transport protein for retinol in blood
We found that injecting 8E10 genomic copies of GFP or RBP4 expressing associated viruses (AAV) into the tail vein of mice led to a ϳ3-fold increase in serum RBP4, when analyzed 2 weeks later (Fig. 1D)
We found that these mice failed to develop signs of glucose intolerance and insulin resistance when compared with their controls

Summary

Results

We generated a model of liver-specific RBP4 overexpression by a nongenomic intervention in adult mice, excluding developmental compensations and potential unspecific effects due to genomic reorganization. Circulating RBP4 levels correlated well with serum retinol, Liver-specific RBP4 overexpression does not impair glucose homeostasis in NC-fed mice. Liver-specific overexpression of RBP4 failed to affect glucose tolerance and serum insulin levels 15 min after the i.p. injection of glucose (0.5 g/kg) in these HFD-fed mice (Fig. 4, F and G). Food intake did not differ between both groups (Fig. 5D) These findings suggest that liver-RBP4 overexpression and a ϳ2-fold increase in circulating RBP4 levels do not affect energy metabolism in mice. Expression was previously shown to be increased in RBP4 injected mice [4] Both gluconeogenic genes were unchanged in mice with liver-specific RBP4 overexpression (Fig. 6A).

Discussion

Experimental procedures

Mice and tail vein injection of AAV

Protein isolation and immunoblotting

Quantification of liver and serum retinoid levels