Coordinate regulation of triacylglycerol synthesis and glucose transport by acylation-stimulating protein

Abstract

Acylation-stimulating protein (ASP) is the most potent recognized stimulant of triacylglycerol synthesis in human adipocytes. However, its mechanism(s) of action have not yet been elucidated in detail. The present study examines the effects of ASP on membrane transport of glucose and fatty acids in cultured human skin fibroblasts. The data demonstrate that ASP stimulated carrier-mediated glucose transport in a time- and concentration-dependent manner, an effect that was greater than and independent of that observed with insulin. ASP increased the V max for glucose transport with no change in the transport K m , suggesting that ASP might result in increased sugar transporters in the plasma membrane. This finding was supported by quantitative Western blot analyses using a monoclonal antibody (G3) that demonstrated an increase in the plasma membrane content of the glucose transporter (Glut 1) with a concomitant decrease in the glucose transporter content of an intracellular membrane fraction. By contrast, ASP had no effect on specific membrane transport of fatty acids. Maximal effects of ASP on triacylglycerol synthesis were demonstrated at saturating levels of both glucose and oleate. Comparable stimulation by ASP in the absence of glucose (with or without pyruvate) was also demonstrated, although the absolute rates of triacylglycerol synthesis were substantially lower. Finally, it was shown that ASP increased the apparent V max for triglyceride synthesis without changing its K m . Since ASP will act in the absence of extracellular glucose, ASP must have additional actions, independent of its effect on specific membrane transport of glucose by which it accelerates intracellular triglyceride synthesis. This conclusion is consistent with our previous observations that ASP increases the activity of the last and rate-limiting step in triglyceride synthesis, diglycerol acyl transferase. It appears therefore that the effect of ASP on a complex biosynthetic pathway is achieved by virtue of a multifaceted interaction with the cell.