Evidence for a Novel Key Regulatory Step in the Triglyceride Synthetic Pathway of Human Adipose Tissue

The triglyceride (TG) synthetic pathway is believed to be regulated at the level of the rate-limiting enzyme acyl-CoA:1,2-diacylglycerol O-acyltransferase (DGAT). Recent reports using rat hepatic tissue suggest a kinase-dependent mechanism for the regulation of DGAT activity. To examine this process further, the present study investigates the regulatory mechanisms involved in the modulation of DGAT in human adipocytes. Adipocytes were fractionated into a microsomal fraction containing DGAT and a cytosolic fraction containing a putative regulatory kinase. DGAT activity was determined bymeasuring the incorporation of 14C-oleoyl-CoA into TG with exogenously supplied 1,2-dioleoyl-sn-glycerol. Kinase activity was assayed by addition of the cytosolic fraction in the presence of Mg2+ and ATP. The results indicate a significant inhibition of human adipose tissue DGAT activity by as much as 43% (avg: 17.5% ± 10.4%, p < 0.01) via a mechanism consistent with a phosphorylation event. Partial purification of the putative cytosolic kinase was achieved by multidimensional chromatography. This study thus provides evidence for a novel and key regulatory step in the human TG biosynthetic pathway. Further research is necessary to determine whether the model outlined here is a physiologic conduit through which extracellular hormones exert a regulatory influence on TG synthesis.