Acyl-CoA:diacylglycerol acyltransferase-1 (DGAT1) catalyzes the final step of 
triglyceride synthesis in mammalian cells. Data obtained from DGAT1-knockout 
mice have indicated that this enzyme plays an important role in energy 
homeostasis. We investigated the regulation of the expression and function of 
DGAT1 in mouse 3T3-L1 cell as a model for mammalian adipocytes. We demonstrated 
that the DGAT1 protein level increased by approximately 90-fold following 
differentiation of 3T3-L1 into mature adipocytes, a change that was accompanied 
by approximately 7-fold increase in DGAT1 mRNA. On the other hand, forced 
overexpression of DGAT1 mRNA by >20-fold via a recombinant adenovirus only 
resulted in approximately 2-fold increase in DGAT1 protein in mature adipocytes 
and little increase in preadipocytes. These results indicated that gene 
expression of DGAT1 in adipocytes is subjected to rigorous posttranscriptional 
regulation, which is modulated significantly by the differentiation status of 
3T3-L1 cells. Protein stability is not a significant factor in the control of 
DGAT1 expression. The steady-state levels of DGAT1 were unaffected by blockage 
of proteolytic pathways by ALLN. However, translational control was suggested by 
sequence analysis of the 5'-untranslated region of human DGAT1 (hDGAT1) mRNA. We 
found that the level of DGAT1 activity was predominantly a function of the 
steady-state level of DGAT1 protein. No significant functional changes were 
observed when the conserved tyrosine phosphorylation site in hDGAT1 was mutated 
by a single base pair substitution. Despite only a approximately 2-fold increase 
in DGAT1 protein caused by recombinant viral transduction, a proportionate 
increase in cellular triglyceride synthesis resulted without affecting the 
triglyceride lipolysis rate, leading to >2-fold increase in intracellular 
triglyceride accumulation. No change in adipocyte morphology or in the 
expression levels of lipoprotein lipase, proxisomal proliferation-activating 
receptor-gamma, and aP2 was evident secondary to DGAT1 overexpression at 
different stages in 3T3-L1 differentiation. These data suggest that 
dysregulation of DGAT1 may play a role in the development of obesity, and 
manipulation of the steady-state level of DGAT1 protein may offer a potential 
means to treat or prevent obesity.