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The synthesis of triglycerides is catalyzed by two known acyl-CoA:diacylglycerol acyltransferase (DGAT) enzymes. Although they catalyze the same biochemical reaction, these enzymes share no sequence homology, and their relative functions are poorly understood. Gene knockout studies in mice have revealed that DGAT1 contributes to triglyceride synthesis in tissues and plays an important role in regulating energy metabolism but is not essential for life. Here we show that DGAT2 plays a fundamental role in mammalian triglyceride synthesis and is required for survival. DGAT2-deficient (Dgat2 ؊/؊ ) mice are lipopenic and die soon after birth, apparently from profound reductions in substrates for energy metabolism and from impaired permeability barrier function in the skin. DGAT1 was unable to compensate for the absence of DGAT2, supporting the hypothesis that the two enzymes play fundamentally different roles in mammalian triglyceride metabolism.Triglycerides (triacylglycerols) are the major storage form of energy in eukaryotic organisms. However, excessive deposition of triglycerides in white adipose tissue (WAT) 1 leads to obesity and in non-adipose tissues (such as pancreatic ␤ cells, skeletal muscle, and liver) is associated with tissue dysfunction referred to as lipotoxicity (1, 2). Therefore, an understanding of the processes that mediate triglyceride synthesis is of significant biomedical importance.Triglycerides are synthesized from diacylglycerol and activated forms of fatty acids (fatty acyl-CoAs) in a reaction catalyzed by acyl-CoA:diacylglycerol acyltransferase (DGAT) enzymes (3-5). The genes for two DGAT enzymes, DGAT1 and DGAT2, have been identified (6, 7). Both DGAT1 and DGAT2 are ubiquitously expressed, with the highest levels of expression found in tissues that are active in triglyceride synthesis, such as WAT, small intestine, liver, and mammary gland (6, 7). Both enzymes are intrinsic membrane proteins, although DGAT1 has 6 -12 putative transmembrane domains, whereas DGAT2 has one. Both also have similarly broad fatty acyl-CoA substrate specificities in in vitro assays (7). However, despite their ability to catalyze similar reactions, DGAT1 and DGAT2 belong to different gene families that share neither DNA nor protein sequence similarity. DGAT1 is homologous to the acylCoA:cholesterol acyltransferase enzymes, ACAT1 and ACAT2, which are involved in cholesterol ester biosynthesis (6), whereas DGAT2 shares homology with acyl-CoA:monoacylglycerol acyltransferase enzymes (8 -13). This raises the question of why two different types of DGAT enzymes have emerged from convergent evolution.Insights into the functions of DGAT1 and DGAT2 in triglyceride metabolism have been provided by studies in yeast. Through deletion and overexpression studies, several groups have demonstrated that DGA1, the yeast homologue of DGAT2, is the major DGAT enzyme contributing to triglyceride synthesis and storage in yeast (14 -16). In contrast, ARE2, a yeast homologue of DGAT1, plays a minor role in triglyceride synthesis. Intere...

Both exposure of stratum corneum to neutral pH buffers and blockade of acidification mechanisms disturb cutaneous permeability barrier homeostasis and stratum corneum integrity/cohesion, but these approaches all introduce potentially confounding variables. To study the consequences of stratum corneum neutralization, independent of hydration, we applied two chemically unrelated superbases, 1,1,3,3-tetramethylguanidine or 1,8-diazabicyclo [5,4,0] undec-7-ene, in propylene glycol:ethanol (7:3) to hairless mouse skin and assessed whether discrete pH changes alone regulate cutaneous permeability barrier function and stratum corneum integrity/cohesion, as well as the responsible mechanisms. Both 1,1,3,3-tetramethylguanidine and 1,8-diazabicyclo [5,4,0] undec-7-ene applications increased skin surface pH in parallel with abnormalities in both barrier homeostasis and stratum corneum integrity/cohesion. The latter was attributable to rapid activation (<20 min) of serine proteases, assessed by in situ zymography, followed by serine-protease-mediated degradation of corneodesmosomes. Western blotting revealed degradation of desmoglein 1, a key corneodesmosome structural protein, in parallel with loss of corneodesmosomes. Coapplication of serine protease inhibitors with the superbase normalized stratum corneum integrity/cohesion. The superbases also delayed permeability barrier recovery, attributable to decreased beta-glucocerebrosidase activity, assessed zymographically, resulting in a lipid-processing defect on electron microscopy. These studies demonstrate unequivocally that stratum corneum neutralization alone provokes stratum corneum functional abnormalities, including aberrant permeability barrier homeostasis and decreased stratum corneum integrity/cohesion, as well as the mechanisms responsible for these abnormalities.