Fig. 5

Lipid synthesis pathway in porcine mammary gland. Note: LCFA the enter porcine mammary gland, facilitated by transport proteins (mainly CD36), and are converted into their activated form LC-acyl-CoA, with the help of ACSL. Cytosolic LC-acyl-CoA is transported to the endoplasmic reticulum membrane by FABP3 and esterified there to glycerol-3-phosphate to produce LPA by GPAM. In the endoplasmic reticulum, PA can be hydrolyzed with LPIN to form DAG, which then is acylated to form TAG by DGAT. Newly-formed TAG forms cytoplasmic lipid droplets in the ER membrane via incorporation. The cytoplasmic lipid droplets are then transported to the apical membrane, and eventually released into porcine milk. A series of enzymes are required to facilitate this process, of which FASN and ACACA are considered the crucial enzymes of cellular fatty acid de novo synthesis. ACACA carboxylates acetyl-CoA to form malonyl-CoA, which is further converted by FASN to fatty acids (C ≤ 16). The synthesized fatty acids then participate in TAG formation. After the initation of lactation, mRNA of enzymes is increased 5-fold or more are shown in light purple; those increasing 2-fold or more are colored light red, and those expression increase or decrease less than 2-fold are shown in light gray. Abbreviations: ACACA, acetyl-CoA carboxylase alpha; ACSL3, acyl-CoA synthetase long-chain family member 3; ACSS2, acyl-CoA synthetase short-chain family member 2; AGPAT6, 1-acyl-sn-glycerol-3-phosphate acyltransferase 6; CD36, fatty acid translocase/CD36; CLD, cytoplasmic lipid droplet; DAG, diacylglycerol; DGAT1, diacylglycerol acyltransferase 1; FABP3, fatty acid binding protein 3; FASN, fatty acid synthase; GPAM, glycerol-3-phosphate acyltransferase, mitochondrial; LCFA, long chain fatty acid; LPA, lysophosphatidic acid; LPIN2, lipin 2; MFG, milk fat globule; PA, phosphatidic acid; PLIN2, perilipin 2; PPARγ, peroxisome proliferator-activated receptor γ; SCFA, short chain fatty acid; SCD, stearoyl-CoA desaturase; TAG, triacylglycerol