Fat metabolism in liver of female yaks by contrast with male yaks
Liver is one of main site in which fat is synthesized. FAS catalyzes the formation of long-chain fatty acids from acetyl-CoA and malonyl-CoA [22, 23]; ACC catalyzes the carboxylation of acetyl-CoA to malonyl-CoA, the first and rate-limiting step of de novo fatty acid biosynthesis [24]; SCD catalyzes the insertion of a double bond at the delta-9 position into fatty acyl-CoA substrates including palmitoyl-CoA and stearoyl-CoA [25]; DGAT-1 catalyze the last and the only committed step of triglyceride biosynthesis [26]. All of above enzymes are the promoters of fat synthesis, and their levels in livers of female yaks were higher than male yaks (p < 0.01). Therefore, the fat synthesis in livers of female yaks increased. CPT1 catalyzes the transfer of long- and medium-chain fatty acids from cytoplasm into mitochondria and is the rate-limiting enzyme in fatty acid oxidation [27], and its level in livers of female yaks was lower than male yaks (p < 0.05). This result showed the fatty acid oxidation in livers of female yaks decreased. HSL catalyzes the release of fatty acids from storage triglycerides in adipocytes, liver and muscle; LPL is a key enzyme in adipolysis, and catalyzes the hydrolysis of triglycerides from circulating chylomicrons and VLDL [28]; ATGL is the rate-limiting enzyme in triacylglycerol hydrolysis. All of above enzymes are the promoter of lipolysis, and there were on differences in their levels in livers of female and male yaks. Therefore, the lipolysis in liver of female yaks was similar with male yaks.
Fat transport in blood of female yaks by contrast with male yaks
Livestock in a normal physiological state can not store fat in liver, and synthetic fat in liver was transferred into blood. CH level in serum of female yaks was higher than male yaks (p < 0.01), and CH synthesis in female yaks increased. VLDL transports endogenous products [29], and its level in serum of female yaks was higher than male yaks (p < 0.05). Meanwhile, HDL and LDL levels in serum of female yaks were also higher than male yaks (p < 0.01). These results were corresponding with the results of enzyme activity in liver of yaks and TG level. IGF-1 has much higher growth-promoting activity than insulin, and lowers total triglyceride, low density lipoprotein triglycerides and LDL cholesterol levels in blood [30], and its level in serum of female yaks was lower than male yaks (p < 0.01); COR promotes the fat synthesis and fat deposition in vivo [31], and its level in female yaks was higher than male yaks (p < 0.01). IGF-1 and COR levels in serum of female yaks were verified by the high levels of TG, VLD, HDL and LDL. These results indicated more fat was transferred in blood of female yaks.
Cholesterol metabolisms in female yaks by contrast with male yaks
CH strongly associates with the fat metabolism especially fat transport in livestock, and many steroid hormones effecting the fat metabolism, such as cortisol, aldosterone, and testosterone, are derived from CH. Most of CH is synthesized in liver, whereas a small part is synthesized in adipocyte, and CH is the ingredient of LDL, VLDL and HDL [32]. The LDL, VLDL and HDL levels in serum of female yaks were higher than male yaks (p < 0.05), and CH level in serum of female yaks was higher than male yaks (p < 0.05) too. Therefore, the cholesterol synthesis in liver of female yak increased.
Prenylcysteine oxidase 1 (PCYOX1) hydrolyzes the thioether bond of prenylcysteines in the final step in the degradation of prenylated proteins [33]; acyl-coenzyme a-cholesterol acyltransferase (ACAT) catalyzes cholesterol and long chain fatty acyl-CoA to produce cholesteryl ester [34]; protein farnesyltransferase subunit beta (FNTB) catalyzes the transfer of farnesyl moiety from farnesyl diphosphate to a cysteine [35]. They involved in terpenoid backbone biosynthesis and the expression levels of their genes all upregulated in subcutaneous fat of female yaks (pFDR < 0.05). Meanwhile, 2-trans, 6-trans-farnesal levels in the pathsway of terpenoid backbone biosynthesis increased in subcutaneous fat of female yaks. Therefore, it can be inferred that terpenoid backbone biosynthesis in subcutaneous fat of female yaks increased. The terpenoid backbone biosynthesis is one of links in the cholesterol and steroid synthesis.
Apolipoprotein E (ApoE) transports cholesterol, and its gene expression downregulated in subcutaneous fat of female yak (pFDR < 0.05), so less cholesterol was transfered out of fat cell in female yaks. The low-density lipoprotein receptor (LDLR) is important for the clearance of VLDL, intermediate density lipoprotein (IDL) and LDL, and LDLR gene expression upregulated in subcutaneous fat of female yaks (pFDR < 0.05); cholesteryl ester hydrolase (LIPA) catalyzes the deacylation of cholesteryl ester core lipids of LDL to generate cholesterol [36], and LIPA gene expression upregulated in subcutaneous fat of female yaks (pFDR < 0.05); LPL positively regulates cholesterol storage, and LPL gene expression upregulated in subcutaneous fat of female yaks ( pFDR <0.05); steroidogenic acute regulatory protein (STAR) controls the rate-limiting step in steroidogenesis, and STAR gene expression upregulated in subcutaneous fat of female yaks (pFDR< 0.05). Meanwhile, glycocholic acid (VIP4.38) involved in the emulsification of fats, and chenodeoxycholic acid glycine conjugate (VIP 1.17) is a bile acid-glycine conugate. They can be generated by cholesterol metabolism, and their levels increased in subcutaneous fat of female yak (p < 0.05). Therefore, the cholesterol synthesis and storage increased in the subcutaneous fat of female yaks.
Steroid metebolisms in female yaks by contrast with male yaks
Delta (24)-sterol reductase (DHCR24) catalyzes sterol ring modifications [37]; squalene monooxygenase (SQLE) is a vital sterol synthesis enzyme across eukaryotic life [38]. It can be found that DEGs LIPA, DHCR24 and SQLE gene expressions in steroid biosynthesis all upregulated in subcutaneous fat of female yaks (pFDR < 0.05). Meanwhile, 25-hydroxyvitamin D3 (VIP3.53) in steroid biosynthesis decreased in subcutaneous fat of female yaks (p < 0.01); 7-dehydrodesmosterol (VIP2.20) is a substrate of 24-dehydrocholesterol reductase which is an important enzyme in the steroid biosynthesis, and its level increased in subcutaneous fat of female yaks (p < 0.01). The existing researches indicate that androgens can be translated into estrogen in fat cell, and both androgens and estrogen belong to steroid. Therefore, it can be inferred that steroid biosynthesis was promoted in subcutaneous fat of of female yak.
Fat synthesis in subcutaneous fat of female yaks by contrast with male yaks
The fat can be synthesized in adipose tissue too, and not only synthetic fat in vivo but also fat from food only deposite in adipose tissue finally. The fat is mainly component of triglyceride, and triglyceride synthesis includes pathways of monoglyceride and 1,2-diglyceride-3-pho-s-3-phate. 1-acyl-sn-glycerol-3-phosphate acyltransferase beta (AGPAT2) is the key catalyst in two pathways; DGAT2 and glycerol-3-phosphateac-yltransferase (GPAT) are two key enzymes promoting the triglyceride synthesis in the pathway of 1,2-diglyceride-3-phosphate [39]. AGPAT2, DGAT2 and GPAT gene expressions upregulated in subcutaneous fat of female yak ( pFDR < 0.01). Meanwhile, glutamate (VIP1.68), malic acid (VIP1.45), citric acid (VIP1.99) and D-2,3-dihydroxypropanoic acid (VIP1.62) all increased in subcutaneous fat of female yaks ( p < 0.05), and these metabolisms involved in the carbon metabolism. D-2,3-dihydroxypropanoic acid can translate into glycerate-3P which can be used to synthesize triglyceride; malic and citric acid are the intermediates of TCA cycle, and can translate into the acetyl-CoA which is initiator in the process of fat acid biosynthesis. Therefore, it was inferred that the triglyceride synthesis in subcutaneous fat of female yak increased.
Lipolysis and fatty acid oxidation in subcutaneous fat of female yaks by contrast with male yaks
ATGL being the rate-limited enzymes in fat catabolism, and its gene PNPLA1 expression downregulated in subcutaneous fat of female yak (pFDR < 0.05). Therefore, fat catabolism in subcutaneous fat of female yak decrease. Protocadherin fat 2 (FAT2) transfers fatty acis into the adipocyte, and FAT2 gene expression upregulated in subcutaneous fat of female yak (pFDR < 0.01), so the fat acid transport was promoted in subcutaneous fat of female yaks. CPT1C gene expression downregulated in subcutaneous fat of female yak (pFDR < 0.01), so the fatty acids oxidation decreased in subcutaneous fat of female yaks.
Fatty acid synthesis in subcutaneous fat of female yaks by contrast with male yaks
Estradiol 17-beta-dehydrogenase 8 (HSD17B8) [40], trifunctional enzyme subunit beta (HADHB) [41], hydroxyacyl-coenzyme A dehydrogenase (HADH) [42], elongation of very long chain fatty acids protein 6 (ELOVL6) [43], very-long-chain (3R)-3-hydroxyacyl-CoA dehydratase 2 (HACD2) [44] and cytosolic acyl coenzyme A thioester hydrolase (ACOT7) [45] were the positive regulation of fatty acid elongation. Their gene HADHB, HADH, ELOVL6, HSD17B8, HACD2 and ACOT7 expressions all upregulated in subcutaneous fat of female yak (pFDR < 0.05). Therefore, the fatty acid synthesis increased in subcutaneous fat of female yak.
Polyunsaturated fatty acids (PUFAs) in livestock can not directly be synthesized in vivo, and must be derived from the precursor compounds, like linoleic acid and linolenic acid from grass or feed [46]. The ω-6 PUFAs like arachidonic acid are synthesized by linoleic acid and ω-3 PUFAs like DHA and EPA are synthesized by linolenic acid [47]. SCD, ACOT7, HACD2, non-specific lipid-transfer protein (SCP2) [48], ELOVL6 [49] and acetyl coenzyme-A acyl-transferase-1 (ACAA1) [50] are all the promoter of unsaturated fatty acid biosynthesis, and their gene expression all upregulated in the subcutaneous fat of female yaks (pFDR<0.05). Meanwhile, α-linolenic acid (VIP3.10), γ-linolenic acid (VIP4.83), EPA (VIP1.63) and DPA (VIP1.43) in unsaturated fatty acids biosynthesis increased in subcutaneous fat of female yak (p < 0.05). γ-linolenic acid is can convert into dihomo-gamma-linolenic acid which is a biosynthetic precursor of monoenoic prostaglandins such as PGE1; α-linolenic acid is an important structural components of cell membranes; EPA and DPA serve as the precursor for the prostaglandin-3 and thromboxane-3 families. Therefore, it was inferred that the unsaturated fatty acids biosynthesis especially EPA, γ-linolenic acid and DPA increased in subcutaneous fat of female yaks.
Five DEMs including PC(15:0/14:0) (VIP1.12), γ-linolenic acid, (9R,10S,12Z)-9, 10-dihydroxy-12-octadecenoic acid (VIP1.53), 13(S)-HODE (VIP1.19) and 9(S)-HpODE (VIP1.21) involved in linoleic acid metabolism, and all increased in subcutaneous fat of female yaks (p < 0.05). γ-linolenic acid is an intermediate in biosynthesis of ω-6 PUFAs like arachidonic acid. Meanwhile, PC(15:0/14:0), thromboxane (VIP2.04) and (+/-)14,15-DiHETrE (VIP1.15) involved in arachidonic acid metabolis, and PC(15:0/14:0) and (+/-) 14,15-DiHETrE increased in subcutaneous fat of female yaks, wheres thromboxane decreased. PC(15:0/14:0) and α-linolenic acid involved in α-linolenic acid metabolism, and increased in subcutaneous fat of female yaks (p < 0.05). Stearidonic acid, conelenic acid, heptadecatrienoic acid and etherolenic acid are all downstream products in alpha-Linolenic acid metabolism. Therefore, many active substances deriving from ω-3 and ω-6 PUFAs increased in subcutaneous fat of female yak.
Summary, as showed in Fig. 7, the fat synthesis and cholesterol synthesis in liver of female yaks increased, whereas the fatty acid oxygen in liver of female yaks decreased; the fat transport in blood of female yaks increased; the fatty acid synthesis, fat synthesis, cholesterol synthesis in subcutaneous fat of female yaks increased, whereas lipolysis in subcutaneous fat of female yaks decreased .
Secretion of adipocytokines in female yaks by contrast with male yaks
Metabolic hormones such as insulin, cortisol or thyroid hormones play an important role in fat metabolism [
51]. Calcium ion is one of the most abundant ions in livestock and many studies have shown calcium signaling plays a major role in increasing exocytosis secretion like hormones, autacoids and neuotransmitters. COR level in serum of female yaks was higher than male yaks (
p < 0.01); meanwhile, myo-inositol (VIP 3.87) increased in subcutaneous fat of female yaks (
p < 0.01). Above compounds can strengthen calcium signaling in fat cell of female yaks, and the secretion of adipocytokinein female yaks in fat cell was promoted by the calcium signaling.
LEP is a key player in the regulation of energy balance, and acts as an appetite-regulating factor [52], and has central and peripheral effects by binding leptin receptor (LEPR), and activate several major signaling pathways like PPARs signaling. In this study, LEP gene expression upregulated in subcutaneous fat of female yaks (pFDR < 0.05), and LEP level in serum of female yaks was also higher than male yaks (p < 0.01); adiponectin(ADPN) is another essential hormone secreted by adipocytes and regulates glucose and lipid metabolism. Adiponectin receptor 2 protein (ADIPOQ2) is the receptor for ADPN, and ADIPOQ2 gene expression upregulated in subcutaneous fat of female yaks (pFDR < 0.01). ADIPOQ-binding activates a signaling cascade that leads to increase PPAR activity, and ultimately to regulate fatty acid metabolism, increase glucose uptake and decrease gluconeogenesis in subcutaneous fat of female yaks.
Regulation of fat deposition in female yaks by the PPAR and SREBF1 signaling by contrast with male yaks (Fig. 8)
PPAR is essentially a class of transcription regulators relying on ligand, and can be activated by fatty acids especially unsaturated fatty acid and their derivatives [53]. Long-chain fatty acids are transferred through adipocyte plasma membrane by long-chain fatty acid transport protein 4 (SLC27A4), and SLC27A4 gene expression upregulated in subcutaneous fat of female yaks (pFDR < 0.05); meanwhile, ω-3 fatty acids can modulate the expression of many genes involved in fatty acid metabolism, by interacting with transcription factors PPARs [54]. 13(S)-HODE is a eicosanoid belong to ω-3 fatty acids, and increased in subcutaneous fat of female yaks (p < 0.01). Therefore, the subcutaneous fat in female yaks can obtain more long-chain fatty acids especially 13(S)-HODE from blood than male yaks, and these acids combined with PPARs in fat cell. On the other hand, LEP and ADIPOQ-binding also activated PPAR, and PPARD gene expression upregulated in female yaks ( FC = 1.43, pFDR < 0.01). Therefore, it can be inferred that the fat metabolism in yaks of different gender was mainly regulated by downstream genes in PPAR signaling, and downstream genes included the gene of NADP-dependent malic enzyme (ME1) [55], SCD, Acyl-CoA-binding protein (ACBP) [56], LPL, CPT1, SLC27A4, ACAA1 and perilipin-5 (PLIN5). ME1 and SCD promote fat synthesis, and ME1 and SCD gene expressions upregulated in the subcutaneous fat of female yaks (pFDR < 0.01); ACBP, LPL and SLC27A4 positively regulates the fatty acid transport, and therie genes DBI, LPL and SLC27A4 expression upregulated in the subcutaneous fat of female yaks (pFDR < 0.01); ACAA1 catalyzes the last step of the mitochondrial beta-oxidation pathway, and CPT1C and ACAA1 gene expression downregulated in the subcutaneous fat of female yaks (pFDR < 0.01); PLIN5 possesses positive regulation of lipid storage, and PLIN5 gene expression upregulated in the subcutaneous fat of female yaks (pFDR < 0.05). On the other hand, sterol regulatory element-binding protein 1 (SREBF1) is a transcriptional activator required for fat homeostasis, and regulates transcription of the lipogenic genes [57], and the higher levels of LEP can lead to the activation of the SREBF1 activity, and SREBF1 gene expression upregulated in subcutaneous fat of female yaks (pFDR < 0.01). SCD1 gene expression is regulated by SREBF1 [58, 59], and upregulated in subcutaneous fat of female yaks (pFDR < 0.05). In a word, the fatty acid synthesis, the fatty acid transport and fat synthesis increased by the regulation of PPAR and SREBF1 signal in subcutaneous fat of female yaks, whereas the fatty acid oxidation decreased, and they resulted in that the amount of fat deposition in female yaks was more than male yaks.