A selected population study reveals the biochemical mechanism of intramuscular fat deposition in chicken meat

BACKGROUND

Increasing intramuscular fat (IMF) is an important strategy to improve meat quality, but the regulation mechanism of IMF deposition needs to be systematically clarified.

RESULTS

A total of 520 chickens from a selected line with improved IMF content and a control line were used to investigate the biochemical mechanism of IMF deposition in chickens. The results showed that the increased IMF would improve the flavor and tenderness quality of chicken meat. IMF content was mainly determined both by measuring triglyceride (TG) and phospholipid (PLIP) in muscle tissue, but only TG content was found to be decisive for IMF deposition. Furthermore, the increase in major fatty acid (FA) components in IMF is mainly derived from TGs (including C16:0, C16:1, C18:1n9c, and C18:2n6c, etc.), and the inhibition of certain very-long-chain FAs would help to IMF/TG deposition.

CONCLUSIONS

Our study elucidated the underlying biochemical mechanism of IMF deposition in chicken: Prevalent accumulation of long-chain FAs and inhibitions of medium-chain FAs and very long chain FA would jointly result in the increase of TGs with the FA biosynthesis and cellular uptake ways. Our findings will guide the production of high-quality chicken meat.

Butyric acid induces spontaneous adipocytic differentiation of porcine bone marrow-derived mesenchymal stem cells

Butyric acid (BA) affects the differentiation of mesenchymal stem cells (MSC) through the activation of different transcriptional pathways. The aim of this study was to determine the effects of BA on proliferation and spontaneous differentiation of porcine bone marrow-derived MSC. Second passage MSC (n = 6) were cultured in either a basal medium (BM, DMEM + 10% FBS), or BM + 2.5 mmol/L BA (BA-2.5) or BM + 5 mmol/L BA (BA-5). Cell proliferation was significantly decreased by both BA-2.5 and BA-5 after 48 h and 72 h (- 55% and - 63%, respectively). To assess the impact of BA on spontaneous differentiation, MSC were cultured for 27 d, with complete media changes every 3 d. At day 27, cells were stained for osteocytic, chondrocytic, and adipocytic differentiation. No terminal differentiation was detected in control MSC, while accumulated small drops of lipids were stained by Oil-Red-O in BA-treated cells. The phenotypic changes were associated with changes in gene expression, determined by qPCR. Treatment with BA modulated the expression of adipocytic differentiation markers: peroxisome proliferator-activated receptor γ and CCAAT/enhancer binding protein α were significantly increased by both BA-2.5 and BA-5 throughout the study, while lipoprotein lipase and fatty acid-binding protein 4 were increased by BA-5 at day 3, and decreased by both BA-5 and BA-2.5 later throughout the study. Osteocalcin and aggrecan mRNA was reduced throughout the experiment by both doses of BA (P < 0.05). In conclusion, our data support that BA promotes the spontaneous differentiation of porcine bone marrow-derived MSC toward an adipocytic lineage in the absence of inducing cocktail media.

Method using a co-culture system with high-purity intramuscular preadipocytes and satellite cells from chicken pectoralis major muscle

Intramuscular fat is important in improving meat quality; however, the lack of high-purity intramuscular preadipocyte (IMP) in vitro has severely limited the in-depth research on the mutual regulation of myocytes and adipocytes in chicken. In this study, we establish a new method by combining the mature adipocyte ceiling method and the transwell co-culture system. Mature intramuscular adipocyte (MIA) and muscle satellite cell (MSC) were obtained from digested pectoralis major, and MIAs were transformed into IMPs by dedifferentiation with ceiling culture. MSCs were then purified by differential adhesion for 2 h. The results by inverted-microscope observation, MTT assay, Oil Red O staining, and q-PCR revealed that the de-differentiated cells from MIA were identified as the IMPs, and had the same the cellular morphology, the capacity on differentiation, proliferation and passage with the abdominal fat preadipocytes (P > 0.05). The applicability of the obtained IMPs in co-cultured experiment with the MSC revealed that it could meet the requirements of the experimental study. Finally, a co-culture system of IMPs and MSCs was established using a transwell chamber. The co-cultured results indicated that MSCs in the proliferative stage tend to accelerate the differentiation of IMPs to induce more fat content in co-cultured IMPs than in single-culture IMPs (P < 0.05), in the non-proliferative stage, the results tend to show the opposite (P < 0.05). The mRNA levels of related genes significantly changed in accordance with the fat content in cells. The results strongly supported the view that the established co-culture system was effective and feasible. In summary, we successfully found a new method to explore the interaction between myocytes and adipocytes of chicken. Our findings can deepen research on the regulation of chicken myocytes and adipocytes.

Knockdown of ubiquitin D inhibits adipogenesis during the differentiation of porcine intramuscular and subcutaneous preadipocytes

OBJECTIVES

Intramuscular fat (IMF) has a significant influence on porcine meat quality. Ubiquitin D (UBD) is involved in the management of diverse intracellular processes. However, its physiological functions in adipose cell differentiation and proliferation are still poorly defined.

MATERIALS AND METHODS

Intramuscular and subcutaneous preadipocytes were isolated from the longissimus dorsi and neck subcutaneous deposits of Chinese native Guanzhong Black piglets (3-5 days old), respectively. Lentivirus with short hairpin RNA (shRNA) for UBD was applied to knockdown UBD expression. We used real-time PCR and Western blot analysis to detect gene expression. Lipid droplets were dyed with Oil Red O, and cell proliferation was assessed using flow cytometry, 5-ethynyl-2'-deoxyuridine incorporation and cell counting assays.

RESULTS

Lipogenesis through the Akt/mTOR pathway was inhibited when preadipocytes were transfected with UBD shRNA. The expression of adipogenic genes and the number of lipid droplets were obviously diminished. Moreover, repression of UBD attenuated cell proliferation. UBD downregulation resulted in cell cycle arrest because of a decreased proportion of S-phase cells, and the expression of positive cell proliferation markers was significantly decreased.

CONCLUSION

These observations illustrated that knockdown of UBD partially suppressed porcine intramuscular and subcutaneous preadipocyte adipogenesis through the Akt/mTOR signalling and inhibited cell proliferation, suggesting the essential role of UBD in the differentiation of preadipocytes.

Genetic variants in IL15 promoter affect transcription activity and intramuscular fat deposition in longissimus dorsi muscle of pigs

Intramuscular fat (IMF) content is a key aspect of pork quality. Elucidation of intramuscular adipocyte regulation mechanisms is important for improving IMF content. Intramuscular adipocytes are dispersed among muscle fibers, so they are inclined to be affected by muscle-derived factors. Interleukin-15 is a major muscle-secreted factor. In this study, the genetic and physiological impacts of IL15 on adipogenesis is investigated. The promoter region of IL15 was scanned by comparative sequencing using two DNA pools of high- and low-IMF individuals. Two SNPs, c.-342C>T (ss2137497757) and c.-334G>A (ss2137497756) (the translation start site is designated as +1), were identified with reverse allele distribution in these two groups. Genotyping by allele-specific PCR revealed that the two SNPs were completely linked. The IMF content of TA/TA individuals was lower than that for CG/CG ones, whereas the IL15 expression level was higher in T-A/T-A individuals. Luciferase assaying also revealed that the T-A haplotype promoter had higher transcription activity. Meanwhile, the effect of interleukin-15 on adipocyte differentiation was further assessed in vitro. Results showed that interleukin-15 suppressed preadipocyte proliferation in a dose-dependent manner. The cell cycle of preadipocytes was arrested, and apoptosis was induced. Oil Red O staining and triglyceride quantification indicated that adipocyte differentiation was also inhibited by interleukin-15. The mRNA levels of PPARG and FABP4 decreased markably upon interleukin-15 treatment. Taken together, we identified two completely linked SNPs in the porcine IL15 promoter region that could alter IL15 transcription activity. As interleukin-15 can inhibit porcine adipocyte differentiation, these promoter mutations could affect IMF deposition by producing differential levels of muscle-derived interleukin-15.

Muscle-specific downregulation of GR levels inhibits adipogenesis in porcine intramuscular adipocyte tissue

Intramuscular adipose is conducive to good pork quality, whereas subcutaneous adipose is considered as waste in pig production. So uncovering the regulation differences between these two adiposes is helpful to tissue-specific control of fat deposition. In this study, we found the sensitivity to glucocorticoids (GCs) was lower in intramuscular adipocytes (IMA) compared with subcutaneous adipocytes (SA). Comparison of glucocorticoid receptor (GR) revealed that IMA had lower GR level which contributed to its reduced GCs sensitivity. Higher methylation levels of GR promotor 1-C and 1-H were detected in IMA compared with SA. GR expression decrease was also found in adipocytes when treated with muscle conditioned medium (MCM) in vitro, which resulted in significant inhibition of adipocytes proliferation and differentiation. Since abundant myostatin (MSTN) was detected in MCM by ELISA assay, we further investigated the effect of this myokine on adipocytes. MSTN treatment suppressed adipocytes GR expression, cell proliferation and differentiation, which mimicked the effects of MCM. The methylation levels of GR promotor 1-C and 1-H were also elevated after MSTN treatment. Our study reveals the role of GR in muscle fiber inhibition on intramuscular adipocytes, and identifies myostatin as a muscle-derived modulator for adipose GR level.

Irisin exerts dual effects on browning and adipogenesis of human white adipocytes

To better understand the role of irisin in humans, we examined the effects of irisin in human primary adipocytes and fresh human subcutaneous white adipose tissue (scWAT). Human primary adipocytes derived from 28 female donors' fresh scWAT were used to examine the effects of irisin on browning and mitochondrial respiration, and preadipocytes were used to examine the effects of irisin on adipogenesis and osteogenesis. Cultured fragments of scWAT and perirenal brown fat were used for investigating signal transduction pathways that mediate irisin's browning effect by Western blotting to detect phosphorylated forms of p38, ERK, and STAT3 as well as uncoupling protein 1 (UCP1). Individual responses to irisin in scWAT were correlated with basal expression levels of brown/beige genes. Irisin upregulated the expression of browning-associated genes and UCP1 protein in both cultured primary mature adipocytes and fresh adipose tissues. It also significantly increased thermogenesis at 5 nmol/l by elevating cellular energy metabolism (OCR and ECAR). Treating human scWAT with irisin increased UCP1 expression by activating the ERK and p38 MAPK signaling. Blocking either pathway with specific inhibitors abolished irisin-induced UCP1 upregulation. However, our results showed that UCP1 in human perirenal adipose tissue was insensitive to irisin. Basal levels of brown/beige and FNDC5 genes correlated positively with the browning response of scWAT to irisin. In addition, irisin significantly inhibited adipogenic differentiation but promoted osteogenic differentiation. We conclude that irisin promotes "browning" of mature white adipocytes by increasing cellular thermogenesis, whereas it inhibits adipogenesis and promotes osteogenesis during lineage-specific differentiation. Our findings provide a rationale for further exploring the therapeutic use of irisin in obesity and exercise-associated bone formation.

Myostatin inhibits porcine intramuscular preadipocyte differentiation in vitro

This study assessed the effect of myostatin on adipogenesis by porcine intramuscular preadipocytes. Intramuscular preadipocytes were isolated from the longissimus dorsi muscle of newborn pigs. Myostatin inhibited intramuscular preadipocyte differentiation in a dose-dependent manner. Myostatin treatment during preadipocyte differentiation significantly (P < 0.05) inhibited the expression of the adipogenic marker genes CCAAT/enhancer-binding protein β, CCAAT/enhancer-binding protein α, peroxisome proliferator-activated receptor γ, sterol regulatory element-binding protein-1c, fatty acid-binding protein, and adiponectin. Myostatin also significantly (P < 0.05) reduced the release of glycerol and decreased both adipose triglyceride lipase and hormone-sensitive lipase expression in intramuscular adipocytes. Our study suggests that myostatin acts as an extrinsic regulatory factor in regulating intramuscular adipogenesis.