Fat has unique distribution characteristics and different economic value in various tissues of animals. In broilers, the intramuscular fat is economically desirable in production. Appropriately increased IMF content can improve meat quality, including tenderness, flavor and juiciness [4–6]. However, excessive abdominal fat deposition has negative impacts on feed efficiency and carcass yield [2, 3], and decreased abdominal fat deposition is beneficial to reduce produces waste and improve consumer acceptance. Lowering abdominal fat and increasing intramuscular fat can effectively increase the economic value of broilers. Therefore, how to change the constitution distribution is an important scientific problem for broilers.
Unlike a marbling distribution of IMF in domestic animals, the IMF of chicken cannot be obtained directly from anatomy. Moreover, muscle tissue of chicken has a variety of cell composition , the preadipocytes of IMF can not be separated by physical methods for the similar density with muscle cells. So, the high purity preadipocytes of IMF can only be obtained by the dedifferentiation of mature adipocytes in vitro as described previously . In this study, the abdominal fat preadipocytes and intramural preadipocytes were respectively obtained from the mature adipocytes of the same chicken to compare their lipogenesis ability with the consistency of experimental conditions in vitro, expecting to establish a theoretical foundation for the body fat distribution of chicken and provide ideas and development direction for chicken production. As known, the adipocytes in different tissues were regulated by the adjacent microenvironment to perform the corresponding physiological function [17, 18]. To eliminate the effect of factors in vivo and in vitro, the second-generation cells used. After the cells were overgrown for 2 days, the lipogenesis of adipocytes were detect, which was different from the usual practice of inducing adipocyte differentiation in vitro, avoiding the possibility that the inducers could conceal the lipogenesis of the cells themselves. The results showed that the lipogenesis of preadipocytes derived from abdominal adipocytes were significantly superior to those derived from muscle tissue, consistent with those results in vivo as previously reported [19, 20].
Genetically, the molecular basis on the difference in lipid deposition between the two cells was explored using RNA-sEq. Transcriptome results revealed a large number of differentially expressed genes involved in the biological processes, such as cell adhesion, tight adhesion, cell differentiation, extracellular matrix, DNA binding, cacium ion binding, et al. Also, the genes related to lipid metabolism were identified, including some representative genes related to lipid metabolism. These supported the differences in lipid deposition between the two types of cells. Further, the expression levels of some classical genes were verified by qRT-PCR. These representative lipid metabolism related genes mainly involved in intracellular fat decomposition (DGKH, DGKQ, and DGKD) [21, 22], extracellular fat decomposition (LPL) , fatty acid synthesis (ELOVL1, ELOVL6, FADS1L1, FADS6, SCD, and SCD5) [22–25], fatty acid transport (FABP3 and FABP4) [26, 27], fat maintenance (CIDEC, MOGAT1, PLIN3, and PLIN4) [28–31] and adipocyte differentiation (PPARG, RBP7, and RXRG) [32, 33]. The correlation analysis on data of RNA-seq and qRT-PCR showed that they had a strong positive correlation, which confirmed the accuracy of RNA-seq data. Among these representative genes, the CEBPA, DGKH, DGKQ, DGKD, FADS1L1, SCD, SCD5, and PPARG expressions were down-regulated in DAFPs compared to DIMFPs, but the CIDEC, ELOVL1, ELOVL6, FABP3, FABP4, FADS6, LPL, MOGAT1, PLIN3, PLIN4, RBP7, and RXRG expressions were up-regulated in DAFPs compared to DIMFPs, showing the consistency with the lipid content. So it was considered that these gene had the important effects on regulating the lipid deposition in adipocyte of chicken.
Based on KEGG database, 47 enriched pathways were screened, including the well-known pathways affecting lipid metabolism (MAPK-, TGF beta-, Wnt-, Calcium-, PPAR signaling pathway). Among them, it was acknowledged that the PPAR signaling pathway plays a key role in mediating lipid metabolism [34, 35]. Here, the representative genes related to lipid metabolism (FABP3, FABP4, LPL, PPARG, RXRG, SCD, and SCD5 et al.) were enriched in the PPAR signaling pathway. In addition, it was reported that MAPK-, TGF beta-, Wnt-, and Calcium signaling pathways had the interaction with the PPAR pathway to regulate the lipid metabolism in the lipogenesis process [36–38], and there also had a large number genes were enriched in MAPK-, Calcium-, and TGF beta signaling pathways. According to the enrichment information of these three signaling pathways in this study, the evidence pointed to these three pathways could mediate the biology function of cell differentiation or metabolism. Then, it was deduced that the MAPK-, Calcium-, and TGF beta signaling pathways also involved the regulation of lipogenesis between DAFPs and DIMFPs.