Expression profiles of key transcription factors involved in lipid metabolism in Beijing-You chickens

Highlights

• Accumulation of IMF in breast and thigh from 8 to 14 wk exceeded that from 14 to 20 wk.

• C/EBPα and PPARγ were important effectors in lipid metabolism in chickens.

• C/EBPβ might play a role in IMF deposition in the early developmental stages.

• PPARα in breast and thigh muscle was significantly higher than that in abdominal fat.

Abstract

Intramuscular fat (IMF) is a crucial factor for the meat quality of chickens. With the aim of studying the molecular mechanisms underlying IMF deposition in chickens, the expression profiles of five candidate transcription factors involved in lipid metabolism in several tissues were examined in Beijing-You (BJY) chickens at five ages (0, 4, 8, 14 and 20 wk). Results showed that accumulation of IMF in breast (IMFbr), thigh (IMFth) and abdominal fat weight increased significantly (P < 0.01) after 8 wk. Accumulation of both IMFbr and IMFth from 8 to 14 wk exceeded that from 14 to 20 wk; IMFth was 4–7 times of IMFbr. As for the expression profiles of key transcription factors: 1) expression of C/EBPα and PPARγ in abdominal fat was significantly higher than that in breast and thigh muscles at all ages. The expression of C/EBPα was positively correlated with PPARγ in both breast and thigh muscles, which indicated that both C/EBPα and PPARγ promoted fat deposition and might act through a unified pathway; 2) the expression of SREBP-1 in 0, 4, and 8 wk in thigh muscle was significantly higher than that in breast; 3) expression of C/EBPβ at 4 and 8 wk was significantly higher than that at 14 and 20 wk; and it was positively correlated with IMFth and IMFbr from 0 to 8 wk; 4) expression of PPARα in breast and thigh muscles was significantly higher than that in abdominal fat. Taken together, all five transcription factors studied play roles in lipid metabolism in chickens with C/EBPα and PPARγ being important effectors.

Introduction

It is commonly known that intramuscular fat (IMF) contributes to flavor and juiciness. It is an important factor for the palatability of chicken meat. The content of IMF is the result of lipogenesis and lipolysis with complex molecular mechanisms. Chickens differ from mammals in having minimal dissectible adipose tissue associated with the connective elements of skeletal muscle and lipid accumulation is more influenced by uptake of blood lipids and subsequent lipogenesis rather than de novo fatty acid synthesis (Griffin et al., 1987, 1992). Liver serves as the main site of fatty acid synthesis and exports lipids as lipoproteins along with those derived from the gut (Griffin et al., 1987, 1992; Leclercq et al., 1984, Leveille et al., 1975). Together, these serve as substrates for use by other tissues, including deposition in muscle as well as in adipose tissue. The regulation of lipid deposition in adipose tissue is incompletely understood but can be expected to involve adipogenesis (differentiation and maturation), lipid transport, lipogenesis and lipolysis; the latter three would be of additional importance in muscle.

There is evidence, from a variety of species, for certain transcription factors playing roles in controlling adipocyte differentiation, lipogenesis and lipolysis (Haraguchi et al., 2003, Hausman et al., 2009, Hummasti et al., 2008). Adipogenesis is a well-regulated process controlled by a highly coordinated activation of various transcription factors. Temporal expression, in a highly coordinated cascade, of transcripts for CCAAT/enhancer-binding protein factors (C/EBPα, C/EBPβ), sterol regulatory element-binding protein 1 (SREBP1), and peroxisome proliferator-activated receptors (PPARα and PPARγ) is especially important (Elam et al., 2001, Koo et al., 2001, Rosen et al., 2000). SREBP1 plays an important role in the early stages of adipogenesis (Kim and Spiegelman, 1996). In addition to C/EBPβ being expressed in the early stages of adipogenesis (Darlington et al., 1998, Timchenko et al., 1996), C/EBPβ as a transcriptional relay might be under the direct control of some effects of insulin and/or SREBP1 in mature fat cells (Le Lay et al., 2002, Timchenko et al., 1996). Subsequently, C/EBPβ induces the expression of C/EBPα and PPARγ at later stages of cell differentiation (Rosen et al., 2002, Timchenko et al., 1996) and these participate in a positive feedback loop, promoting and maintaining the differentiated state (Yeh et al., 1995a). The two transcription factors, C/EBPα and PPARγ, play essential roles in activating terminal differentiation of adipocytes, lipid synthesis and other specific programs (Hausman et al., 2009, Olofsson et al., 2008, Rosen et al., 2002). PPARα is another important contributor to lipid metabolism by increasing fatty acid β-oxidative and lipid oxidation, hence reducing lipid accumulation (Muoio et al., 2002, Tsuchida et al., 2005, Ye et al., 2001).

The dynamics of these transcription factors (SREBP1, C/EBPβ, C/EBPα, PPARγ and PPARα) in important tissues across maturation in chickens are still unclear, as are the possible effects of the transcription factors on fat deposition. Beijing-You (BJY) chickens are a local variety, with a high fat content and an excellent flavor. The objective of this study was to describe developmental changes and explore possible relationships between expression of transcription factors and fat deposition in economically important tissues during growth of chickens.