Lipogenic activity of intramuscular and subcutaneous adipose tissues from steers produced by different generations of angus sires

Main regulatory factors of marbling level in beef cattle

The content of intramuscular fat (IMF), that determines marbling levels is considered as one of the vital factors influencing beef sensory quality including tenderness, juiciness, flavour and colour. The IMF formation in cattle commences around six months after conception, and continuously grows throughout the life of the animal. The accumulation of marbling is remarkably affected by genetic, sexual, nutritional and management factors. In this review, the adipogenesis and lipogenesis process regulated by various factors and genes during fetal and growing stages is briefly presented. We also discuss the findings of recent studies on the effects of breed, gene, heritability and gender on the marbling accumulation. Various research reported that feeding during pregnancy, concentrate to roughage ratios and the supplementation or restriction of vitamin A, C, and D are crucial nutritional factors affecting the formation and development of IMF. Castration and early weaning combined with high energy feeding are effective management strategies for improving the accumulation of IMF. Furthermore, age and weight at slaughter are also reviewed because they have significant effects on marbling levels. The combination of several factors could positively affect the improvement of the IMF deposition. Therefore, advanced strategies that simultaneously apply genetic, sexual, nutritional and management factors to achieve desired IMF content without detrimental impacts on feed efficiency in high-marbling beef production are essential.

Characterization of β-adrenergic receptors in bovine intramuscular and subcutaneous adipose tissue: comparison of lubabegron fumarate with β-adrenergic receptor agonists and antagonists

Chinese hamster ovary cell constructs expressing either the β ₁-, β ₂- or β ₃-adrenergic receptor (AR) were used to determine whether a novel β-AR modulator, lubabegron fumarate (LUB; Experior, Elanco Animal Health) might exert greater potency for a specific β-AR subtype. EC₅₀ values calculated based on cAMP accumulation in dose response curves indicate that LUB is highly selective for the β ₃-AR subtype, with an EC₅₀ of 6 × 10^–9 M, with no detectible agonistic activity at the β ₂-AR. We hypothesized that the accumulation of lipolytic markers would reflect the agonist activity at each of the β-receptor subtypes of the specific ligand; additionally, there would be differences in receptor subtype expression in subcutaneous (s.c.) and intrmuscular (i.m.) adipose tissues. Total RNA was extracted from adipose tissue samples and relative mRNA levels for β ₁-, β₂-, and β ₃-AR were measured using real-time quantitative polymerase chain reaction. Fresh s.c. and i.m. adipose tissue explants were incubated with isoproterenol hydrochloride (ISO; β-AR pan-agonist), dobutamine hydrochloride (DOB; specific β ₁-AA), salbutamol sulfate (SAL; specific β ₂-AA), ractopamine hydrochloride (RAC), zilpaterol hydrochloride (ZIL), BRL-37344 (specific β ₃-agonist), or LUB for 30 min following preincubation with theophylline (inhibitor of phosphodiesterase). Relative mRNA amounts for β ₁-, β ₂-, and β ₃-AR were greater (P < 0.05) in s.c. than in i.m. adipose tissue. The most abundant β-AR mRNA in both adipose tissues was the β ₂-AR (P < 0.05), with the β ₁- and β ₃-AR subtypes being minimally expressed in i.m. adipose tissue. ISO, RH, and ZH stimulated the release of glycerol and nonesterified fatty acid (NEFA) from s.c. adipose tissue, but these β-AR ligands did not alter concentrations of these lipolytic markers in i.m. adipose tissue. LUB did not affect glycerol or NEFA concentrations in s.c. or i.m. adipose tissue, but attenuated (P < 0.05) the accumulation of cAMP mediated by the β ₁- and β ₂-AR ligands DOB and SAL in s.c. adipose tissue. Collectively, these data indicate that bovine i.m. adipose tissue is less responsive than s.c. adipose tissue to β-adrenergic ligands, especially those that are agonists at the β ₁- and β₃-receptor subtypes. The minimal mRNA expression of the β ₁- and β ₃ subtypes in i.m. adipose tissue likely limits the response potential to agonists for these β-AR subtypes.

Gene expression identifies metabolic and functional differences between intramuscular and subcutaneous adipocytes in cattle

Background

This study used a genome-wide screen of gene expression to better understand the metabolic and functional differences between commercially valuable intramuscular fat (IMF) and commercially wasteful subcutaneous (SC) fat depots in Bos taurus beef cattle.

Results

We confirmed many findings previously made at the biochemical level and made new discoveries. The fundamental lipogenic machinery, such as ACACA and FASN encoding the rate limiting Acetyl CoA carboxylase and Fatty Acid synthase were expressed at 1.6–1.8 fold lower levels in IMF, consistent with previous findings. The FA elongation pathway including the rate limiting ELOVL6 was also coordinately downregulated in IMF compared to SC as expected. A 2-fold lower expression in IMF of ACSS2 encoding Acetyl Coenzyme A synthetase is consistent with utilisation of less acetate for lipogenesis in IMF compared to SC as previously determined using radioisotope incorporation. Reduced saturation of fat in the SC depot is reflected by 2.4 fold higher expression of the SCD gene encoding the Δ9 desaturase enzyme. Surprisingly, CH25H encoding the cholesterol 25 hydroxylase enzyme was ~ 36 fold upregulated in IMF compared to SC. Moreover, its expression in whole muscle tissue appears representative of the proportional representation of bovine marbling adipocytes. This suite of observations prompted quantification of a set of oxysterols (oxidised forms of cholesterol) in the plasma of 8 cattle exhibiting varying IMF. Using Liquid Chromatography-Mass Spectrometry (LC-MS) we found the levels of several oxysterols were significantly associated with multiple marbling measurements across the musculature, but (with just one exception) no other carcass phenotypes.

Conclusions

These data build on our molecular understanding of ruminant fat depot biology and suggest oxysterols represent a promising circulating biomarker for cattle marbling.

Glucose and acetate metabolism in bovine intramuscular and subcutaneous adipose tissues from steers infused with glucose, propionate, or acetate

We hypothesized that abomasal infusion of glucose would promote de novo fatty acid biosynthesis from glucose in vitro in bovine intramuscular (i.m.) and subcutaneous (s.c.) adipose tissues to a greater extent than ruminal infusion of acetate, propionate, or glucose. Angus crossbred steers (n = 24), 22 mo of age, were fitted with ruminal cannulas, and steers were adapted to another corn/sorghum finishing diet over a 2-wk period while recovering from the placement of the cannulas. After the adaptation period, the steers were fed the second finishing diet at 130% of their voluntary intake and were infused with isocaloric amounts (3.76 Mcal/d) of glucose, propionate, or acetate for 35 d. Glucose was infused either into the rumen or into the abomasum, whereas propionate and acetate were infused into the rumen. Acetate infusion decreased DM and DE intakes (P < 0.05). The 5th to 8th longissimus muscle section was removed immediately and transported to the laboratory within 10 min post-exsanguination in 38 °C, oxygenated Krebs Henseleit buffer containing 5 mM glucose and 5 mM acetate. Intramuscular and s.c. adipose tissues were dissected from the muscle and incubated in vitro in 5 mM glucose plus 5 mM acetate (containing [U-14C]glucose or [1-14C]acetate). Lipid content was lower (P = 0.04) in i.m. adipose tissue of the acetate-infused steers than in the other treatment groups, and i.m. adipocytes from acetate-infused steers were smaller (P = 0.01) than those from propionate-infused steers. The rate of incorporation of acetate into glyceride-fatty acids (GFA) in i.m. and s.c. adipose tissues was greater (P < 0.03) in steers receiving ruminal or abomasal infusions of glucose than in adipose tissues from steers infused with acetate. The greatest rates of GFA synthesis were observed in s.c. adipose tissue from steers infused ruminally with propionate or abomasally infused with glucose (P < 0.001). In i.m. and s.c. adipose tissues, the proportion of acetyl units from acetate incorporated into GFA was greater in steers receiving glucose infusion in the rumen or abomasum than in steers receiving acetate or propionate infusion (P < 0.05). Contrary to our hypothesis, abomasal glucose infusion did not promote greater fatty acid biosynthesis from glucose in i.m. adipose tissue than ruminal glucose infusion. However, glucose infusion caused the greatest production of acetyl units from acetate in i.m. and s.c. adipose tissues.

Review: Nutrigenomics of marbling and fatty acid profile in ruminant meat

The present review will present the recent published results and discuss the main effects of nutrients, mainly fatty acids, on the expression of genes involved in lipid metabolism. In this sense, the review focuses in two phases: prenatal life and finishing phase, showing how nutrients can modulate gene expression affecting marbling and fatty acid profile in meat from ruminants. Adiposity in ruminants starts to be affected by nutrients during prenatal life when maternal nutrition affects the differentiation and proliferation of adipose cells enhancing the marbling potential. Therefore, several fetal programming studies were carried out in the last two decades in order to better understand how nutrients affect long-term expression of genes involved in adipogenesis and lipogenesis. In addition, during the finishing phase, marbling becomes largely dependent on starch digestion and glucose metabolism, being important to create alternatives to increase these metabolic processes, and modulates gene expression. Different lipid sources and their fatty acids may also influence the expression of genes responsible to encode enzymes involved in fat tissue deposition, influencing meat quality. In conclusion, the knowledge shows that gene expression is a metabolic factor affecting marbling and fatty acid profile in ruminant meat and diets and their nutrients have direct effect on how these genes are expressed.

Conjugated linoleic acid (t-10, c-12) reduces fatty acid synthesis de novo, but not expression of genes for lipid metabolism in bovine adipose tissue ex vivo

We hypothesized that exogenous fatty acids, and especially or 18:2 trans-10, cis-12 conjugated linoleic acid (CLA), would decrease adipogenic and lipogenic gene expression and de novo fatty acid biosynthesis in intramuscular (i.m.) and subcutaneous (s.c.) adipose tissues. Fresh i.m. and s.c. adipose tissues were collected from the longissimus thoracis muscle of Angus steers at 12, 14, and 16 months of age (n = 4 per time point). Adipose tissue explants were incubated in duplicate for 48 h with 40 μM α-linolenic (ALA), oleic, stearic, trans-vaccenic, or CLA. Adipocyte size, acetate and glucose incorporation into fatty acids in vitro and mRNA levels for C/EBPβ, CPT1β, GPR43, PPARγ, PRKAA1 (AMPKα) and SCD1 were measured following the incubations. PRKAA1 and SCD1gene expression were greater (P < 0.001) in s.c. adipose tissue than in i.m. adipose tissue and acetate incorporation into lipids and C/EBPβ, PPARγ, and SCD1gene expression were greater at 16 months of age than at 12 months of age in i.m. adipose (P < 0.01). C/EBPβ gene expression increased by 16 months of age and PRKAA1 gene expression decreased by 16 months of age in s.c. adipose tissue. All fatty acids increased s.c. adipocyte volumes whereas CLA decreased acetate incorporation into lipids in s.c. adipose tissue (P < 0.05), but none of the fatty acids affected gene expression in i.m. or s.c. adipose tissue (P > 0.10). Thus, CLA depressed de novo fatty acid biosynthesis from acetate but neither CLA nor other fatty acids significantly affected adipogenic or lipogenic gene expression.

Board-invited review: the biology and regulation of preadipocytes and adipocytes in meat animals

The quality and value of the carcass in domestic meat animals are reflected in its protein and fat content. Preadipocytes and adipocytes are important in establishing the overall fatness of a carcass, as well as being the main contributors to the marbling component needed for consumer preference of meat products. Although some fat accumulation is essential, any excess fat that is deposited into adipose depots other than the marbling fraction is energetically unfavorable and reduces efficiency of production. Hence, this review is focused on current knowledge about the biology and regulation of the important cells of adipose tissue: preadipocytes and adipocytes.

Computer image analysis of intramuscular adipocytes and marbling in the longissimus muscle of cattle1

The deposition of fat in muscle, recognized by the consumer as marbling, is an important meat quality trait. The objective of the study was to provide additional insights into the quantitative extent of marbling by means of computer image analysis. Fifty-one F(2) generation German Holstein and Charolais crossbreed cattle, 18 mo of age, were used to determine relationships among marbling traits, adipocyte size, and the amount of adipose tissue in different depots. Differences were recorded among the size of i.m. adipocytes in different groups of marbling flecks, divided according to the location in the muscle cross-section and to the size of the marbling flecks. The results showed positive correlation between i.m. adipocyte size and the weight of s.c. fat, intestinal fat, omental fat, and perirenal fat (r = 0.50, 0.61, 0.70, and 0.63, respectively, P < 0.001). The i.m. adipocyte size was correlated with i.m. fat content, number of marbling flecks, proportion of marbling fleck area, and total length of marbling flecks (r = 0.71, 0.44, 0.62, and 0.55, respectively, P < 0.01). The number of marbling flecks was also correlated with i.m. fat content, proportion of marbling fleck area, and total length of marbling flecks (r = 0.58, 0.62, and 0.91, P < 0.01, respectively). The ventral marbling flecks had a 5-fold larger fleck area, 4-fold more adipocytes, and larger adipocytes (P < 0.001). Larger marbling flecks contained larger adipocytes (P < 0.001). Moreover, compared with the small marbling flecks, there was a 48-fold larger fleck area and 26-fold more adipocytes in the large marbling flecks. The results indicate that i.m. fat deposition increases concurrently with the other fat depots but is still independent. Furthermore, the i.m. fat is preferentially deposited in the ventral area of LM. Although the i.m. adipocyte size has an important effect on the traits of marbling flecks, cell number plays a greater role in i.m. fat deposition than cell size.

Lipid metabolism and secretory function of porcine intramuscular adipocytes compared with subcutaneous and perirenal adipocytes

The function of adipocytes interspersed between myofiber fasciculi in skeletal muscle physiology and physiopathology is poorly documented. Because regional differences in adipocyte features have been reported in various species, we hypothesized that lipid metabolism and secretory function of intramuscular (IM) adipocytes differ from that of nonmuscular adipocytes. In the present study, adipocytes isolated from trapezius muscle were compared with subcutaneous and perirenal adipocytes in growing pigs. Between 80 and 210 days of age, gene expressions and/or activities of enzymes involved in lipogenesis or lipolysis were much lower (P < 0.05) in adipocytes isolated from muscle than in those from other locations. Insulin-induced lipogenesis and lipolytic efficiency after catecholamine addition were also the lowest (P < 0.05) in IM adipocytes. In these cells, the age-related increase (+300%) in the ratio of mRNA levels of fatty acid synthase to hormone-sensitive lipase paralleled the enlargement of adipocyte diameters (+70%, P < 0.05) and the increase in lipid content in muscle (+135%, P < 0.05) during growth. Expressions of genes coding for leptin, adiponectin, and IGF-I, as well as for various hormonal receptors, were lower (P < 0.05) in IM adipocytes than in other adipocytes, whereas levels of TNF-alpha mRNA did not differ between sites. Interestingly, IGF-II mRNA levels were higher (P < 0.05) in IM adipocytes than in other adipocytes. These data support the view that IM fat is not just an ectopic extension of other fat locations but displays specific biological features during growth.

Carcass, sensory, and adipose tissue traits of Brangus steers fed casein-formaldehyde-protected starch and/or canola lipid

We predicted that providing rumen-protected starch to the small intestine would increase adiposity of intramuscular adipose tissue, and hence marbling scores. Eighteen 15-mo-old Brangus steers were assigned randomly to one of three dietary treatment groups: 1) cracked corn (Corn); 2) casein-formaldehyde-protected lipid (Canola Lipid); or 3) casein-formaldehyde-protected starch (Marble Plus). All diets were equally balanced for ME (2.91 Mcal/kg), CP (12.5%), and DM (89%). Ether extract was 3.7, 6.9, and 6.9% for the Corn, Canola Lipid, and Marble Plus diets, respectively, and the Marble Plus also contained 3.7% protected starch. Steers were fed the diets for 126 d before slaughter. Average daily feed intake (as-fed basis), ADG, and feed:gain ratio (P > or = 0.23) did not differ among treatments. Carcasses across treatments did not differ (P = 0.26) in adjusted fat thickness, longissimus muscle area, hot carcass weight, dressing percentage, marbling scores, or USDA quality grade. Percentage of kidney, pelvic, and heart fat was higher (P < 0.01) and USDA yield grade tended (P = 0.08) to be higher, for carcasses from Canola Lipid- and Marble Plus-fed steers than for carcasses from Corn-fed steers. Of the descriptive meat sensory attributes, connective tissue amount (P = 0.06) and painty flavor (P = 0.12) tended to be greater in meat from Marble Plus steers than from Canola Lipid steers. Percentages of 18:2n-6 and 18:3n-3 were higher (P < 0.01), and 15:0, 16:0, and 17:0 were lower (P < or = 0.07) in tissues from Canola Lipid- and Marble Plus-fed steers than in Corn-fed steers. Mean adipocyte volume was greater (P = 0.02) in i.m. adipose tissue and tended (P = 0.11) to be greater in s.c. adipose tissue of Canola Lipid steers (848 pL) vs. Corn steers (536 pL). Glucose incorporation into total lipids, glyceride-glycerol, and fatty acid fractions was highest (P < 0.01) in s.c. adipose tissue from steers fed Marble Plus but was unaffected (P > or = 0.33) by diet in i.m. adipose tissue. Fatty acid synthetase activity tended (P = 0.08) to be higher in s.c. adipose tissue of Marble Plus steers, and NADP-malic dehydrogenase activity was higher (P = 0.03) in i.m. adipose tissue of Canola Lipid steers. We conclude that Marble Plus did not improve carcass quality, but also did not reduce beef sensory attributes. Any differences we observed in carcass characteristics, adipose tissue cellularity, or lipogenesis apparently were caused by the protected lipid rather than the protected starch.

Restricted feed intake during fattening reduces intramuscular lipid deposition without modifying muscle fiber characteristics in rabbits

The present study was conducted to determine the effects of feed restriction during fattening on muscle fiber characteristics and intramuscular lipid traits. From 11 wk of age onward, rabbits were given free access to feed (control group), or received 70% of the control feed intake (restricted group). At the same weight at slaughter, restricted-fed rabbits were 3 wk older than controls (18 vs. 15 wk). The longissimus lumborum (LL, white loin), biceps femoris (BF, white thigh) and semimembranosus proprius (SMP, red thigh) muscles were then removed, and biochemical and histochemical assays were performed. In the three muscles, there was no effect of feed restriction on mean fiber size or percentage of the different fiber types. Restricted vs. control feeding resulted in a significant reduction (P<0.001) in total lipid content in all three muscles. This reduction was paralleled by a decline (P<0.001) in the activities of the malic enzyme and glucose-6 phosphate dehydrogenase (G6PDH), generating NADPH for the support of fatty acid synthesis. The diet-induced variations in lipid concentration and enzyme activities were larger (P<0.05) in the pure oxidative SMP muscle than in the predominantly fast-twitch glycolytic LL and BF muscles. Whatever feeding status, the ratio of malic enzyme to G6PDH activities was sharply lower (P<0.001) in SMP than in BF and LL muscles (averaging 1.5 vs. 9 and 15, respectively). These data indicate that nutritional status regulates intramuscular lipid deposition, without changing fiber-type composition. Further studies are necessary to determine the role of G6PDH in the lipogenic process of oxidative vs. glycolytic muscles.

Starvation depresses acylglycerol biosynthesis in bovine subcutaneous but not intramuscular adipose tissue homogenates

The primary objective of this study was to document the effects of starvation on acylglycerol biosynthesis in homogenates of intramuscular and subcutaneous adipose tissues. Adipose tissue samples were obtained from 8th-13th thoracic rib sections from 12 Angus cattle (six steers plus six heifers). Three steers and three heifers were starved for 72 h prior to slaughter while the remainder were slaughtered 4 h after food was withheld. Fat-free 700 x g centrifugal fractions were used to measure the esterification of radiolabeled sn-glycerol 3-phosphate (G-3-P) into acylglycerols at 1.0 mM palmitic or stearic acid, or 0.2 mM oleic, linoleic, or alpha-linolenic acid. There were significant tissue x fatty acid interactions for rates of incorporation into diacylglycerols and triacylglycerols; in subcutaneous, but not intramuscular homogenates, palmitic > stearic = oleic = linoleic = alpha-linolenic acid. Subcutaneous homogenates incorporated a greater percentage of G-3-P into triacylglycerols, and a lesser percentage into phospholipid, than intramuscular homogenates (P < 0.05). In intramuscular homogenates, the primary product of G-3-P esterification to saturated fatty acids was phospholipids. When unsaturated fatty acid served as substrates, triacylglycerols and phospholipids were produced in equal proportions in intramuscular homogenates, and triacylglycerols were the predominant product in subcutaneous homogenates. Intramuscular adipose tissue homogenates exhibited no response to starvation, whereas triacylglycerol and diacylglycerol synthesis was depressed by approximately 50% in subcutaneous adipose tissue homogenates. Similarly, phosphatidic phosphohydrolase activity, initially greater in subcutaneous than in intramuscular adipose tissue, was decreased by approximately 50% by starvation in subcutaneous adipose tissue, but not in intramuscular adipose tissue. We conclude that differences in rates of diacylglycerol and triacylglycerol biosynthesis, and response to starvation, between intramuscular and subcutaneous adipose tissues were due to dissimilarities in the activity of phosphatidic phosphohydrolase.