Identification and expression pattern of cationic amino acid transporter-1 (CAT-1) mRNA in small intestinal epithelia of Angus steers at four production stages

¹ Department of Animal and Food Sciences, University of Kentucky, Lexington 40546-0215

^* To whom correspondence should be addressed. E-mail: jmatthew{at}uky.edu.

	Abstract

Although it is known that dietary supplementation of cationic AA (CAA), especially L-lysine, is essential for optimal growth of beef cattle, proteins responsible for absorption of CAA by bovine intestinal epithelia have not been described. CAT-1 is a major intestinal CAA transporter demonstrating a high-affinity (µM) transport activity for L-lysine in other mammals, and is widely expressed by small intestinal epithelia of non-ruminants, but neither sequence nor expression pattern data exist for CAT-1 in cattle. Therefore, the goal of this research was to compare the relative expression (putative) of CAT-1 mRNA by duodenal, jejunal, or ileal small intestinal epithelia both across and within commercially relevant beef cattle production/development stages. Twenty-four Angus steers were assigned randomly (n = 6) to 1 of 4 treatments (suckling, weanling, growing, and finishing) after all animals were born. Duodenal, jejunal, and ileal epithelia were scraped and total RNA extracted after steers were killed at 32, 184, 248, and 423 d of age. Average daily gains of animals did not differ (1.09 ± 0.05 kg/d) among stages, whereas the small intestine length relative to BW decreased (P < 0.01) with animal development. Using standard reverse transcription-PCR cloning techniques, a partial-length bovine CAT-1 complimentary DNA (695 bp; GenBank, DQ399522) was generated from jejunal mRNA samples, which possessed 89% and 87% identities to pig and human CAT-1 orthologs, respectively. Based on this bovine-specific genetic data, a real-time PCR-based assay of reverse transcribed mRNA was developed and used to measure relative changes in bovine CAT-1 mRNA abundance in epithelia as steers developed. CAT-1 mRNA was expressed by the duodenum, jejunum, and ileum of all 4 production stages. In contrast to expression by duodenal or ileal epithelium, jejunal expression of CAT-1 mRNA by growing steers was greater (P = 0.005) than by suckling, weanling, and finishing steers. In terms of the expression of CAT-1 mRNA within production stage, jejunal expression was greater (P = 0.002) than that by duodenum or ileum for growing steers. In contrast, no site difference was found for suckling, weanling, or finishing steers. These data indicate that previously reported Na⁺-independent uptake of lysine by jejunal and ileal epithelia likely occurred by CAT-1, and that the potential capacity for CAT-1-mediated uptake of CAA for beef steers may be greatest for the "growing" phenotype.

Key Words: Amino acid transport, Bovine, CAT-1, Regulated gene expression, SLC7A1, Small intestine

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