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

Abstract

Although dietary supplementation of cationic AA (CAA), especially l-Lys, is known to be essential for optimal growth of beef cattle, the proteins responsible for absorption of CAA by bovine intestinal epithelia have not been described. Cationic AA transporter-1 (CAT-1) is a major intestinal CAA transporter, demonstrating a high-affinity (muM) transport activity for l-Lys in other mammals, and is widely expressed by small intestinal epithelia of nonruminants, 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 across and within commercially relevant beef cattle production and development stages. Twenty-four Angus steers were assigned randomly (n = 6) to 1 of 4 treatments (suckling, weanling, growing, and finishing) after all steers were born. Duodenal, jejunal, and ileal epithelia were scraped, and total RNA was extracted after the steers were killed at 32, 184, 248, or 423 d of age. Average daily gains of the steers did not differ (1.09 +/- 0.05 kg/d) among stages, whereas the small intestinal length relative to BW decreased (P < 0.01) with steer development. Using standard reverse transcription-PCR cloning techniques, we generated a partial-length bovine CAT-1 complementary DNA (695 bp; GenBank accession no. DQ399522) from jejunal mRNA samples, which possessed 89 and 87% identities to pig and human CAT-1 orthologs, respectively. On the basis of 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 intestinal epithelia as steers developed. The 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 that by suckling, weanling, or 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 intestinal site difference was found for suckling, weanling, or finishing steers. These data indicate that previously reported Na(+)-independent uptake of Lys 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.