Temporal response of liver signal transduction elements during in vivo endotoxin challenge in cattle: Effects of growth hormone treatment

We quantified the changes in abundance of inducible nitric oxide synthase (iNOS) and associated tissue signal transduction pathway elements (STPEs) in the bovine liver in response to lipopolysaccharide (LPS) challenge and further assessed the impact on the LPS-driven variable responses as affected by daily treatment with recombinant growth hormone (GH) prior to LPS challenge. Twenty-four cross-bred beef steers were divided into GH-treated (recombinant bovine GH, Monsanto Inc., St. Louis, MO; 0.1 mg/kg BW, i.m., daily for 12 days) and non-GH-treatment (control) groups (n = 12/group). Liver biopsy samples were obtained from all animals at 0, 3, 6, and 24 h after LPS challenge (E. coli 055:B5, 2.5 μg/kg BW, i.v. bolus) for Western blot analyses of iNOS and STPEs. In response to LPS, tissue levels of iNOS increased significantly (P < 0.001) in the first 3 h and persisted at levels greater than those at time 0 until 24 h. GH further augmented levels of iNOS at 0, 3, and 6 h resulting in an overall significant increase in the iNOS protein level (P < 0.01). AKT/protein kinase B (AKT/PKB) phosphorylation levels at time 0 were not different between GH-treated and control animals; LPS increased the phosphorylation of AKT/PKB with GH treatment stimulating a four-fold further increase of AKT/PKB phosphorylation. Effects similar to those on AKT/PKB were also observed on signal transducer and activator of transcription 5b (STAT5b). The family of mitogen-activated protein kinase (MAPK) showed different pattern of response. ERK1/2 phosphorylation increased 3 h after LPS challenge but only in GH-treated group (P < 0.01). Compared to 0 h, SAPK/JUN phosphorylation increased in both experimental groups 3, 6 h (P < 0.01), and 24 h (P < 0.05) after LPS. However, at 3 h the increase was greater (P < 0.01) in GH-treated than in control animals. No effect of LPS challenge or GH treatment on p38MAPK was observed. These results suggest that GH treatment has a significant impact on the differential activation of STPEs in the clinical response to LPS.