Implanting cattle with steroids significantly enhances feed efficiency, rate of gain, and muscle growth. However, the mechanisms responsible for these improvements in muscle growth have not been fully elucidated. Trenbolone acetate (TBA), a testosterone analog, has been shown to increase proliferation rate in bovine satellite cell (BSC) cultures. The classical genomic actions of testosterone have been well characterized; however, our results indicate that TBA may also initiate a quicker, nongenomic response that involves activation of G protein– coupled receptors (GPCR) resulting in activation of matrix metalloproteinases 2 and 9 (MMP2 and MMP9) that release membrane-bound heparinbinding epidermal growth factor–like growth factor (hbEGF), which then binds to and activates the epidermal growth factor receptor (EGFR) and/or erbB2. Furthermore, the EGFR has been shown to regulate expression of the IGF-1 receptor (IGF-1R), which is well known for its role in modulating muscle growth. To determine whether this nongenomic pathway is potentially involved in TBA-stimulated BSC proliferation, we analyzed the effects of treating BSC with guanosine 5′-O-2-thiodiphosphate (GDPβS), an inhibitor of all GPCR; a MMP2 and MMP9 inhibitor (MMPI); CRM19, a specific inhibitor of hbEGF; AG1478, a specific EGFR tyrosine kinase inhibitor; AG879, a specific erbB2 kinase inhibitor; and AG1024, an IGF-1R tyrosine kinase inhibitor on TBA-stimulated proliferation rate (3H-thymidine incorporation). Assays were replicated at least 9 times for each inhibitor experiment using BSC cultures obtained from at least 3 different animals. Bovine satellite cell cultures were obtained from yearling steers that had no previous exposure to androgenic or estrogenic compounds. As expected, BSC cultures treated with 10 nM TBA showed (P < 0.05) increased proliferation rate when compared with control cultures. Additionally, treatment with 5 ng hbEGF/mL stimulated proliferation in BSC cultures (P < 0.05). Treatment with GDPβS, MMPI, CRM197, AG1024, AG1478, and/or AG879 all suppressed (P < 0.05) TBA-induced increases in proliferation. These data indicate that TBA likely initiates a nongenomic response involving GPCR, MMP2 and MMP9, hbEGF, EGFR, erbB2, and IGF-1R, which may play a role in TBA-mediated increases in BSC proliferation.
Bibliographical noteFunding Information:
1This research was supported by USDA Cooperative State Research, Education and Extension Service Program (USDA National Research Initiative Competitive Grant), contract grant number 2012-67015-19447, and by the Minnesota Agricultural Experiment Station.
- Epidermal growth factor receptor
- G protein–coupled receptors
- Satellite cell
- Trenbolone acetate