Decorin, a growth hormone regulated protein in humans.

Context: Growth hormone (GH) stimulates connective tissue and muscle growth, an effect that is potentiated by testosterone. Decorin, a myokine and a connective tissue protein, stimulates connective tissue accretion and muscle hypertrophy. Whether GH and testosterone regulate decorin in humans is not known. Objective: To determine whether decorin is stimulated by GH and testosterone. Design: Randomized, placebo-controlled, double-blind study. Participants and Intervention: 96 recreationally trained athletes (63 men, 33 women) received 8 weeks of treatment followed by a 6-week washout period. Men received placebo, GH (2 mg/day), testosterone (250 mg/week) or combination. Women received either placebo or GH (2 mg/day). Main outcome measure: Serum decorin concentration. Results: GH treatment significantly increased mean serum decorin concentration by 12.7 +/- 4.2%; P < 0.01. There was a gender difference in the decorin response to GH, with greater increase in men than in women (Delta 16.5 +/- 5.3%; P < 0.05 compared to Delta 9.4 +/- 6.5%; P = 0.16). Testosterone did not significantly change serum decorin. Combined GH and testosterone treatment increased mean decorin concentration by 19.5 +/- 3.7% (P < 0.05), a change not significantly different from GH alone. Conclusion: GH significantly increases circulating decorin, an effect greater in men than in women. Decorin is not affected by testosterone. We conclude that GH positively regulates decorin in humans in a gender-dimorphic manner.