Progestins abrogate estrogen-induced changes in the insulin-like growth factor axis

OBJECTIVE: We sought to investigate the effect of oral progestins on estrog en-mediated changes in the insulin-like growth factor axis in the periphera l circulation. STUDY DESIGN: Oral conjugated equine estrogen alone or in combination with medroxyprogesterone acetate, desogestrel, or norethindrone was given in a r andomized triple-crossover fashion to 10 healthy postmenopausal women, and the effects on the insulin-like growth factor axis were determined. RESULTS: Baseline circulating insulin-like growth factor I levels were sign ificantly reduced by conjugated equine estrogen (359 +/- 54 vs 225 +/- 44 n g/mL; P = .0001). This effect was reversed by progestins (medroxyprogestero ne acetate, 254 +/- 44 ng/mL; desogestrel, 266 +/- 50 ng/mL; norethindrone, 286 +/- 48 ng/mL; F = 12.2; P = .0015). Free insulin-like growth factor I was reduced by conjugated equine estrogen (1.00 +/- 0.15 ng/mL vs 2.10 +/- 0.39 ng/mL; P = .004), but addition of progestogens had no further effect. Insulin-like growth factor II and insulin levels were unaffected by conjuga ted equine estrogen or progestins. Plasma insulin-like growth factor bindin g protein 1 concentration increased significantly from baseline with conjug ated equine estrogen alone (44.1 +/- 6.0 vs 154 +/- 30 mu g/L; P = .003). T his rise was opposed by progestins of increasing androgenicity (medroxyprog esterone acetate, 130 +/- 26 mu g/L; desogestrel, 100 +/- 16 mu g/L; noreth indrone, 78.0 +/- 12 mu g/L; F = 12.5; P = .0015). Insulin-like growth fact or binding protein 3 levels fell with conjugated equine estrogen, and this was reversed by progestins (conjugated equine estrogen, 2.17 +/- 0.13 mg/L; vs norethindrone and conjugated equine estrogen, 2.41 +/- 0.12 mg/L; F = 7 .6; P = .01). Insulin-like growth factor binding protein 4 levels increased with conjugated equine estrogen with or without progestins, whereas insuli n-like growth factor binding protein 2 levels were unchanged. CONCLUSIONS: Coadministration of androgenic progestins abrogates estrogen-r elated changes in circulating insulin-like growth factor I, insulin-like gr owth factor binding protein 1, and insulin-like growth factor binding prote in 3. Such hormone replacement therapy-induced changes may have significant consequences for the development of cardiovascular disease and osteoporosi s and implications for the use of insulin-like growth factor I in monitorin g growth hormone replacement.