Comparison of three serum assays for bone collagen formation during postmenopausal estrogen-progestin therapy

Postmenopausal hormone replacement therapy (HRT) lowers the turnover rate of the mineralized bone matrix, the predominant organic component of which is type I collagen. The effect of estrogen on bone metabolism has been monitored by measuring the circulating concentration of the carboxy-terminal propeptide of type I procollagen (PICP), which decreases during HRT. We have recently developed assays for the intact amino-terminal propeptide (PINP) of type I procollagen, a protein set free from the other end of the same gene product. PICP and PINP, both derived from the synthesis of type I collagen, but differing in their further metabolism, were assessed in 47 postmenopausal women, aged 45-66 years, undergoing postmenopausal HRT. Estradiol-gel applied daily was combined to a continuous progestin administered by three different routes. Serum samples obtained before the treatment and 6 and 12 months after its commencement were analyzed for PICP, PINP, PINP Col 1 (assay variant measuring also the degradation product of PINP) and PIIINP (amino-terminal propeptide of type III procollagen). During HRT the circulating concentration of PICP decreased by 20%, that of PINP by 42% and that of PINP Col 1 by 32% in 12 months. The correlation between the two propeptides, which was 0.676 before the treatment, increased to 0.851 in 6 months and to 0.815 in 12 months. The correlations between PINP and PINP Col 1 were 0.872 before the treatment and increased to 0.925 and 0.941 after 6 and 12 months of treatment, respectively. The serum concentration of PIIINP, which reflects the turnover of the soft tissue collagens, did not change remarkably. Our findings indicate that the intact PINP is a more dynamic marker of bone metabolism than PICP and can therefore be recommended as a marker reflecting the effect of estrogen on bone collagen formation during HRT.