A peptidyl derivative structurally based on the inhibitory center of cystatin C inhibits bone resorption in vitro

Abstract

Human cystatin C is a cysteine proteinase inhibitor belonging to the cystatin superfamily, which previously has been shown to inhibit bone resorption in bone organ culture. The aminoterminal segment, Arg 8-Leu 9-Val 10-Gly 11 (RLVG), of the single polypeptide chain of cystatin C constitutes an essential part of its inhibitory center. In the present study, the effect of benzyloxycarbonyl-Arg 8-Leu 9-Val 10-Gly 11-diazomethane (Z-RLVG-CHN 2) on bone resorption in vitro was compared with the effects of cystatin C and calcitonin. Bone resorption was assessed by the release of 45Ca and 3H from mouse calvarial bones prelabeled with [ 45Ca]CaCl 2 and [ 3H]-proline, respectively. Z-RLVG-CHN 2 concentration-dependently inhibited the release of 45Ca and 3H in bones stimulated by parathyroid hormone (PTH), with half-maximal inhibition obtained at 1 μmol/L. The inhibitory actions of Z-RLVG-CHN 2 and cystatin C were persistent, whereas action induced initially by calcitonin was lost with time. The inhibition caused by Z-RLVG-CHN 2 and cystatin C on PTH-stimulated 45Ca release was observed after 6 h, whereas inhibition by calcitonin was seen already after 2 h. In contrast, the inhibitory effects of Z-RLVG-CHN 2 and cystatin C, as well as that of calcitonin, on 3H release was seen already after 2 h. Z-RLVG-CHN 2, in which the reactive carboxyterminal diazomethane was substituted by nonreactive groups [−OH, −NH 2, or −N(CH 3) 2], resulted in peptidyl derivatives, which, in contrast to Z-RLVG-CHN 2 and cystatin C, inhibited neither cysteine proteinases nor bone resorption. In contrast to wild-type cystatin C, recombinant human cystatin C with Gly substitutions for residues Arg 8, Leu 9, Val 10, and Trp 106, and with low or nonexistent affinity for cysteine proteinases, did not display any inhibitory effect on bone resorption. These data strongly indicate that Z-RLVG-CHN 2 inhibits bone resorption in vitro by a mechanism that seems primarily to be due to an inhibition of bone matrix degradation via cysteine proteinases. The data also corroborate the hypothesis that cystatin C inhibits bone resorption by virtue of its cysteine proteinase inhibitory capacity.