title: Regulation of insulin-like growth factor-binding protein-4 protease activity by estrogen and parathyroid hormone in SaOS-2 cells: implications for the pathogenesis of postmenopausal osteoporosis.
abstract: The cellular mechanisms involved in the accelerated bone loss occurring in association with estrogen deprivation as seen following the menopause are not fully understood. Insulin-like growth factor-I (IGF-I) is the local regulator of osteoblasts and one of its binding proteins, insulin-like growth factor-binding protein-4 (IGFBP-4), binds to IGF-I and suppresses biological activity. Previous studies have shown that the binding activity of IGFBP-4 in the conditioned medium of parathyroid hormone (PTH)-treated SaOS-2 osteoblastic-like cells is enhanced twofold and that this PTH-enhanced IGFBP-4 binding activity is abolished by 17 beta-estradiol. Levels of IGFBP-4 in the conditioned medium have been reported to be regulated not only at the level of production but also at the level of degradation which is catalyzed by a protease that specifically cleaves IGFBP-4. We have, therefore, studied the effects of 17 beta-estradiol and PTH on IGFBP-4 protease activity using SaOS-2 cells. SaOS-2 cells produce a protease that specifically cleaves IGFBP-4 into two fragments of approximately 18 and 14 kilodaltons. IGFBP-4 protease activity in the conditioned medium from PTH-treated cells was suppressed, while this PTH-induced suppression of protease activity was reversed by the addition of 17 beta-estradiol to the cultures. IGFBP-4 proteolytic activity was stimulated by IGF-I or IGF-II added exogenously and was inhibited by EDTA or protease inhibitors. IGFBP-4 proteolyzed in the conditioned medium from cells treated with PTH and 17 beta-estradiol was less effective at inhibiting IGF-I-stimulated [3H]thymidine incorporation into DNA compared with that proteolyzed in the conditioned medium from PTH-treated cells. The simplest explanation is that 17 beta-estradiol suppressed the inhibitory effect of PTH on osteoblastic activity by inhibiting the PTH-induced suppression of IGFBP-4 protease activity.
authors.affiliation: Department of Obstetrics and Gynecology, Tokyo Women's Medical College, Japan.
authors.forename: Y
authors.initials: Y
authors.lastname: Kudo
authors.forename: M
authors.initials: M
authors.lastname: Iwashita
authors.forename: S
authors.initials: S
authors.lastname: Itatsu
authors.forename: T
authors.initials: T
authors.lastname: Iguchi
authors.forename: Y
authors.initials: Y
authors.lastname: Takeda
mesh_terms: D002460:Cell Line; D004247:DNA / Q000096:biosynthesis; D003864:Depression, Chemical; D004958:Estradiol / Q000494:pharmacology*; D005260:Female; D006801:Humans; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology; D007335:Insulin-Like Growth Factor II / Q000494:pharmacology; D008666:Metalloendopeptidases / Q000378:metabolism*; D008875:Middle Aged; D010006:Osteoblasts / Q000187:drug effects / Q000201:enzymology* / Q000378:metabolism; D015663:Osteoporosis, Postmenopausal / Q000209:etiology* / Q000378:metabolism; D010281:Parathyroid Hormone / Q000494:pharmacology*; D011266:Pregnancy-Associated Plasma Protein-A; D013936:Thymidine / Q000378:metabolism
chemical_list: D010281:Parathyroid Hormone; D004958:Estradiol; D007334:Insulin-Like Growth Factor I; D007335:Insulin-Like Growth Factor II; D004247:DNA; D008666:Metalloendopeptidases; D011266:Pregnancy-Associated Plasma Protein-A; D013936:Thymidine