Regulation of insulin-like growth factor binding protein 4 by a specific insulin-like growth factor binding protein 4 proteinase in normal human osteoblast-like cells: Implications in bone cell physiology

Abstract

Insulin-like growth factor binding protein 4 (IGFBP-4) is secreted by normal human osteoblast-like cells (hOB) and is a potent inhibitor of insulin-like growth factor (IGF) action in vitro. In previous studies, IGF treatment of hOB in culture led to markedly reduced medium levels of IGFBP-4 as detected by western ligand blotting. In the present study, incubation of hOB-conditioned medium (hOB-CM) with IGF under cell-free conditions resulted in a similar loss of IGFBP-4. Both IGF-I and IGF-II were capable of inducing a decrease in IGFBP-4; however, IGF-II was more effective. When the six characterized IGFBP were added to hOB-CM, only IGFBP-4 disappeared in response to IGF-II addition. This IGF-regulated loss of IGFBP-4 was inhibited by metalloproteinase inhibitors and appeared to be due to a proteinase that cleaved IGFBP-4 in 18 and 14 kD fragments identified by western immunoblotting. Conditioned media from eight of eight different donor hOB lines tested exhibited IGFBP-4 proteinase activity. To assess the biologic consequences of IGF-II-induced IGFBP-4 proteolysis, we treated hOB with IGF-II for 5 h, which decreased medium IGF-BP-4 by 70%, and then measured IGF-I and insulin stimulation of [3H]thymidine incorporation. IGF-II itself was not mitogenic and had no effect on insulin-stimulated [3H]thymidine incorporation. However, pretreatment of cultured hOB with IGF-II enhanced IGF-I-stimulated [3H]thymidine incorporation threefold.(ABSTRACT TRUNCATED AT 250 WORDS)

Pregnancy-associated plasma protein-A (PAPP-A): A local regulator of IGF bioavailability through cleavage of IGFBPs

“…A signature characteristic of PAPP-A's interaction with IGFBP-4 is IGF-dependence [6]. In a cell-free assay, IGF is required for PAPP-A's proteolytic activity against IGFBP-4, with IGF-II generally being more effective than IGF-I [3,4,37]. IGF-II is not a cofactor for the enzyme, but rather its binding to IGFBP-4 renders the substrate more susceptible to proteolysis by PAPP-A [38].…”

“…IGFBP-4 is an inhibitory IGFBP and binds IGFs with high affinity, thus preventing their interaction with the IGF-I receptor, which mediates cell growth and survival signals. PAPP-A cleaves IGFBP-4 in the middle of the protein, markedly reducing its affinity for IGFs and, thus, ''freeing'' them for receptor binding and activation [4,6,7,[34][35][36]. A signature characteristic of PAPP-A's interaction with IGFBP-4 is IGF-dependence [6].…”

“…However, PAPP-A 1 found clinical use as a marker in screening for Down's syndrome pregnancies [2]. In the early 1990s several laboratories reported novel protease activity against insulin-like growth factor binding protein-4 (IGFBP-4) in media conditioned by several cell types [3][4][5]. This activity was unique in that it appeared to require the presence of IGF-I or IGF-II for cleavage of IGFBP-4 to occur.…”

Human matrix metalloproteinases: An ubiquitarian class of enzymes involved in several pathological processes

“…PAPPA seems to be a critical determinant of growth and development through the proteolysis of insulin-growth-factor-binding proteins (IGFBPs), which bind IGFI and IGFII, thus preventing their interaction with IGF receptors (Oxvig, 2001). PAPPA-mediated IGFBP cleavage ''releases'' IGF for receptor activation, modulating the local availability of IGF (Byun et al, 2001;Durham et al, 1994;Laursen et al, 2000). Emerging preclinical, clinical and histopathological evidences sustain that PAPPA may serve as a marker of cardiovascular risk, reflecting the atherosclerotic plaque instability (Bayes-Genis et al, 2001;Lund et al, 2003;Consuegra-Sanchez et al, 2009a).…”

Insulin-like growth factor-binding proteins and bone metabolism

“…1B). An IGFBP-4 protease produced by cultured cells, including osteoblasts, was shown to cleave IGFBP-4 mid-molecule and potentiate the effectiveness of exogenous IGF-stimulated growth (17,25,10). Studies using wild-type and protease-resistant IGFBP-4 provided evidence that IGFBP-4 proteolysis may be important for bone formation in vitro and in vivo (31,39).…”