Cheryl A. Conover

Insulin-like growth factor binding protein-1: recent findings and new directions.

Abstract In 1988, insulin-like growth factor-binding protein-1 (IGFBP-1) became the first characterized member of a group of structurally related soluble proteins which specifically bind and modulate the actions of the IGFs. Since then, a wealth of information has accumulated regarding the physiology of this dynamic serum protein. In this review, we update our 1993 summary (Lee PDK et al. Proc Soc Exp Biol Med 204:4-29) of the status of IGFBP-1 research. The IGFBP-1 protein sequence contains 12 N-terminal and 6 C-terminal cysteine residues which are conserved in other mammalian IGFBP-1 sequences and amongst other IGFBPs; both of the cysteine-rich regions are required for optimal IGF binding. The nonconserved IGFBP-1 midregion may act as both a hinge which defines ligand binding characteristics and as a specific target for protease activity. Integrin-binding and phosphorylation sites within the IGFBP-1 sequence have functional significance in vitro, but their physiologic relevance in vivo have not been defined. The human IGFBP-1 and IGFBP-3 genes are contiguous and located in close proximity to the homeobox A (HOXA) gene cluster on chromosome 7. The other IGFBP genes, located on chromosomes 2, 12, and 17, are also associated with HOX clusters, suggesting evolutionary linkage of the IGFBP and HOX gene families. Similarities between the hlGFBP-1 and phosphoenolpyruvate kinase (PEPCK) promoters, including regions conferring insulin, glucocorticoid, and cyclic adenosine-monophosphate responses, are consistent with our previous hypothesis that IGFBP-1 is involved in regulation of glucose metabolism. The tissue-specific patterns of IGFBP-1 gene expression in liver, kidney, decidua, and ovary may be due to stimulation of IGFBP-1 transcription by hepatic nuclear factor 1 (HNF1) proteins. Clinical and basic studies of IGFBP-1 physiology have been aided by several recently developed assay methods. Numerous investigations have confirmed that insulin, via inhibition of IGFBP-1 transcription, is the primary determinant of IGFBP-1 expression both in vitro and in viva IGF-I and IGF-II also have specific inhibitory effects on IGFBP-1 expression. Glucocorticoids and cAMP stimulate IGFBP-1 transcription, but these effects are observed only in conditions of low or absent insulin effect. Other stimulants of IGFBP-1 expression include thyroid hormones and epidermal growth factor. Phorbol ester stimulation of IGFBP-1 expression can supersede the effects of insulin in vitro; however, the mechanism and in vivo correlates of this effect have not been determined. Cytokines and, perhaps, growth hormones may affect IGFBP-1 expression, perhaps by altering the regulatory actions of insulin; this effect may have important clinical relevance. IGFBP-1 expression is upregulated in liver and (nonhuman) kidney during postinjury regeneration. The IGF-inhibitory actions of IGFBP-1 has been confirmed by numerous in vitro studies and several in vivo animal investigations, including administration of recombinant IGFBP-1 and IGFBP-1 transgenic models. IGFBP-1 has been shown to inhibit somatic linear growth, weight gain, tissue growth, and glucose metabolism. Moreover, IGFBP-1 appears to be a primary determinant of free IGF-I levels in serum. Excess levels of IGFBP-1 may contribute to growth failure in intrauterine growth restriction and in pediatric chronic renal failure, while low IGFBP-1 levels are associated with obesity and with cardiovascular risk factors in insulin resistance syndromes. Serum IGFBP-1 measurements may be useful biochemical marker in these pathologic conditions. IGFBP-1 is expressed in decidualized stromal cells of the uterine endometrium and in ovarian granulosa cells. IGFBP-1, together with IGFs, insulin, ovarian steroids, cytokines, and other factors, is involved in a complex system which regulates menstrual cycles, ovulation, decidualization, blastocyst implantation, and fetal growth. Models for the role of IGFBP-1 in female reproductive physiology are presented, and evidence for pathophysiologic roles in pre-eclampsia, polycystic ovarian syndrome, and uterine malignancy are reviewed. Very recent data indicates that IGFBP-1 undergoes regulated expression in human osteoblasts. Limited information also suggests that IGFBP-1 may be present in peripheral neurons, and that serum IGFBP-1 may increase during exercise and in critical ilIness. In summary, two major roles for IGFBP-1 in normal physiology can be constructed from current data: (i) As an “endocrine” factor, IGFBP-1 regulates the bio-availability of serum IGF-I, thereby modulating IGF-mediated tissue metabolism. The dominant regulation of IGFBP-1 expression by meal-related changes in hepatic insulin concentrations provides a dynamic link to substrate availability. (ii) As an autocrine/paracrine factor, IGFBP-1 appears to play a crucial role in the female reproductive system and, in particular, the sequence of events leading from ovulation to implantation to successful fetal outcome. Future investigations will further delineate the manner in which IGFBP-1 participates in these and other physiologic processes, and the mechanisms by which IGFBP-1 may be involved in clinical pathophysiology.

Insulin-like growth factor binding proteins from cultured human fibroblasts. Characterization and hormonal regulation

Specific, high affinity insulin-like growth factor (IGF) binding proteins are secreted by human fibroblasts in culture. By multiple criteria, the species of IGF binding proteins produced by human fibroblasts are distinct from the HepG2/amniotic fluid IGF binding protein, but share many characteristics with the growth hormone-dependent IGF binding protein forms predominant in normal adult human plasma. Treatment of cultured human fibroblasts with growth hormone produced an increase in IGF binding protein activity in the medium, while addition of glucocorticoids markedly diminished IGF binding activity. Insulin, epidermal growth factor, platelet-derived growth factor, and progesterone had no effect on IGF binding activity in fibroblast media. In comparison, HepG2 IGF binding activity was enhanced by progesterone, decreased by insulin, and unaffected by growth hormone or glucocorticoid treatment. Five molecular forms of IGF binding proteins were identified by Western ligand blots in human fibroblast conditioned medium, with Mr = 41,500, 37,000, 32,000, 28,000, and 23,000. In human fibroblast conditioned medium, the Mr = 41,500 and 37,000 IGF binding protein species were abundant, as in normal human plasma, with a major Mr = 23,000 form which was a minor component in plasma.

Effect of an anti-insulin-like growth factor I receptor antibody on insulin-like growth factor II stimulation of DNA synthesis in human fibroblasts

To investigate the role of insulin-like growth factor II in the control of DNA synthesis in human fibroblasts, dose-response curves for insulin-like growth factor I and II stimulation of [3H]thymidine incorporation were compared in the absence and presence of alpha IR-3, a highly specific monoclonal antibody directed against the type I insulin-like growth factor receptor. Specific binding of [125I]insulin-like growth factor I to human fibroblast monolayer cultures was inhibited 60-70% in the presence of alpha IR-3. alpha IR-3 had no effect on [125I]insulin-like growth factor II binding to human fibroblasts. However, alpha IR-3 inhibited both insulin-like growth factor I and II stimulated [3H]thymidine incorporation. These data indicate that the type II insulin-like growth factor receptor does not function as a transducer of insulin-like growth factor IIs mitogenic effect in human fibroblasts.

Serum glucocorticoids have persistent and controlling effects on insulinlike growth factor i action under serum-free assay conditions in cultured human fibroblasts

SummaryThe biological actions of insulinlike growth factor I (IGF-I) measured under serum-free assay conditions were found to be significantly influenced by prior subculture conditions for adult human fibroblasts. Glucocorticoids seemed to be the major medium variable affecting IGF-I action. IGF-I added to serum-free cultures had little or no effect on [14C]aminoisobutyric acid uptake or [3H]thymidine incorporation in human fibroblasts previously maintained in media containing serum with low glucocorticoid levels or in serum stripped of endogenous steroids. However, a 48-h preincubation with dexamethasone resulted in a marked synergistic increase in IGF-I stimulation of [14C]aminoisobutyric acid uptake and [3Hthymidine incorporation in these cultures. In contrast, IGF-I in serum-free medium seemed to be a potent mitogenic and metabolic stimulus for human fibroblasts which had been subcultured in media with a high glucocorticoid content, either endogenous, or supplemented. After these culture conditions, a 48-h preexposure to dexamethasone had no further enhancing effect on IGF-I action. Dexamethasone also potentiated IGF-I, insulin, and epidermal growth factor stimulation of fibroblast replication depending on the earlier subcultivation conditions. Thus, glucocorticoids are important modulators of IGF-I bioactivity in cultured human fibroblasts. Serum glucocorticoids can exert a profound influence on the biological phenomena measured in cell culture, even when the serum has been removed before the actual experiment, and must be carefully taken into account for accurate evaluation of the biological function of IGF-I and other growth factors.

Impaired synergism between somatomedin C/Insulin-like growth factor i and dexamethasone in the growth of fibroblasts from a patient with insulin resistance

ABSTRACT. To explore a possible mechanism for the diminished growth potential in a patient with an unusual form of insulin resistance, somatomedin C/insulin-like growth factor I (SM-C/IGF-I) and insulin stimulation of [3H]thymidine incorporation and cell replication were compared in skin fibroblasts from the patient (DF) and normal controls. There appeared to be no generalized abnormality in cellular responsiveness to growth factors. In both DF and control cells, SM-C/IGF-I (50 ng/ml), insulin (100 ng/ml), and epidermal growth factor (5 ng/ml) stimulated [3H]thymidine incorporation 5-, 2-, and 6-fold, respectively. Low concentrations of human hypopituitary serum (0.25%) enhanced the effectiveness of SM-C/IGF-I and insulin to a similar extent in both DF and control cells. On the other hand, 10% calf serum stimulated [3H]thymidine incorporation 37-fold in control cells, while DF cells were only 50% as responsive. Preincubation of control cells with dexamethasone (10-7 M) caused a marked synergistic increase in SM-C/IGF-I stimulated [3H]thymidine incorporation (15- to 20-fold in serum-free medium; 50- to 80-fold in 0.25% human hypopituitary serum). In contrast, preexposure to dexamethasone did not augment SM-C/IGF-I stimulation of thymidine incorporation into DNA of DF cells. Furthermore, the stimulation of cell replication by SM-C/IGF-I and insulin was potentiated by dexamethasone in control but not DF cultures. These data suggest that impairment of the synergistic action of glucocorticoids with SM-C/IGF-I and insulin regulation of fibroblast growth may be involved in the pathology of this insulinresistant growth disorder.