The recent availability of reagents to study the IGFs, their receptors, and binding proteins has led to an explosive growth in the study of IGF physiology. However, most studies to date have been descriptive, and studies delineating mechanisms of action are limited. It is apparent that most organ systems synthesize several components of the IGF system necessary for IGF to function in an autocrine or paracrine fashion and that regulation of this system occurs at the local level. However, the relative importance of locally produced IGF vs circulating IGF remains unclear. The mechanisms by which the IGFBPs modulate IGF activity are crucial to understanding this system, and identification of specific roles for each of these proteins will be required. Of critical importance is the identity of the intracellular signal transduction system by which the IGF receptor mediates the effects of the IGFs, and the delineation of mechanisms by which the IGFBPs interact with the receptor at the cellular level. It is also of interest to determine what role, if any, the IGF-II receptor plays in mediating the growth-promoting effects of the IGFs. The ubiquitous distribution of the IGFs, IGFBPs, and IGF receptors indicates that they may play a role in the regulation of coordinate growth among several tissues and cell types. Understanding the mechanisms by which these components interact to coordinate growth responses between different cell types should greatly enhance our understanding of normal growth and development.

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Phytoecdysteroids, structurally similar to insect molting hormones, produce a range of effects in mammals, including increasing growth and physical performance. In skeletal muscle cells, phytoecdysteroids increase protein synthesis. In this study we show that in a mouse skeletal muscle cell line, C2C12, 20-hydroxyecdysone (20HE), a common phytoecdysteroid in both insects and plants, elicited a rapid elevation in intracellular calcium, followed by sustained Akt activation and increased protein synthesis. The effect was inhibited by a G-protein Coupled Receptor (GPCR) inhibitor, a phospholipase C (PLC) inhibitor, and a phosphoinositide kinase-3 (PI3K) inhibitor.

Background Altered fetal programming due to a suboptimal in utero environment has been shown to increase susceptibility to many diseases later in life. This study examined the effect of alcohol exposure in utero on N-nitroso-N-methylurea (NMU)-induced mammary cancer risk during adulthood. Methods Study 1: Pregnant Sprague-Dawley rats were fed either a liquid diet containing 6.7% ethanol (alcohol-fed), an isocaloric liquid diet (pair-fed), or rat chow ad libitum (ad lib-fed) from day 11 to 21 of gestation. At birth, female pups were cross-fostered to ad lib-fed control dams. Adult offspring were given an I.P. injection of NMU at a dose of 50 mg/kg body weight. Mammary glands were palpated for tumors twice a week and rats were euthanized at 23 weeks post-injection. Study 2: To investigate the role of estradiol (E2), animals were exposed to the same in utero treatments, but were not given NMU. Serum was collected during the pre-ovulatory phase of the estrous cycle. Results At 16 weeks post-injection, overall tumor multiplicity was greater in the offspring from the alcohol-fed group compared to the control groups, indicating a decrease in tumor latency. At study termination 70% of all animals possessed tumors. Alcohol-exposed animals developed more malignant tumors and more estrogen receptor-α negative tumors relative to the control groups. In addition, IGF binding protein-5 (IGFBP-5) mRNA and protein were decreased in tumors of alcohol-exposed animals. Study 2 showed that alcohol-fed animals had significantly increased circulating E2 when compared to either control group. Conclusions These data indicate that alcohol exposure in utero increases susceptibility to mammary tumorigenesis in adulthood and suggest that alterations in the IGF and E2 systems may play a role in the underlying mechanism.

Vascular smooth muscle cells (SMC) synthesize insulin-like growth factor-I (IGF-I), which is a mitogen for this cell type in vitro. Since IGF binding proteins (IGFBP) modulate IGF bioactivity, we determined which IGFBPs were secreted by porcine SMC. Porcine SMC secreted 34,000 and 24,000 M(r) forms of IGFBPs which were identified as IGFBP-2 and IGFBP-4, respectively, by immunoblotting. Northern blot analysis showed single transcripts of 1.6 kb and 2.4 kb for IGFBP-2 and IGFBP-4, respectively. Secretion of IGFBP-2 was not regulated to a significant degree, with insulin, IGF-II, IGF-I, forskolin, and dibutyryl cyclic adenosine monophosphate (cAMP) inducing minimal changes in IGFBP-2 secretion of less than 30% by radioimmunoassay (RIA). Insulin increased (2.8 +/- 0.1-fold) the abundance of IGFBP-4 protein in conditioned media (CM) and increased IGFBP-4 mRNA levels. Growth factors for SMC such as platelet-derived growth factor (PDGF), fibroblast growth factor (FGF), epidermal growth factor (EGF), and transforming growth factor beta-1 (TGF beta-1) were without effect on either IGFBP-2 or -4. IGF-I treatment decreased the amount of IGFBP-4 present in CM, but a corresponding decrease in IGFBP-4 mRNA levels was not observed. In order to determine if IGFBP-4 could modulate IGF-I bioactivity, IGFBP-4 was added to pSMCs with and without IGF-I. IGF-I alone (20 ng/ml) induced a 1.6 to threefold increase in 3H-thymidine incorporation. Addition of IGFBP-4 (between 50 and 250 ng/ml) to cultures containing IGF-I (20 ng/ml) had no effect on DNA synthesis compared to that observed with IGF-I alone, while 500 ng/ml consistently caused a small decrease (15 +/- 5%; mean +/- SE). Immunoblotting of the CM obtained at the end of the 3H-thymidine assay showed a loss of intact IGFBP-4 in the cultures containing IGF-I. This corresponded with an increase in the abundance of a 16,000 M(r) immunoreactive fragment that did not bind IGF-I. Coincubation with insulin had no effect on the amount of IGFBP-4 that was converted to fragment, suggesting that the reaction was dependent upon IGF-I binding to IGFBP-4. In contrast, addition of IGFBP-4 (500 ng/ml) to human fibroblast cultures with IGF-I (20 ng/ml) almost completely inhibited the stimulatory effect of IGF-I on DNA synthesis and no increase in fragment was detected in the CM. In summary, SMC secrete IGFBP-2 and IGFBP-4, both of which have been shown to regulate IGF-mediated DNA synthesis.(ABSTRACT TRUNCATED AT 400 WORDS)