The Insulin-Like Growth Factors (IGFs) and IGF Binding Proteins in Postnatal Development of Murine Mammary Glands

The insulin-like growth factors are mitogens and survival factors for normal mammary epithelialcells in vitro. Data reviewed here demonstrate that mRNAs for IGF-I and IGF-II, the IGFtype I receptor and the IGFBPs are expressed locally in mammary tissue during pubertal andpregnancy-induced growth and differentiation of murine mammary glands. IGF-I, IGF-II andthe IGF-IR were expressed in terminal end buds (TEBs) in virgin glands during ductal growth.In addition, IGF-II and IGF-IR mRNAs were expressed in ductal and alveolar epithelium inglands throughout postnatal development. Consistent with these results, IGF-I promoted ductalgrowth and proliferation in mouse mammary glands in organ culture. In addition to endogenousexpression of the IGFs and IGF-IR, the IGFBPs showed a varied pattern of expression inmammary tissue during postnatal development. For example, IGFBP-3 and -5 mRNAs wereexpressed in TEBs and ducts while IGFBP-2 and -4 mRNAs were expressed in stromal cellsimmediately surrounding the epithelium. These results support a role for the IGFs and IGFBPsas local mediators of postnatal mammary gland growth and differentiation.     

Circulating insulin-like growth factor-I and benign prostatic hyperplasia--a prospective study.

OBJECTIVE: The aim of this study was to investigate the role of insulin-like growth factor-I (IGF-I), a strongly mitogenic and anti-apoptotic factor, in the development of benign prostatic hyperplasia (BPH). The bioactivity of IGF-I within tissues depends on circulating levels, as well as on the local production of IGF-I and the presence of IGF-binding proteins (IGFBPs). The IGFBPs regulate the efflux of IGF-I to the extravascular space and the bioavailability of IGF-I within tissues. MATERIAL AND METHODS: Within the Northern Sweden Health and Disease Study, 60 cases of BPH defined by a history of prostate resection were identified, and two controls per case were selected. IGF-I, IGFBP-1, IGFBP-3 and insulin were measured by immuno-radiometric assays in stored plasma samples drawn a mean of 3.2 years before surgery. RESULTS: The risk of BPH increased with increasing quartile levels of IGF-I adjusted for IGFBP-3 (p(trend) = 0.10) up to a relative risk of 2.16 (95% confidence interval 0.83-5.64) for the highest quartile. The risk decreased with increasing levels of IGFBP-1 (p(trend) = 0.10). CONCLUSIONS: Our results suggest that elevated IGF-I bioactivity may stimulate the development of BPH; however, they were not statistically significant and require confirmation from larger studies.     

Insulin-like growth factors and risk of benign prostatic hyperplasia

BACKGROUND: Insulin-like growth factors (IGFs) have potent growth mitogenic and anti-apoptotic effects on prostate tissue, whereas IGF binding proteins (IGFBPs) inhibit growth of prostatic tissue. The IGF axis has been implicated in prostate cancer risk, but its role in benign prostatic hyperplasia (BPH) is unclear. METHODS: Plasma levels of IGF-I, IGF-II, IGFBP-1, and IGFBP-3 were determined from the fasting bloods of 206 BPH cases admitted for treatment and 306 randomly selected population controls in Shanghai, China. RESULTS: Relative to the lowest tertile, men in the highest tertile of IGF-I levels had a significantly elevated risk of BPH (odds ratio [OR] = 2.80, 95% confidence interval [95% CI] = 1.60-4.92; P(trend) < 0.001). Results for IGF-I were more pronounced after adjustment for serum androgens. In contrast, men in the highest IGFBP-3 tertile had a significantly reduced risk (OR = 0.40; 95% CI = 0.23-0.69; P(trend) < 0.001). No associations of BPH with IGF-II and IGFBP-1 were observed. CONCLUSION: As has been previously observed for prostate cancer, we found that IGF-I and IGFBP-3 are associated with BPH risk in China. Further investigation is needed to elucidate the role of the IGF axis in BPH etiology.     

Circulating levels of insulin-like growth factor binding protein 3 (IGFBP-3) and insulin-like growth factor I (IGF-I) in perimenopausal women

The study investigated possible menopause-related changes in circulating insulin-like growth factor binding protein 3 (IGFBP-3) levels and their relationship with insulin-like growth factor I (IGF-I) plasma levels. Forty-three healthy women, aged 45–55 years, were studied (22 premenopausal and 21 postmeno-pausal, matched for age and body mass index); in all subjects plasma IGF-I and IGFBP-3 levels were measured by radioimmunoassay. No difference was found between mean IGFBP-3 plasma levels in the two groups studied (premenopausal 3.42±0.49 v postmenopausal 3.46±0.58 mg/l), while mean IGF-I levels were significantly lower in postmenopausal as compared with premenopausal women (136.7±37.86 v 175.7±51.91 ng/ml,p<0.02). Multiple regression analysis showed no significant effect of age, body mass index and years since menopause on IGFBP-3 levels; however, considering the IGF-I/IGFBP-3 ratio as a possible parameter of circulating free somatomedin C, an inverse correlation was found with years since menopause (n=43,r=–0.499,p<0.001). We conclude that lack of oestrogen induces different effects on circulating IGF-I and IGFBP-3, possibly reflecting a real decrease in IGF-I activity.     

Evaluation of IGF system component levels and mitogenic activity of uremic serum on normal human osteoblasts

To test the hypothesis that impairment in bone formation in renal osteodystrophy in adults with chronic renal failure (CRF) might be mediated in part by alterations in circulating levels of the insulin-like growth factor (IGF) system components, we compared serum levels of IGF-I, IGF-II, IGF-binding protein (IGFBP)-3, IGFBP-4 and IGFBP-5 in adults with CRF (CRF patients with parathyroid hormone (PTH) < 100 pg/ml, PTH > 300 pg/ml and end-stage renal failure (ESRF) patients) versus age-matched controls. To evaluate the biological significance of alterations in circulating level of IGF system components, we compared the mitogenic activity of the sera on proliferation of normal human osteoblasts in vitro by using [(3)H]thymidine incorporation. We found severalfold increased serum levels of IGFBP-3 (2-fold), IGFBP-4 (5-fold) and slightly increased IGF-II levels in ESRF patients as well as a 2.6-fold increase in free IGF-I in CRF patients with PTH < 100 pg/ml. The mitogenic activity was found to be increased in serum of kidney failure patients compared to controls. This was most pronounced in CRF patients with PTH < 100 pg/ml showing also a significant increase in free IGF-I and the lowest levels of the IGF-inhibitory IGFBP-4. Our data support the hypothesis that alterations in serum levels of stimulating (i.e. free IGF-I) and inhibitory IGF system components (i.e. IGFBP-4) may influence osteoblastic cell proliferation in renal osteodystrophy.     

Serum insulin-like growth factor-I and insulin-like growth factor-binding protein-3 in localized, metastasized prostate cancer and benign prostatic hyperplasia

OBJECTIVE: Insulin-like growth factors (IGF-I and IGF-II) are important mitogenic peptides and are thought to be significant factors involved in normal and malignant cellular proliferation including benign prostatic hyperplasia (BPH) and prostate cancer (PC). In particular, the association between IGF-I and PC has received much attention. Insulin-like growth factor binding protein-3 (IGFBP-3) is the major carrier protein in serum for the IGF-I, thus is an important functional modulator of it. On the other hand, one of the functions of prostate-specific antigen (PSA) is to cleave IGFBP-3. Epidemiological studies have shown that decreased levels of serum IGFBP-3 are associated with increased PC risk. Controversial results have also been reported on the value of serum IGF-I and/or IGFBP-3 in the detection of PC, especially of metastatic PC; as increased, decreased or unchanged when compared to BPH. The aim of the present study was to investigate whether serum IGF-I and IGFBP-3 levels change in localized and metastasized PC cases compared with BPH as the control cases. METHOD: The study included 45 BPH, 24 localized PC and 19 metastasized PC cases. Serum IGF-I and IGFBP-3 levels were measured by two-site immunoradiometric assay kits, and serum total and free PSA levels were assayed by chemiluminescence method. RESULTS: Serum IGF-I levels in both localized and metastasized PC cases were similar to BPH cases (138.3 +/- 58.2, 137.7 +/- 39.0 and 147.7 +/- 44.2 ng/ml, respectively), whereas serum IGFBP-3 levels were lower in metastasized PC group than in BPH group (1,795.6 +/- 305.6 and 2,196.0 +/- 505.7 ng/ml; p = 0.005). In localized PC, serum IGFBP-3 levels (1,911.00 +/- 349.58 ng/ml) were similar to metastasized PC. There were significant correlations between serum IGFBP-3 and serum free PSA in three groups (r = -0.46, p = 0.02 for localized PC; r = -0.56, p = 0.01 for metastasized PC, and r = -0.31, p = 0.03 for BPH). CONCLUSION: These data reveal that serum IGF-I levels may not change either in localized or metastasized PC, and that decreased serum IGFBP-3 levels may be attributed to its proteolysis by PSA which is increased in PC.     

The role of IGF-I and IGFBP-1 status and secondary hyperparathyroidism in relation to osteoporosis in elderly Swedish women

Summary IGFBP-1 showed a strong inverse relation to the BMD values. The IGF-I values had a significant positive relation to the BMD values at all sites with the exception of the lumbar spine. The use of loop diuretics was a more important cause of secondary hyperparathyroidism than vitamin D status. Introduction Our aim was to investigate among elderly women the relationship to osteoporosis of calcium-regulating hormones and insulin-like growth factor-I (IGF-I) and insulin-like growth factor binding protein-1 (IGFBP-1). Methods A population-based cross-sectional study of 350 elderly women (mean age 73 years). Measurements of bone mineral density (BMD) of the left hip, lumbar spine and heel and risk markers for osteoporosis were studied. Results The BMD values showed significant inverse relationship with the values of IGFBP-1 at all sites of measurement and significant positive relationship with the values of IGF-I at all sites with the exception of the lumbar spine. There was no significant association between the values of BMD and the values of 25-hydroxy vitamin D (25(OH)D). The use of loop diuretics was strongly and significantly associated with elevated levels of PTH >65 pg/ml (OR 4.4, P < 0.001). Conclusions The anabolic growth factor IGF-I and its modulating binding protein IGFBP-1 showed a stronger association with the BMD values than the calcium regulating hormones 25(OH)D and PTH. In this study the use of loop diuretics was a more important cause of secondary hyperparathyroidism than vitamin D status.     

IGF and Insulin Action in the Mammary Gland: Lessons from Transgenic and Knockout Models

Transgenic and knockout mice have become valuable experimental systems with which tostudy specific molecular events within the mammary gland of an intact animal. These modelshave provided a wealth of information about the effects of a number of oncogenes and growthfactors. This review focuses on results obtained from the application of transgenic and knockoutmodels to determine the roles of insulin and insulin-like growth factors (IGF)³ in the regulationof mammary gland development, lactation and tumorigenesis. Transgenic models whichoverexpress IGF-I or -II display specific alterations in mammary gland development and an increasedincidence of mammary tumors. Analysis of mammary gland development in knockout micewhich are deficient in IGF-I or the IGF-I receptor supports the conclusion that the IGF systemis important for normal mammary gland development. This review discusses these observationsin detail and attempts to fit them into a larger picture of IGF and insulin action in themammary gland.     

Insulin-Like Growth Factor I Is a Determinant of Hip Bone Mineral Density in Men Less Than 60 years of Age: MINOS Study

Several studies show that in elderly men bone mineral density (BMD) is not correlated with the insulin-like growth factor (IGF-I) level, but data are scanty in young men. Results of studies correlating insulin-like growth factor binding protein 3 (IGFBP-3) and BMD in men are discordant. As different hypotheses can explain the discordant results, we evaluated the correlation of BMD with serum IGF-I, IGFBP-3, and IGF-I/IGFBP-3 index in a large cohort of 721 men aged 19–85 years taking into account age, body weight, 17-estradiol, free testosterone, and parathyroid hormone. Serum IGF-I and IGFBP-3 decreased with age (r = –0.44 and r = –0.36, P = 0.0001). After adjustment for confounding variables, IGF-I correlated weakly positively with BMD and with bone mineral apparent density (BMAD) of hip as well as with cortical thickness of femoral neck, both of which are determined mainly by bone resorption, but not with bone size determined by periosteal apposition. IGF-I correlated weakly positively with BMD at the whole body and at the third lumbar vertebra IGFBP-3 and IGF-I/IGFBP-3 index did not correlate with densitometric parameters. In men aged 19–60 years, IGF-I correlated with BMD and BMAD of total hip and with cortical thickness of femoral neck positively and more strongly than in the entire cohort but not with the size of proximal femur. BMD of total hip was 6% higher in men in the highest quartile of IGF-I than in men in the lowest quartile. IGF-I, IGFBP-3, and IGF-I/IGFBP-3 index did not correlate with densitometric parameters of other sites. In the men aged more than 60 years, neither IGF-I nor IGFBP-3 nor IGF-I/IGFBP-3 index correlated with BMD, BMAD, or bone size. In men aged 19–60 years, the most significant hormonal determinants of BMD and BMAD of the hip and of the cortical thickness of femoral neck were 17-estradiol and IGF-I (P < 0.05–0.0001). In men aged more than 60 years, the most significant determinants of hip BMD were 17-estradiol and PTH. In conclusion, IGF-I seems to contribute to the inhibition of bone resorption and to maintaining bone mass of the proximal femur during the phase of slow bone loss in men aged less than 60 years. IGFBP-3 and IGF-I/IGFBP-3 index were not correlated with BMD or bone size. Supported by a contract with INSERM/Merck Sharp & Dohme Chibret, Paris, France     

SERUM LEVELS OF INSULIN-LIKE GROWTH FACTOR BINDING PROTEIN-3 IN BENIGN AND MALIGNANT BREAST DISEASE

Background : The insulin-like growth factors IGF-I and IGF-II and their major binding protein IGFBP-3 influence the growth of breast cancer cells in vitro. Some benign non-breast tumours appear to be associated with increased serum IGFBP-3 levels which would tend to reduce bioactive-free IGF concentrations. The present study investigates whether this pattern also occurs in neoplastic breast disease. Methods : Serum IGF-I, IGF-II and IGFBP-3 were measured by specific radioassay in 12 women with benign breast disease, 31 patients with breast cancer and in age-matched controls. Results : The mean (± SD) serum IGFBP-3 concentration was higher in benign breast disease (3.6 ± 0.7 mg/L) than in controls (2.7 ± 0.6 mg/L) or in breast cancer patients (2.7 ± 0.5 mg/L) (P = 0.001). Serum IGF-I and IGF-II levels were not significantly different among the groups. However, the index of free unbound IGF measured as the molar ratio of IGF-I plus IGF-II divided by IGFBP-3 was significantly lower in benign breast disease than in the other subjects. Conclusions : Either the production or clearance of IGFBP-3 is altered in benign breast disease so that there is less free IGF available to cells. This may serve to protect against malignant transformation in patients with benign breast disorders.     

Serum insulin-like growth factor binding protein-1 levels and bone mineral density in older adults: The Rancho Bernardo Study

Insulin-like growth factor-I (IGF-I) stimulates osteoblast function, inhibits collagen matrix degradation, and is positively associated with bone mineral density in most but not all studies. We previously reported that IGF-I was positively associated with BMD at the spine and hip in women but not men. Insulin-like growth factor binding protein-1 (IGFBP-1), a potent modulator of IGF-I, is expressed in normal osteoblasts and inhibits collagen gene expression in bone, but little is known about the relationship between IGFBP-1 and bone mineral density (BMD). We report a cross-sectional study of serum IGFBP-1 levels and BMD in 1,139 community-dwelling men and postmenopausal women (not using estrogen), aged 44–98. In both sexes, IGFBP-1 levels increased linearly with age (p<0.001) and decreased with body mass index (BMI) quartile (p<0.001). After adjusting for age and BMI, there was no significant association between IGFBP-1 and BMD at the hip or spine. IGF-I and IGFBP-1 were weakly and inversely associated with each other. These findings suggest that if there is an important role for IGFBP-1 in bone metabolism, it is mediated or confounded by weight. Studies are needed to further investigate the relationship between inhibitory components of IGF-1 and bone loss.     

Specific growth factor activity identifies and predicts murine mammary tumor

In milk from substrains of mice with varying incidences of developing mammary tumor, we have isolated a specific mitogenic activity, which is unique in that it identifies mice with mammary tumor and predicts those mice that will eventually develop mammary tumor. None of the milk samples from control mice, who never developed mammary tumor, contained this specific predictive mitogenic activity. Chemical characterization has shown this specific mitogenic activity to be acid- and heat-stable and resistant to reducing agents. Partial purification, by ion-exchange, high-performance liquid chromatography size-exclusion, and isoelectrofocusing techniques, of this specific mitogenic activity from milk of mice that had or eventually developed mammary tumor identifies several peptide growth factor components in a 6-10 kDa molecular weight range. Of known growth factors, radioassay techniques identify an insulin-like growth factor-1-like peptide as a major component. Small amounts of platelet-derived growth factor and transforming growth factor-beta activities also were present. Our results suggest that a subset of growth factors that are diagnostic of the presence of murine mammary tumor and predictive of eventual tumor development may be early indicators of the transition of a competent cell to a progressively malignant state. Similar studies of a secreted body fluid from women at risk for breast cancer may lead to the identification of a specific biologic tumor marker for breast cancer.     

Selective osteoblast mitogens can be extracted from prostatic tissue

Extracts of human prostatic carcinoma and of human benign prostatic hyperplasia were found to exhibit mitogenic activity in freshly isolated rat osteoblasts, rat fibroblasts, and osteoblast-derived osteosarcoma cells. This activity was markedly reduced by tryptic digestion of extracts, indicating its protein nature. Results of bioassay studies suggested that the prostatic material was different from a number of known growth factors. Analysis by reversed-phase high performance liquid chromatography revealed that extracts contained mitogenic activity for osteoblasts that was distinct from fibroblast-stimulating activity. Such osteoblast growth factors could form the basis of the skeletal osteogenic response to metastatic prostatic adenocarcinoma.     

EGF-Related Peptides in the Pathophysiology of the Mammary Gland

Normal mammary gland development is the result of complex interactions between a number of hormones and growth factors. Normal and malignant human mammary epithelial cells are able to synthesize and to respond to various different, locally acting growth factors and growth inhibitors. Among these, the EGF-related peptides play an important role in regulating the proliferation and differentiation of human mammary epithelial cells. EGF⁴ and TGF are able to stimulate the lobulo-alveolar development of the mammary gland in vivo as well they are involved in the pathogenesis of human breast cancer. Experimental evidence suggests that estrogen-induced proliferation of breast carcinoma cells is mediated in part by EGF-related growth factors. It has also been demonstrated that activation of certain cellular protooncogenes such as c-Ha-ras in human mammary epithelial cells results in cellular transformation and in an increased production of several EGF-related growth factors such as TGF and amphiregulin. Coexpression of both EGF-related peptides and their own receptors frequently occurs in human breast carcinomas and in human breast cancer cell lines, suggesting that an autocrine pathway of uncontrolled cell growth sustains neoplastic transformation.     

Insulin-like growth factors and their binding proteins in prostate cancer: Cause or consequence?☆

Insulin-like growth factors (IGFs) promote growth and survival of many types of tumor cells. Epidemiologic studies have implicated carcinogenesis with high levels of IGFs in circulation or in tissues. The levels of IGF binding proteins (IGFBPs) have been associated with reduced risk for prostate and other cancers. Experimental studies have implicated high levels of IGF-I directly and IGFBP-3 inversely in prostate cancer growth, survival, and progression. However, recent evidence suggests a much weaker association of IGF-I with prostate cancer development and a stronger antagonistic association of IGFBP-3 with prostate cancer progression. Considering the clonal heterogeneity and unpredictable progression pattern of prostate cancer, the role of any single growth factor or its regulator (IGFBP) as a single determining factor is limited. This review is a critical appraisal of the role of IGFs, IGFBP, and IGF-I receptor (the IGF axis) in both experimental and clinical prostate cancer genesis and progression.     

Growth factor and sex steroid interactions in breast cancer

Mitogenic and inhibitory growth factors and steroid ovarian hormones play important roles as selective modulators of normal mammary development and in the onset and the progression of human breast cancer. The focus of this article is to review past and current research on the interactions of these two classes of effectors in mammary gland development and neoplasia. Steroid hormones regulate synthesis of growth stimulatory and inhibitory growth factors, growth factor receptors, and growth factor binding proteins. In turn, growth factor pathways may modulate phosphorylation and function of steroid receptors and potentiate or inhibit the mitogenic actions of steroids. Ultimately, during the progression of the malignant mammary epithelial cell to hormonal autonomy, overexpression, mutation, or disregulation of key elements of growth factor signal transduction pathways all may play critical roles.     

Growth hormone stimulates proliferation and differentiation of normal human osteoblast-like cells in vitro

Summary In this study we investigated the direct, shortterm effects of human growth hormone (hGH) on the biology of normal adult human osteoblast-like (hOB) cells cultured from trabecular bone explants. In Subconfluent cultures, hGH stimulated hOB proliferation in a dose-dependent fashion (P<0.001, n=15) with half-maximal effects at a concentration of 10 ng/ml. These mitogenic effects were detectable within 24 hours as shown by bromodeoxyuridine labeling. In confluent cultures containing mainly quiescent cells, hGH increased levels of alkaline phosphatase (P<0.05, n=10) and to a lesser degree levels of procollagen type I carboxyterminal propeptide (PICP) (P=0.07, n=9). Effects on osteocalcin (bone GLa protein, BGP) levels were highly variable among different cell strains and only 7 of 10 cell strains showed a stimulatory response (P=0.16). We also studied the effects of hGH on osteoblastic production of insulin-like growth factor I (IGF-I) and IGF-II as well as the production of GH-dependent, insulin-like growth factor binding protein 3 (IGFBP-3). Under basal conditions, human osteoblasts produced IGF-II and IGFBP-3 in the conditioned medium. When stimulated with hGH, minor insignificant increase in both IGF-II and IGFBP-3 (125% and 126% of control, respectively) were detectable. No IGF-I was detectable in the conditioned medium under basal conditions or after stimulation with hGH. In conclusion, the results obtained in this study suggest that GH exerts direct anabolic effects on human osteoblasts.     

The IGF/IGFBP system in relation to macroscopic bone architecture in pediatric renal transplant patients

The post-transplant bone disease of the peripheral skeleton in pediatric renal transplant recipients is characterized by an inadequately thin bone cortex in relation to muscular force. A major hormonal modulator of periosteal growth is the insulin-like growth factor (IGF)/IGF binding protein (IGFBP) system. We therefore hypothesized that the reduced cortical thickness in these patients may be due to functional IGF deficiency. To test this hypothesis, we investigated 55 patients (mean estimated glomerular filtration rate 86.3 ± 30.0 ml/min/1.73 m²) in a cross-sectional study. Parameters of macroscopic bone architecture and forearm muscle size were analyzed by peripheral quantitative computed tomography (pQCT), and serum IGF/IGFBP system components were measured by specific radioimmunoassays. The mean (± standard deviation) standardized serum IGF-I (0.20 ± 1.16 score) level was normal, while the mean IGF-II (1.16 ± 0.11 score) level was significantly elevated. Serum IGFBP-1 and IGFBP-2 levels were not altered, whereas the IGFBP-3 (1.34 ± 0.15 score) level was significantly increased. The serum IGFBP-4 level was slightly elevated (by 11%), the IGFBP-6 level was markedly (2.3-fold) elevated, while the IGFBP-5 level was comparable to that of the control. The respective age-adjusted cortical thickness at both the proximal (r = 0.407, P < 0.005) and distal (r = 0.383, P < 0.01) forearm was positively correlated with the standardized serum IGF-I level. In conclusion, the serum IGF/IGFBP system in pediatric renal transplant recipients is characterized by an increase in the levels of the inhibitory IGFBPs, IGFBP-3, -4 and -6, resulting in a functional IGF deficiency. The positive correlation of IGF-I with cortical thickness underlines the importance of this hormonal system in the modeling of bone, particularly periosteal growth.     

Insulin-Like Growth Factor (IGF) System in the Bovine Mammary Gland and Milk

Primary bovine mammary cells express the two IGF receptors (IGF-IR, IGF-IIR),⁴ insulinreceptor, and four IGFBPs (IGFBP-2, -3, -4, and -5). Examination of the IGF-IR during themammary gland lactation cycle shows that IGF-IR number declines at parturition, a changethat coincides with decreases in the blood level of its ligand, IGF-I. IGF-II and IGF-IIR arelargely unchanged. IGFBP-3 is the predominant mammary IGFBP and its concentration alsodeclines in blood and milk during lactation compared to prepartum and involution periods.Time of lactation and pregnancy were the main determinants of milk but not bloodIGFBP-3 levels. IGFBP-3 binds to membrane proteins of bovine mammary tissue; an IGFBP-3 bindingprotein has been identified as bovine lactoferrin. Lactoferrin has the capacity to compete withIGF binding to IGFBP-3. Appearance of both IGFBP-3 and lactoferrin in conditioned mediaof primary cultures of bovine mammary cells was stimulated by all trans retinoic acid (atRA).Furthermore, atRA was necessary for the entry of exogenously added lactoferrin into themammary cell nucleus, while IGFBP-3 entry into the nuclei of atRA treated cells requiredthe presence of lactoferrin. These findings reveal a novel role for lactoferrin, suggesting thatlactoferrin is critically involved in the regulation of the IGF system during the involution period.