Breast cancer risk in BRCA1 mutation carriers: insight from mouse models.

Since its identification 20 years ago, the biological basis for the high breast cancer risk in women who have germline BRCA1 mutations has been an area of intense study for three reasons. First, BRCA1 was the first gene shown to associate with breast cancer risk, and therefore serves as model for understanding genetic susceptibility. Second, the type of breast cancer that occurs in these women has specific features that have engendered new hypotheses about the cancer biology. Third, it is hoped that understanding the origins of this disease may provide the means to prevent disease. Resolving this question has proven extremely challenging because the biology controlled by BRCA1 is complex. Our working model is that the high frequency of basal-like breast cancer in BRCA1 mutation carriers is the result of a self-perpetuating triad of cellular phenotypes consisting of: (i) intrinsic defects in DNA repair and centrosome regulation that lead to genomic instability and increases spontaneous transformation; (ii) aberrant lineage commitment; and (iii) increased proliferation due to in large part to increased IGF-1 activity. We propose that the last is key and is a potential entree for preventing breast cancer in BRCA1 mutation carriers.

Oral octreotide absorption in human subjects: comparable pharmacokinetics to parenteral octreotide and effective growth hormone suppression.

CONTEXT: Oral administration of a novel octreotide formulation enabled its absorption to the systemic circulation, exhibiting blood concentrations comparable to those observed with injected octreotide and maintaining its biological activity. OBJECTIVES: The aim of the study was to determine oral octreotide absorption and effects on pituitary GH secretion compared to sc octreotide injection. DESIGN: Four single-dose studies were conducted in 75 healthy volunteers. INTERVENTION: Oral doses of 3, 10, or 20 mg octreotide and a single sc injection of 100 µg octreotide were administered. MAIN OUTCOME MEASURE: We measured the pharmacokinetic profile of orally administrated octreotide and the effect of octreotide on basal and stimulated GH secretion. RESULTS: Both oral and sc treatments were well tolerated. Oral octreotide absorption to the circulation was apparent within 1 h after dose administration. Escalating oral octreotide doses resulted in dose-dependent increased plasma octreotide concentrations, with an observed rate of plasma decay similar to parenteral administration. Both 20 mg oral octreotide and injection of 0.1 mg sc octreotide resulted in equivalent pharmacokinetic parameters [mean peak plasma concentration, 3.77 ± 0.25 vs. 3.97 ± 0.19 ng/ml; mean area under the curve, 16.2 ± 1.25 vs. 12.1 ± 0.45 h × ng/ml); and median time ≥ 0.5 ng/ml, 7.67 vs. 5.88 h, respectively). A single dose of 20 mg oral octreotide resulted in basal (P < 0.05) and GHRH-stimulated (P < 0.001) mean GH levels suppressed by 49 and 80%, respectively. CONCLUSIONS: The results support an oral octreotide alternative to parenteral octreotide treatment for patients with acromegaly.

New roles and challenges in neurosurgery of acromegaly.

Acromegaly is a disease that shortens life expectancy (1-3) and causes severe systemic problems during life (4). It can arise and be recognized quickly if the onset is rapid, as in gigantism. Unfortunately there is usually a delay in diagnosis, on average 9 yr (4). The longer the delay the more likely patients are to develop partially or completely irreversible systemic problems, such as sleep apnea (5) and arthritis. Although some of the signs and co-morbidities of acromegaly are permanent, there is good evidence that cure of the disease reverses early mortality (3, 6, 7). It has been estimated that life expectancy is shortened by about 10 yr overall, and longer when diabetes or heart disease are already present at the time of diagnosis.

Diagnosis and treatment of acromegaly complications.

The Pituitary Society in conjunction with the European Neuroendocrine Association held a consensus workshop to develop guidelines for diagnosis and treatment of the co-morbid complications of acromegaly. Fifty nine pituitary specialists (endocrinologists, neurosurgeons and cardiologists) assessed the current published literature on acromegaly complications in light of recent advances in maintaining tight therapeutic control of GH hypersecretion. The impact of elevated GH levels on cardiovascular disease, hypertension, diabetes, sleep apnea, colon polyps, bone disease, reproductive disorders, and neuropsychologic complications were considered. Guidelines are proposed for effective management of these complications in the context of overall acromegaly control. When appropriate, requirements for prospective evidence-based studies and surveillance database development are enunciated. Effective management of co-morbid acromegaly complications will lead to improved morbidity and mortality in acromegaly.

Diagnosis and management of growth hormone deficiency in adults.

In adults, GHD is a clinical syndrome that occurs in patients with pituitary or hypothalamic disease. It may be asymptomatic or present with relatively nonspecific constitutional symptoms. Most patients have abnormal body composition, consisting of increased fat mass and decreased lean mass. Life expectancy is significantly decreased in hypopituitary patients with GHD, with cardiovascular disease a common cause of death. Treatment with growth hormone reverses abnormalities in body composition and may reduce cardiovascular risk factors; however, the long-term treatment outcomes regarding mortality, the incidence of cardiovascular disease, bone fractures, tumor development, and recurrence are not known. Longer prospective clinical studies are needed. The major manufacturers of growth hormone have initiated postmarketing surveillance databases to monitor the safety of growth hormone treatment.

Rheumatic manifestations of pituitary tumors.

Pituitary tumors may cause rheumatologic problems as a result of under production or overproduction of one pituitary hormone. Excessive growth hormone causes destruction of cartilage by a direct action. Facial and acral changes and arthralgias may be some of the first symptoms of acromegaly. The arthritis associated with acromegaly is often devastating. Carpal tunnel syndrome is very common in patients with acromegaly. Adrenocorticotropin (ACTH) has indirect effects via the action of glucocorticoid on bones, muscles, and the immune system. Proximal muscle weakness is a characteristic feature of Cushing's syndrome. Patients with Cushing's syndrome commonly have osteopenia and osteoporosis that lead to an increase in bone fractures. Avascular necrosis is associated with exogenous steroid administration. The effects of too much glucocorticoid or too rapid withdrawal can be severe. Gonadotropins act via the gonadal steroids and protect bone mass from loss. Prolactin is less involved in rheumatologic disease; the data for which are limited in humans. Pituitary tumors can have manifestations similar to rheumatologic disorders and should be included in the differential diagnosis of these diseases.

IGF-I: an essential factor in terminal end bud formation and ductal morphogenesis.

Growth hormone (GH) is essential for rodent mammary gland development during puberty. It binds to GH receptors in the stromal compartment of the mammary gland and stimulates IGF-I mRNA expression. These findings lead to the hypothesis that GH acts through locally produced IGF-I, which in turn, causes development of terminal end buds (TEBs), the structures that lead the process of mammary gland development during puberty. Subsequent studies have in large measure proven this hypothesis. They include the observations that mammary development was grossly impaired in female mice deficient in IGF-I (IGF-I(-/-) knockout mice), and treatment of these mice with IGF-I plus estradiol (E2) restored pubertal mammary development while treatment with GH + E2 did not. Thus, the full phenotypic action of GH in mammary gland development is mediated by IGF-I. We have demonstrated one effect of GH on the mammary gland that does not appear to be mediated by the action of IGF-I. GH increased the level of estrogen receptor (ER) mRNA and protein in the nuclei of mammary fat pad cells, but IGF-I did not. In addition to the critical role of the GH/IGF-I axis during pubertal mammary development, other data suggest that IGF-I might also be of importance during pregnancy and lactation. In summary, the earliest phase of pubertal mammary development (formation of TEBs) requires IGF-I or GH in IGF-I sufficient animals. No other hormones have been shown to stimulate formation of TEBs unless GH or IGF-I is present. GH-induced IGF-I is of major importance in ductal morphogenesis, and may, in fact, be necessary for later stages of mammary development, as well.

Treatment of acromegaly with the growth hormone-receptor antagonist pegvisomant.

BACKGROUND: Patients with acromegaly are currently treated with surgery, radiation therapy, and drugs to reduce hypersecretion of growth hormone, but the treatments may be ineffective and have adverse effects. Pegvisomant is a genetically engineered growth hormone-receptor antagonist that blocks the action of growth hormone. METHODS: We conducted a 12-week, randomized, double-blind study of three daily doses of pegvisomant (10 mg, 15 mg, and 20 mg) and placebo, given subcutaneously, in 112 patients with acromegaly. RESULTS: The mean (+/-SD) serum concentration of insulin-like growth factor I (IGF-I) decreased from base line by 4.0+/-16.8 percent in the placebo group, 26.7+/-27.9 percent in the group that received 10 mg of pegvisomant per day, 50.1+/-26.7 percent in the group that received 15 mg of pegvisomant per day, and 62.5+/-21.3 percent in the group that received 20 mg of pegvisomant per day (P<0.001 for the comparison of each pegvisomant group with placebo), and the concentrations became normal in 10 percent, 54 percent, 81 percent, and 89 percent of patients, respectively (P<0.001 for each comparison with placebo). Among patients treated with 15 mg or 20 mg of pegvisomant per day, there were significant decreases in ring size, soft-tissue swelling, the degree of excessive perspiration, and fatigue. The score fortotal symptoms and signs of acromegaly decreased significantly in all groups receiving pegvisomant (P< or =0.05). The incidence of adverse effects was similar in all groups. CONCLUSIONS: On the basis of these preliminary results, treatment of patients who have acromegaly with a growth hormone-receptor antagonist results in a reduction in serum IGF-I concentrations and in clinical improvement.

Clinical and reimbursement issues in growth hormone use in adults.

Data published in the past decade have demonstrated that adults who are deficient in growth hormone (GH) experience deleterious clinical consequences without treatment. In 1996, the Food and Drug Administration approved the use of GH in adults who were GH deficient as a result of hypothalamic or pituitary disease. However, there are other conditions in adults for which GH treatment has also been approved (acquired immune deficiency syndrome [AIDS]-related wasting) or for which it is being considered, such as aging, catabolic states, and cardiomyopathy. Clinical issues revolve around the rationale for treatment; the diagnostic evaluation; the effects of GH therapy on body composition, bone density, lipids, and cardiac function; and appropriate dosing and follow up. Clearly, the use of GH in adults raises reimbursement issues as well. In this article, Dr. Beverly M.K. Biller provides an overview of the rationale for the treatment of adult-onset GH deficiency and reviews its etiology and clinical features as well as reimbursement and utilization issues related to treatment. Dr. Mary Lee Vance discusses various assays and criteria used in the diagnostic evaluation of the patient with adult-onset GH deficiency. Dr. David L. Kleinberg focuses on the effects of GH therapy on body composition, bone density, lipid profiles, and cardiac function, as well as on reimbursement issues regarding body composition studies. To complete the clinical portion of this session, Dr. David M. Cook addresses dosing and follow up. To address economic implications, Dr. Terry Gordon provides the payer's perspective on the diagnosis and treatment of adult-onset GH deficiency.

The effect of GH on estrogen receptor expression in the rat mammary gland.

Both GH and insulin-like growth factor I (IGF-I) synergize with estrogen to induce normal mammary gland development. However, the nature of this synergy has not been explored. To gain insight into the mechanism of these interactions we have examined the effects of these substances on the estrogen receptor (ER). ER levels in the mammary gland cytosols from hypophysectomized and oophorectomized rats, were measured using two assay systems: a dextran-coated charcoal procedure to measure binding to radiolabeled steroid, and an immunologic assay employing a specific antibody to the receptor. In both assays, levels of ER were at or near baseline detection (approximately 1-2 ng/mg protein). Treating animals with either bovine or human GH significantly increased ER activity (P<0.001), whereas prolactin (PRL) and/or estradiol treatment had no effect. That this increase was at the level of transcription was demonstrated by reverse transcriptase/polymerase chain reaction. Following a single injection of GH (50 microgram), a substantial increase in ER mRNA was observed by 10 h, with levels returning to baseline within 24 h; a concomitant increase in ER itself was also observed at the 10 h time point. The effect of GH appeared to occur mainly in the mammary stroma, because there were no differences in GH stimulation of ER between gland-free and gland containing mammary fat pads. Furthermore, analysis of mammary gland ER by immunocytochemistry demonstrated that while ER was present in the epithelial cells of non-treated animals, only GH treated animals had ER clearly visible in both glandular and fat cells of the tissue. In contrast, treating animals with des(1-3)-IGF-I did not result in reproducible increases in ER, nor in the staining of fat cell nuclei for ER. These data demonstrate a specific GH effect on the ER in the mammary fat cell.

Insulin-like growth factor I is essential for terminal end bud formation and ductal morphogenesis during mammary development.

Previous studies from this laboratory have emphasized the essential role of GH in pubertal mammary development and shown that insulin-like growth factor I (IGF-I) was capable of substituting for GH in this process in rats and mice. The present study shows that, even when GH is present, no mammary development is possible unless IGF-I is present. IGF-I(-/-) null female animals were found to have significantly less mammary development than age matched wild-type controls (P <0.006) using several endpoints including the number of terminal end buds or TEBs (1.3 vs. 7.3), percent of the fat pad occupied by glandular elements (6.5 vs. 100), and number of ducts (15 vs. too numerous to count). That the deficiency in mammary gland development was related to the absence of IGF-I was underscored by the observation that des (1-3) IGF-I administration to IGF-I(-/-) null animals for 5 days caused significant mammary gland development as measured by TEB formation and branching of ducts. The number of TEBs rose from a mean of 1.3 in controls to 20.5 without added E2 (P < 0.009), and from 1.7 to 21 when des (1-3) IGF-I was given together with E2 (P < 0.006). The number of ducts increased significantly from a mean of 12 to 27 in response to IGF-I and E2, and from 15 to 24.5 with IGF-I alone. In contrast, administration of human GH with E2 had no stimulatory effect on mammary development in these animals, indicating that the full effect of GH in this process is mediated by IGF-I. To determine whether IGF-I was also responsible for further ductal morphogenesis, we administered des (1-3) IGF-I + E2 to the knockout animals for 14 days and compared the effects of this combination of hormones on mammary development with those observed after 5 days. We found that there was a significant increase from 5 to 14 days in the number of TEBs (mean: 21 vs. 41) and the area of the mammary fat pad occupied by glands (mean: 10 vs. 20%). There was elongation and thickening of the ducts which accounted for the increased area that was occupied by ductal structures. There was no significant increase in the number of ducts. However, there was the appearance of a large number of buds along the length of the ductal structures, suggesting the beginning of side branching. These results suggest that IGF-I, when given along with E2, is responsible for ductal morphogenesis.

Evidence that insulin-like growth factor I and growth hormone are required for prostate gland development.

Insulin-like growth factor I (IGF-I) has been implicated as a factor that may predispose one to prostate cancer. However, no specific relationship between IGF-I and prostate development or cancer in vivo has been established. To determine whether IGF-I was important in prostate development, we examined prostate architecture in IGF-I(-/-) null mice and wild-type littermates. Glands from 44-day-old IGF-I-deficient animals were not only smaller than those from wild-type mice, but also had fewer terminal duct tips and branch points and deficits in tertiary and quaternary branching (P < 0.0001), indicating a specific impairment in gland structure. Administration of des(1-3)-IGF-I for 7 days partially reversed the deficit by increasing those parameters of prostate development (P < 0.006). That IGF-I production probably mediates an effect of GH in this process was indicated by the observations that GH antagonist transgenic mice also had significantly impaired prostate development (P < 0.0002) and that bovine GH had no independent effect on stimulating prostate development in IGF-I null animals. The data indicate that IGF-I deficiency is the proximate cause of impaired prostate development and give credence to the idea that, like testosterone, GH and IGF-I may be involved in prostate cancer growth as an extension of a normal process.

Prolactin levels and adverse events in patients treated with risperidone.

Hyperprolactinemia is a common clinical disorder that may lead to sexual dysfunction or galactorrhea. It may arise from a variety of etiologies, including the use of antipsychotic agents, presumably because of a dopamine receptor blockade. This analysis was designed to characterize the relationship between risperidone, serum prolactin levels, and possible clinical sequelae. All data from randomized, double-blind studies of risperidone in patients with chronic schizophrenia were analyzed. The two largest studies (the North American and multinational trials) included 841 patients (259 women, 582 men) with paired prolactin level data and 1,884 patients (554 women, 1,330 men) with data on six adverse events possibly associated with increased prolactin levels (amenorrhea, galactorrhea, and decreased libido in women; erectile dysfunction, ejaculatory dysfunction, gynecomastia, and decreased libido in men). Both risperidone and haloperidol produced dose-related increases in plasma prolactin levels in men and women. Among women, the risperidone dose was not correlated with adverse events, nor were the adverse events correlated with endpoint prolactin levels. Among men, the incidence of adverse events was positively correlated with risperidone dose; however, at risperidone doses of 4 to 10 mg/day the incidence of adverse events was not significantly higher than that observed in patients receiving placebo. Furthermore, adverse events in men were unrelated to plasma prolactin levels. Risperidone-associated increase in serum prolactin levels was not significantly correlated to the emergence of possible prolactin-related side effects.

Octreotide as primary therapy for acromegaly.

The effects of octreotide (up to 5 yr) as primary treatment in 26 patients with acromegaly were compared with those in 81 patients with acromegaly who received octreotide as secondary or adjunctive therapy after previous surgery and/or pituitary radiation. These patients were part of a multicenter study that took place between 1989-1995. The study was divided into 3 phases beginning with a 1-month placebo-controlled treatment period followed by a 1-month washout period. In the second phase, patients were randomized to treatment with either 100 or 250 micrograms octreotide, sc, every 8 h for 6 months. Octreotide was then discontinued for 1 month and reinitiated at the lower dose for a total mean treatment duration of 39 months. The dose was titrated by each investigator to improve each patient's individual response, which included improvement in symptoms and signs of acromegaly as well as reduction of GH and insulin-like growth factor I (IGF-I) into the normal range. In the second phase of the study, in which patients were randomized to either 100 or 250 micrograms octreotide, three times daily, mean integrated GH and IGF-I concentrations after 3 and 6 months were equivalent in the primary and secondary treatment groups. During long term open label treatment, mean GH fell from 32.7 +/- 5.2 to 6.0 +/- 1.7 micrograms/L 2 h after octreotide injection in the primary therapy group and remained suppressed for a mean period of 24 months (range, 3-60 months). The mean final daily dose was 777 micrograms. In the patients receiving secondary treatment, mean GH fell from 30.2 +/- 7.6 to 5.6 +/- 1.1 micrograms/L after 3 months and remained suppressed for the remainder of the study (average dose, 635 micrograms daily). Mean IGF-I concentrations fell from 5.2 +/- 0.5 x 10(3) U/L (primary treatment group) and 4.7 +/- 0.4 x 10(3) U/L (secondary treatment group) to a mean of 2.2 +/- 0.3 x 10(3) U/L in both groups after 3 months of open label treatment and remained suppressed. IGF-I was reduced into the normal range during at least half of the study visits in 68% of the primary treatment group and in 62% of the secondary treatment group. Patients whose GH levels fell to at least 2 SD below the baseline mean GH were considered responders. There was no significant difference in the percentage of responders in the primary and secondary treatment groups (70% vs. 61%), nor was there a statistical difference in the mean GH concentrations between the groups. Symptoms of headache, increased perspiration, fatigue, and joint pain were reported at baseline by 46%, 73%, 69%, and 85%, respectively, of patients in the primary therapy group and improved during 3 yr of octreotide treatment in 50-100%. Similarly, these acromegaly-related symptoms were reported by 62%, 58%, 78%, and 60% of patients in the secondary therapy group, and improvement was noted in 62-88%. Pituitary magnetic resonance imaging scans were available in 13 of 26 patients in the primary treatment group before and after 6 months of octreotide treatment. Tumor shrinkage was observed in 6 of 13 patients, with reduction in tumor volume greater than 25% in only 3. Of 6 patients with documented tumor shrinkage, IGF-I was reduced into the normal range in 4 patients. Of the 7 remaining patients in whom tumor shrinkage was less than 10%, IGF-I was reduced into the normal range in 4 patients. Of the 7 remaining patients in whom tumor shrinkage was less than 10%, IGF-I was reduced into the normal range in 5 patients. The degree of tumor shrinkage did not correlate with the percent reduction in IGF-I or GH. In summary, octreotide was equally effective in 26 previously untreated acromegalic patients (primary treatment group) and 81 patients previously treated with either surgery or pituitary radiation (secondary treatment group). These observations call into question the current practice of surgical resection of all newly diagnosed GH-secreting pituitary adenomas regardless of the likelihood of cure. (AB

Role of IGF-I in normal mammary development.

Growth hormone (GH) is now believed to be the pituitary factor that is responsible for mammary ductal morphogenesis. Mammary development at puberty occurs because of synergy between GH and estrogen on formation of terminal end buds (TEBs). TEBs extend into the substance of the mammary gland fat pad, resulting in ductal morphogenesis. Ultimately, the whole mammary fat pad accommodates a complex network of ducts. IGF-I or des(1-3) IGF-I mimic the actions of GH on TEB formation in hypophysectomized, gonadectomized rats. Since GH stimulates IGF-I mRNA within the mammary gland synergistically, we hypothesize that IGF-I partially mediates actions of GH in mammary gland development. Studies in transgenic mice overexpressing IGF-I, des(1-3) IGF-I, or IGFBP-3 show that IGF-I causes ductal hypertrophy in the lactating mouse and prevention of post-lactational mammary gland involution. One of the mechanisms for this effect involves apoptosis. The potential role of GH or IGF-I in mammary carcinogenesis, and the applicability of animal studies to humans, are discussed.

Evidence that the mammary fat pad mediates the action of growth hormone in mammary gland development.

Recent evidence from our laboratory suggests that GH and insulin-like growth factor I (IGF-I) mediate glandular mammary development together with estrogen. It has also been well established that both stromal and epithelial elements must interact for mammary glandular development to occur. To determine whether the effect of GH is mediated by the stromal or epithelial tissue, we set up the following experiment. Bovine GH (bGH; 100 microg) or BSA (as a control), without or with estradiol (E2), was injected i.p. into sexually immature female rats that were hypophysectomized and oophorectomized. Mammary glands and subscapular fat pads were removed from the animals. The mammary glands were divided into two parts: a gland-free fat pad and remaining glandular tissue. The end point of bGH activity was induction of IGF-I messenger RNA (mRNA). This was determined quantitatively by solution hybridization and also by RT-PCR. We found that the effects of GH on stimulation of IGF-I mRNA in the gland-free mammary fat pad and the remainder of the mammary gland were similar (3.6- vs. 3.9-fold, respectively; P < 0.001). In both sorts of mammary tissue, bGH was found to synergize with E2 in the induction of IGF-I mRNA (5.8- vs. 5.3-fold; P < 0.001). There was also an increase in IGF-I mRNA in subscapular fat pads in response to 100 microg bGH (5.3-fold; P < 0.001); however, no synergism between bGH and E2 was found. These data indicate that bGH works as well on mammary stromal tissue as on tissue with glands and suggests that GH acts on the stromal compartment of the mammary gland to induce IGF-I mRNA and possibly IGF-I itself, which, in turn, causes differentiation of epithelial ducts into terminal end buds. These data also might explain why mammary epithelium is also able to differentiate in nonmammary fat pads when transplanted there.

Early mammary development: growth hormone and IGF-1.

The first step in pubertal mammary development is the appearance of terminal end buds arising from pleuropotent stem cells present in the immature ductal tree of the prepubertal animal. Work from this laboratory indicates that growth hormone is the pituitary hormone responsible for terminal end bud development. Growth hormone likely acts through the production of IGF-1. This minireview focuses on the hormonal control of early mammary development with special emphasis on the roles of growth hormone and IGF-1.

Estradiol enhances the stimulatory effect of insulin-like growth factor-I (IGF-I) on mammary development and growth hormone-induced IGF-I messenger ribonucleic acid.

Pubertal mammary development in the rat is largely dependent upon GH and estrogen. We recently showed that insulin-like growth factor-I (IGF-I) can substitute for GH in inducing mammary development in male rats, suggesting that IGF-I mediates GH action. The present study investigated whether IGF-I, like GH, required estradiol (E2) to act or whether IGF-I could substitute for both GH and E2. The effects of IGF-I were tested in the presence and absence of E2. Elvax pellets containing IGF-I or des(1-3) IGF-I were implanted into right lumbar mammary glands of sexually immature, hypophysectomized, oophorectomized female rats, with control BSA-containing pellets in the contralateral glands. After 5 days, both lumbar mammary glands were removed and examined in whole mounts for mammary development by counting terminal end buds and alveolar structures. E2, administered in SILASTIC brand capsules, had no independent effect on mammary development. In the absence of E2, des(1-3) IGF-I had a small, but significant, independent effect on mammary development; native IGF-I was ineffective. The addition of E2 significantly enhanced the effects of IGF-I and des(1-3) IGF-I on mammary development, similar to that noted when E2 was given along with GH. We also studied the effects of E2 and/or hGH on mammary gland IGF-I messenger RNA (mRNA) in hypophysectomized castrated male animals. E2 alone did not increase mammary gland IGF-I mRNA concentrations, but E2 enhanced the effect of hGH on IGF-I mRNA by 4- to 6-fold. These studies indicate that IGF-I can have a small independent effect on mammary development, but like GH, E2 is required for a full effect. They also indicate that E2 is capable of synergizing with GH in the production or expression of IGF-I mRNA, and that the action of E2 on mammary development may take place at multiple sites. If locally produced IGF-I does indeed mediate the action of GH in mammary development, then although E2 is capable of enhancing the effect of GH on IGF-I mRNA, its major effect in mammary development occurs after IGF-I is produced.