Faculty Bibliography

Insulin-like growth factors (IGFs) play a crucial role in regulating cell proliferation and differentiation. The aim of this study was to examine the potential relationship between serum IGF-I levels and breast cancer risk. To do this, we studied liver-specific IGF-I gene-deleted (LID) mice, in which circulating IGF-I levels are 25% of that in control mice. Mammary tumors were induced in two ways: (a) by exposing mice to the carcinogen 7,12-dimethybenz (a)anthracene; and (b) by crossing LID mice with C3(1)/SV40 large T-antigen transgenic mice. In both models, LID mice exhibited a delayed latency period of mammary tumor development. In the 7,12-dimethybenz (a)anthracene-induced mammary tumor model, the incidence of palpable mammary tumors was significantly lower in LID mice (26% versus 56% in controls), and the onset of the tumors was delayed (74 +/- 1.2 days in LID mice versus 59.5 +/- 1.1 days in controls). Histological analysis showed extensive squamous metaplasia in late-stage mammary tumors of control mice, whereas late-stage tumors from LID mice exhibited extensive hyperplasia, but little metaplasia. In control mice, the onset of C3(1)/SV40-large T-antigen-induced mammary tumors occurred at 21.6 +/- 1.8 weeks of age, whereas in LID mice the average age of onset was 30.2 +/- 1.7 weeks. In addition, 60% of the mice in the control group developed two or more mammary tumors per mouse, whereas in the LID mice only 30% developed more than one mammary tumor per mouse. Our data demonstrate that circulating IGF-I levels play a significant role as a risk factor in the onset and development of mammary tumors in two well-established animal models of breast cancer.

Abnormalities in insulin action are the characteristics of type 2 diabetes. Dominant-negative muscle-specific IGF-I receptor (MKR) mice exhibit elevated lipid levels at an early age and eventually develop type 2 diabetes. To evaluate the role of elevated lipids in the progression of the diabetic state, MKR mice were treated with WY14,643, a peroxisome proliferator-activated receptor (PPAR)-alpha agonist. WY14,643 treatment markedly reduced serum fatty acid and triglyceride levels within a few days, as well as muscle triglyceride levels, and subsequently normalized glucose and insulin levels in MKR mice. Hyperinsulinemic-euglycemic clamp analysis showed that WY14,643 treatment enhanced muscle and adipose tissue glucose uptake by improving whole-body insulin sensitivity. Insulin suppression of endogenous glucose production by the liver of MKR mice was also improved. The expression of genes involved in fatty acid oxidation was increased in liver and skeletal muscle, whereas gene expression levels of hepatic gluconeogenic enzymes were decreased in WY14,643-treated MKR mice. WY14,643 treatment also improved the pattern of glucose-stimulated insulin secretion from the perfused pancreata of MKR mice and reduced the beta-cell mass. Taken together, these findings suggest that the reduction in circulating or intracellular lipids by activation of PPAR-alpha improved insulin sensitivity and the diabetic condition of MKR mice.

Liver-specific IGF-I gene deficient (LID) mice exhibit pancreatic islet hyperplasia and insulin resistance. To clarify their causal relationship, we studied age-dependent changes in these two aspects and the response to beta-cell damage caused by streptozotocin in adult mice. As a result, the onset of insulin resistance in LID mice was detectable as early as 1-month of age, while hyperinsulinemia was developed after a significant delay at 2.5-month. Upon streptozotocin administration, control mice exhibited significant hyperglycemia after 9 days, and glucose levels continued to rise at 12-15 days. LID mice developed diabetes much more rapidly, with hyperglycemia after 6 days and higher glucose levels up to 15 days. They also exhibited significant weight loss and 6/19 died. Serum insulin assay, insulin mRNA analysis and immunohistochemistry revealed that the more severe diabetes in LID mice was not due to more damage to their beta-cells. Thus LID mice are more sensitive to streptozotocin-induced beta-cell damage, due to a primary defect in insulin responsiveness. The pancreatic islet hyperplasia observed in these mice seems to represent a compensatory response to insulin resistance, therefore, offers no protection against beta-cell damage.

The insulin-like growth factor system (IGF) has been linked to the process of bone acquisition through epidemiologic analyses of large cohorts and in vitro studies of bone cells. But the exact relationship between expression of IGF-I in bone and skeletal homeostasis or pathologic conditions, such as osteoporosis, remains poorly defined. Recent advances in genomic engineering have resulted in the development of better in vivo models to test the role of IGF-I during development and maintenance of the adult skeleton. It is now established that skeletal expression of IGF-I is critical for differentiative bone cell function. It may also be essential for the full anabolic effects of parathyroid hormone on trabecular bone and for some component of biomineralization. Evidence from conditional mutagenesis studies suggests that serum IGF-I may represent more than a storage depot or permissive factor during the final phase of skeletal acquisition. This work re-examines the original tenets of the "somatomedin hypothesis" in light of these newer mouse models and their remarkable skeletal phenotypes. The implications are far reaching and suggest that newer approaches for manipulating the IGF regulatory system may one day be useful as therapeutic adjuncts for the treatment of osteoporosis.

Insulin-like growth factors (IGF-I and IGF-II) play a crucial role in regulating cell proliferation and differentiation. The IGFs have mitogenic and antiapoptotic effects on normal and transformed cells. These peptide growth factors are produced by virtually all tissues and act in an endocrine, autocrine, and paracrine fashion. The endocrine form of IGF-I originates mostly (75%) from the liver and IGF-binding proteins regulate its bioactivity. Compared to other peptide growth factors, the IGFs are in abundant supply in circulation. The role of this large reservoir of IGFs has been debated for many years. In the last few years substantial progress has been made in understanding the function of the endocrine IGF-I using new animal models. This review will revisit the IGF system with particular attention to the role of circulating IGF-I in growth regulation, metabolism, and cancer.

IGF-1 is a growth-promoting polypeptide that is essential for normal growth and development. In serum, the majority of the IGFs exist in a 150-kDa complex including the IGF molecule, IGF binding protein 3 (IGFBP-3), and the acid labile subunit (ALS). This complex prolongs the half-life of serum IGFs and facilitates their endocrine actions. Liver IGF-1-deficient (LID) mice and ALS knockout (ALSKO) mice exhibited relatively normal growth and development, despite having 75% and 65% reductions in serum IGF-1 levels, respectively. Double gene disrupted mice were generated by crossing LID+ALSKO mice. These mice exhibited further reductions in serum IGF-1 levels and a significant reduction in linear growth. The proximal growth plates of the tibiae of LID+ALSKO mice were smaller in total height as well as in the height of the proliferative and hypertrophic zones of chondrocytes. There was also a 10% decrease in bone mineral density and a greater than 35% decrease in periosteal circumference and cortical thickness in these mice. IGF-1 treatment for 4 weeks restored the total height of the proximal growth plate of the tibia. Thus, the double gene disruption LID+ALSKO mouse model demonstrates that a threshold concentration of circulating IGF-1 is necessary for normal bone growth and suggests that IGF-1, IGFBP-3, and ALS play a prominent role in the pathophysiology of osteoporosis.

Nutritional status is a critical factor that modulates the responsiveness of the liver to GH and the resulting production of endocrine (mostly liver-derived) IGF-I. Using a conditional Cre/lox P system, we have established a liver-specific IGF-I-deficient mouse model. Despite the reduction in the circulating IGF-I (75%), the growth parameters are normal, except for the reduced spleen size, providing a unique model to study the effect of protein restriction on the autocrine/paracrine GH/IGF-I axis. To determine the effects of protein calorie malnutrition on the spleen, liver-specific IGF-I-deficient mice were assigned to one of four isocaloric diets, differing in the protein content (20, 12, 4, and 0%), for a period of 10 d. A low protein intake decreased the nonhepatic IGF-I secretion into the circulation, whereas it caused an increase in the level of circulating GH. This supports the view that nonhepatic IGF-I production contributes to circulating IGF-I levels. The lack of dietary protein led to an up-regulation of GH and IGF-I receptors expression in the spleen, whereas the IGF-I mRNA remained unchanged, as was demonstrated by flow cytometry and ribonuclease protection assay. B lymphocytes seem to be responsible for the up-regulated GH/IGF-I receptor expression. Northern blot analysis showed an up-regulation of IGF-binding protein-3 mRNA levels, which suggests that the protein deprivation may lead to an increased sequestration of circulating or locally synthesized IGF-I. These results support the hypothesis that the splenic GH/IGF-I axis responds to the nutritional stress caused by a low protein intake, to maintain the tissue homeostasis.

Insulin-like growth factor-I (IGF-I) is essential for normal growth; igf-1 gene mutations are associated with extreme growth retardation in mice and, very rarely, in humans. The relative contributions of tissue vs. endocrine (hepatic) IGF-I to the regulation of growth has been a fundamental question. New gene targeting technologies are providing answers for these questions.

Ghrelin activates GH release and is implicated in growth and metabolic regulation. The regulation of its biosynthesis has not been well studied. The current investigation was designed to examine some of the factors that may influence ghrelin gene expression in the stomach. Thus, in C57BL/6 mice, ghrelin mRNA was detectable by Northern blots throughout the age groups studied, but the levels changed markedly over time. Levels were low at E18.5 and increased rapidly after birth to 6-fold at P14 before peaking to 8-fold at P21. The levels then exhibited a gradual decline at P60 (75% of the peak level) and at 6 months (67%) and a drastic decrease as the animals aged to 19 months (only 5%). Furthermore, sexual dimorphic gene expression, the effect of liver-derived IGF-I deficiency, as well as ghrelin secretion were studied. Our results support a role of ghrelin in growth/metabolism in juvenile and young adult mice of both sexes and in sexually dimorphic regulation of GH secretion in aged mice.

It has been shown previously that slight elevations in serum levels of insulin-like growth factor-I (IGF-I) are correlated with an increased risk for developing prostate, breast, colon, and lung cancer. The aim of this study was to determine the role of serum IGF-I levels in the process of stimulating tumor growth and metastasis in a mouse model of colon cancer. Colon 38 adenocarcinoma tissue fragments were orthotopically transplanted by attachment to the surface of the cecum in control and liver-specific IGF-I-deficient (LID) mice in which serum IGF-I levels are 25% of that in control mice. A total of 156 male mice at 5 weeks of age (74 control mice and 82 LID mice) received tumor transplants. Mice were divided randomly into two groups; one group was injected i.p. with recombinant human IGF-I (2 mg/kg) twice daily for 6 weeks, and the other group received saline injections. IGF-I treatment increased the serum levels of IGF-I and IGFBP-3 in both control and LID mice. In the saline-injected group, the incidence of tumor growth on the cecum as well as the frequency of hepatic metastasis was significantly higher in control mice as compared with LID mice. Both control and LID mice treated with recombinant human IGF-I displayed significantly increased rates of tumor development on the cecum and metastasis to the liver, as compared with saline-injected mice. The number of metastatic nodules in the liver was significantly higher in control mice as compared with LID mice. The expression of vascular epithelial growth factor (VEGF) as well as vessel abundance in the cecum tumors was dependent on the levels of serum IGF-I. This study supports the hypothesis that circulating IGF-I levels play an important role in tumor development and metastasis.