Faculty Bibliography

The discovery of the growth hormone (GH)-mediated somatic factors (somatomedins), insulin-like growth factor (IGF)-I and -II, has elicited an enormous interest primarily amongst endocrinologists who study growth and metabolism. The advancement of molecular endocrinology over the past four decades enables investigators to re-examine and refine the established somatomedin hypothesis. Specifically, gene deletions, transgene overexpression, or more recently, cell-specific gene-ablations, have enabled investigators to study the effects of the Igf-I and Igf-II genes in temporal and spatial manners. The GH/IGF axis, acting in an endocrine and autocrine/paracrine fashion, is the major axis controlling skeletal growth. Studies in rodents have clearly shown that IGFs regulate bone length of the appendicular skeleton evidenced by changes in chondrocytes of the proliferative and hypertrophic zones of the growth plate. IGFs affect radial bone growth and regulate cortical and trabecular bone properties via their effects on osteoblast, osteocyte, and osteoclast function. Interactions of the IGFs with sex steroid hormones and the parathyroid hormone demonstrate the significance and complexity of the IGF axis in the skeleton. Finally, IGFs have been implicated in skeletal aging. Decreases in serum IGFs during aging have been correlated with reductions in bone mineral density and increased fracture risk. This review highlights many of the most relevant studies in the IGF research landscape, focusing in particular on IGFs effects on the skeleton.

The growth of cancer metastases in the liver depends on a permissive interaction with the hepatic microenvironment and neutrophils can contribute to this interaction, either positively or negatively, depending on their phenotype. Here we investigated the role of IGF-I in the control of the tumor microenvironment in the liver, using mice with a conditional, liver-specific, IGF-I deficiency (iLID) induced by a single tamoxifen injection. In mice that had a sustained (3 weeks) IGF-I deficiency prior to the intrasplenic/portal inoculation of colon carcinoma MC-38 cells, we observed an increase in neutrophil accumulation in the liver relative to controls. However, unlike controls, these neutrophils did not acquire the (anti-inflammatory) tumor-promoting phenotype, as evidenced by retention of high ICAM-1 expression and nitric oxide production and low CXCR4, CCL5, and VEGF expression and arginase production, all characteristic of the (pro-inflammatory) phenotype. This coincided with an increase in apoptotic tumor cells and reduced metastasis. Neutrophils isolated from these mice also had reduced IGF-IR expression levels. These changes were not observed in iLID mice with a short-term (2 days) IGF-I depletion, despite a 70% reduction in their circulating IGF-I levels, indicating that a sustained IGF-I deficiency was necessary to alter the neutrophil phenotype. Similar results were obtained with the highly metastatic Lewis lung carcinoma subline H-59 cells and in mice injected with an IGF-Trap that blocks IGF-IR signaling by reducing ligand bioavailability. Our results implicate the IGF axis in neutrophil polarization and the induction of a pro-metastatic microenvironment in the liver.

Laron syndrome (LS), or primary growth hormone (GH) insensitivity, is the best-characterized entity among the congenital insulin-like growth factor 1 (IGF1) deficiencies. Life-long exposure to minute endogenous IGF1 levels is linked to low stature as well as a number of endocrine and metabolic abnormalities. While elevated IGF1 is correlated with increased cancer incidence, epidemiological studies revealed that patients with LS do not develop tumors. The mechanisms associated with cancer protection in LS are yet to be discovered. Recent genomic analyses identified a series of metabolic genes that are overrepresented in patients with LS. Given the augmented expression of these genes in a low IGF1 milieu, we hypothesized that they may constitute targets for IGF1 action. Thioredoxin-interacting protein (TXNIP) plays a critical role in cellular redox control by thioredoxin. TXNIP serves as a glucose and oxidative stress sensor, being commonly silenced by genetic or epigenetic events in cancer cells. Consistent with its enhanced expression in LS, we provide evidence that TXNIP gene expression is negatively regulated by IGF1. These results were corroborated in animal studies. In addition, we show that oxidative and glucose stresses led to marked increases in TXNIP expression. Supplementation of IGF1 attenuated TXNIP levels, suggesting that IGF1 exerts its antiapoptotic effect via inhibition of TXNIP Augmented TXNIP expression in LS may account for cancer protection in this condition. Finally, TXNIP levels could be potentially useful in the clinic as a predictive or diagnostic biomarker for IGF1R-targeted therapies.

Hepatic osteodystrophy is multifactorial in its pathogenesis. Numerous studies have shown that impairments of the hepatic growth hormone/insulin-like growth factor-1 axis (GH/IGF-1) are common in patients with non-alcoholic fatty liver disease, chronic viral hepatitis, liver cirrhosis, and chronic cholestatic liver disease. Moreover these conditions are also associated with low bone mineral density (BMD) and greater fracture risk, particularly in cortical bone sites. Hence, we addressed whether disruptions in the GH/IGF-1 axis were causally related to the low bone mass in states of chronic liver disease using a mouse model of liver-specific GH-receptor (GHR) gene deletion (Li-GHRKO). These mice exhibit chronic hepatic steatosis, local inflammation, and reduced BMD. We then employed a crossing strategy to restore liver production of IGF-1 via hepatic IGF-1 transgene (HIT). The resultant Li-GHRKO-HIT mouse model allowed us to dissect the roles of liver-derived IGF-1 in the pathogenesis of osteodystrophy during liver disease. We found that hepatic IGF-1 restored cortical bone acquisition, microarchitecture, and mechanical properties during growth in Li-GHRKO-HIT mice, which was maintained during aging. However, trabecular bone volume was not restored in the Li-GHRKO-HIT mice. We found increased bone resorption indices in vivo as well as increased basal reactive oxygen species and increased mitochondrial stress in osteoblast cultures from Li-GHRKO and the Li-GHRKO-HIT as compared to control mice. Changes in systemic markers such as inflammatory cytokines, osteoprotegrin, osteopontin, parathyroid hormone, osteocalcin, or carboxy-terminal collagen crosslinks, could not fully account for the diminished trabecular bone in the Li-GHRKO-HIT mice. Thus, the reduced serum IGF-1 associated with hepatic osteodystrophy is a main determinant of low cortical but not trabecular bone mass.

Aging is associated with dysregulation of glucose and lipid metabolism. Various factors that contribute to the dysregulation include both modifiable (e.g. obesity, insulin resistance) and non-modifiable risk factors (age-associated physiologic changes). Although there is no linear relationship between aging and prevalence of non-alcoholic fatty liver disease, current data strongly suggests that advanced age leads to more severe histological changes and poorer clinical outcomes. Hepatic lipid accumulation could lead to significant hepatic and systemic consequences including steatohepatitis, cirrhosis, impairment of systemic glucose metabolism and metabolic syndrome, thereby contributing to age-related diseases. Insulin, leptin and adiponectin are key regulators of the various physiologic processes that regulate hepatic lipid metabolism. Recent advances have expanded our understanding in this field, highlighting the role of novel mediators such as FGF 21, and mitochondria derived peptides. In this review, we will summarize the mediators of hepatic lipid metabolism and how they are altered in aging.

Eph receptors belong to a subfamily of receptor tyrosine kinases that are activated by membrane-spanning ligands called ephrins. Previously, we demonstrated that the ephrinB1-EphB2 interaction regulates odontogenic/osteogenic differentiation from dental pulp cells (DPCs) in vitro. The goal of this study was to identify the molecular mechanisms regulated by the EphB2/ephrinB1 system that govern tertiary dentin formation in vitro and in vivo. During tooth development, ephrinB1, and EphB2 were expressed in preodontoblast and odontoblasts at postnatal day 4. EphrinB1 was continuously expressed in odontoblasts and odontoblastic processes until the completion of tooth eruption. In addition, ephrinB1 was expressed in odontoblastic processes 2 wk following tooth injury without pulp exposure, whereas EphB2 was expressed in the center of pulp niches but not odontoblasts. In a model of tooth injury with pulp exposure, ephrinB1 was strongly expressed in odontoblasts 4 wk postinjury. In vitro studies with human and mouse DPCs treated with calcium hydroxide (CH) or mineral trioxide aggregate (MTA) showed an increased expression of insulin-like growth factor 1 (IGF-1). Experiments using several inhibitors of IGF-1 receptor signaling revealed that inhibiting the Ras/Raf-1/MAPK pathway inhibited EphB2 expression, and inhibiting the PI3K/Akt/mTOR pathway specifically inhibited ephrinB1 gene expression. Tooth injury in mice with odontoblast-specific IGF-1 receptor ablation exhibited a reduced tertiary dentin volume, mineral density, and ephrinB1 expression 4 wk following injury. We conclude that the IGF-1/ephrinB1 axis plays significant roles in the early stages of tooth injury. Further research is needed to fully understand the potential of targeting ephrinB1 as a regenerative pulp therapy.