Mechanism of the diabetogenic action of growth hormone. I. Effect of polypeptides derived from growth hormone on glycolysis in muscle

Growth Hormone Upregulates Mediators of Melanoma Drug Efflux and Epithelial-to-Mesenchymal Transition In Vitro and In Vivo

Simple Summary

Growth hormone (GH) action is strongly implicated in the progression and therapy resistance in several types of solid tumors which overexpress the GH receptor (GHR). The aim of our study was to characterize the effects of GH and its downstream effector insulin-like growth factor 1 (IGF-1) on melanoma using in vitro and in vivo models. We confirmed an IGF-1-independent role of elevated circulating GH in upregulating key mechanisms of therapy resistance and malignancy with analyses conducted at the molecular and cellular level. We identified that GH upregulates key mechanisms of therapy resistance and metastases in melanoma tumors in an IGF-1 dependent and independent manner by upregulating multidrug efflux pumps and EMT transcription factors. Our study reveals that GH action renders an intrinsic drug resistance phenotype to the melanoma tumors—a clinically crucial property of GH verifiable in other human cancers with GHR expression.

Abstract

Growth hormone (GH) and the GH receptor (GHR) are expressed in a wide range of malignant tumors including melanoma. However, the effect of GH/insulin-like growth factor (IGF) on melanoma in vivo has not yet been elucidated. Here we assessed the physical and molecular effects of GH on mouse melanoma B16-F10 and human melanoma SK-MEL-30 cells in vitro. We then corroborated these observations with syngeneic B16-F10 tumors in two mouse lines with different levels of GH/IGF: bovine GH transgenic mice (bGH; high GH, high IGF-1) and GHR gene-disrupted or knockout mice (GHRKO; high GH, low IGF-1). In vitro, GH treatment enhanced mouse and human melanoma cell growth, drug retention and cell invasion. While the in vivo tumor size was unaffected in both bGH and GHRKO mouse lines, multiple drug-efflux pumps were up regulated. This intrinsic capacity of therapy resistance appears to be GH dependent. Additionally, epithelial-to-mesenchymal transition (EMT) gene transcription markers were significantly upregulated in vivo supporting our current and recent in vitro observations. These syngeneic mouse melanoma models of differential GH/IGF action can be valuable tools in screening for therapeutic options where lowering GH/IGF-1 action is important.

The Effects of 20-kDa Human Placental GH in Male and Female GH-deficient Mice: An Improved Human GH?

A rare 20K isoform of GH-V (here abbreviated as GHv) was discovered in 1998. To date, only 1 research article has characterized this isoform in vivo, observing that GHv treatment in male high-fat fed rats had several GH-like activities, but unlike GH lacked diabetogenic and lactogenic activities and failed to increase IGF-1 or body length. Therefore, the current study was conducted to further characterize the in vivo activities of GHv in a separate species and in a GH-deficient model (GH-/- mice) and with both sexes represented. GHv-treated GH-/- mice had significant increases to serum IGF-1, femur length, body length, body weight, and lean body mass and reduced body fat mass similar to mice receiving GH treatment. GH treatment increased circulating insulin levels and impaired insulin sensitivity; in contrast, both measures were unchanged in GHv-treated mice. Since GHv lacks prolactin receptor (PRLR) binding activity, we tested the ability of GH and GHv to stimulate the proliferation of human cancer cell lines and found that GHv has a decreased proliferative response in cancers with high PRLR. Our findings demonstrate that GHv can stimulate insulin-like growth factor-1 and subsequent longitudinal body growth in GH-deficient mice similar to GH, but unlike GH, GHv promoted growth without inhibiting insulin action and without promoting the growth of PRLR-positive cancers in vitro. Thus, GHv may represent improvements to current GH therapies especially for individuals at risk for metabolic syndrome or PRLR-positive cancers.

Effect of synthetic C-terminal fragments of hGH on insulin release by isolated islets

Synthetic fragments representing the C-terminal end of the growth hormone molecule have been tested for their direct in vitro effects on insulin release by isolated rat islets of Langerhans. hGH 177-191 caused a dose-related potentiation of glucose-induced insulin release, whereas the peptide by itself caused no stimulation of insulin release from the islets. The rate curves constructed for insulin secretion as a function of extracellular glucose concentration showed that the Km for glucose is not altered in the presence of the peptide, but that the Vmax of secretion is increased. Significant potentiation of insulin release by the peptide was seen only at high extracellular concentrations of glucose. Measurement of cAMP levels in islets showed that the peptide caused no significant alteration of cAMP levels while still potentiating insulin release. It was therefore concluded that the mechanism of potentiation of insulin release by the peptide may be independent of the changes in cAMP levels in islets. hGH 172-191, too, caused potentiation of glucose-stimulated insulin release from islets, whereas hGH 179-191 was not active in this report.

Hyperglycemic action of synthetic C-terminal fragments of human growth hormone

The synthetic peptides corresponding to amino acids 172-191, 176-191, 177-191, 178-191, 179-191, and 180-191 of human growth hormone (hGH) have been studied for their in vivo effects in normal rats. Four of the peptides (hGH 172-191, 176-191, 177-191, and 178-191) produced a short-lived rise in blood glucose and a more sustained rise in plasma insulin, whereas the other two (hGH 179-191 and 180-191) were inert in the systems tested. A single dose (5 nmol/kg body wt) of the peptides containing the amino acids sequence 178-191 of the hGH molecule significantly reduced insulin sensitivity of the animals in intravenous insulin tolerance tests. The findings also indicate that the biologically active peptides must not only have the minimum of the informational sequence but also that this must be in correct physical configuration.

Insulin antagonist of pituitary origin in plasma of normal and diabetic subjects

Studies on ultrafiltrates of the plasma of normal and diabetic subjects have shown that a polypeptide of similar characteristics to the growth-hormone-derived polypeptide In-G is present in higher concentrations in diabetic subjects, thus indicating a possible role in the pathogenesis of diabetes mellitus. The polypeptide is absent from the plasma of hypophysectomized diabetic patients.