Targeting the GH/IGF-1 axis with novel, small molecule inhibitors

Abstract

The growth hormone (GH) / insulin-like growth factor (IGF) family of ligands, binding proteins and receptors play multiple roles in cell growth, metabolism and development. In addition, numerous studies have demonstrated the pathophysiological importance of the GH/IGF-1 axis. In particular, the impact of IGF-1 receptor (IGF-1R) in cancer has attracted increasing attention during the last decade. Several classes of pharmacological agents that inhibit GH/IGF-1 signaling at different levels have shown anticancer activity both in vitro and in vivo. GH receptor (GHR) antagonists have proven the most effective and safe way to pharmacologically treat overproduction of GH (acromegaly) and antibodies against the IGF-1R cause massive apoptosis in vitro and tumor regression in animal models. However, there is a need to develop low molecular weight compounds targeting the GH/IGF-1 axis which can be administered perorally and have increased bioavailability compared to protein drugs. In paper I, we present a new mRNA quantification method which was used to test a number of low molecular weight compounds for their ability to reduce GH-induced IGF-1 mRNA in primary hepatocytes. One such potential GHR antagonist, BVT-A, was selected, and in paper II its attenuating effect on several markers for GH/IGF-1 overactivity was verified in an animal model of acromegaly. Picropodophyllin (PPP) was discovered some years ago as an effective inhibitor of IGF-1R signaling in cell lines and tumor repressor in vivo. The mechanism by which PPP caused this effect has not been fully delineated. In paper III, we show that IGF-1R knockout cells at late passages can acquire IGF-1R expression and dependency and therefore become sensitive to PPP treatment. Paper IV describes the inhibitory effect of PPP on IGF-1 induced vascular endothelial growth factor (VEGF) production as well as on neovascularization of the choroid in a model of macular degeneration, the most common cause of blindness. In paper V we show that PPP induces downregulation of the IGF-1R. This effect is important since it is known that downregulation, not only deactivation of the receptor is necessary for induction of massive apoptosis. Finally, in paper VI we show that PPP recruits the E3 ligase Mdm2 and â-Arrestin1 to the IGF-1R. This action was found to be involved in receptor downregulation and inhibition of AKT signaling and suggests that PPP acts as a â-Arrestin1-biased IGF-1R agonist. Hopefully, the substances identified and evaluated in this thesis will function as lead compounds in the development of improved pharmaceutical agents against GH/IGF-1 dependent diseases.