Biased signalling at the IGF-1R : pitfalls and potential of the GRK/beta-arrestin system in cancer therapeutics

Abstract

Embedded into the boundary between the living machinery and the external space, plasma membrane receptors allow cells to perceive their environment and elicit appropriate responses. Two of the largest classes of receptors are the G protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs). Distinct in their structural and functional characteristics, each lend themselves to important cellular operations. While crosstalk between members of either family is a general phenomenon, a growing body of evidence suggests there may be a more direct overlap. This thesis explores the insulin-like growth factor type 1 receptor (IGF-1R), as an intermediate between these two receptor families. Supporting growth and survival in many human cancers, the IGF-1R has long been considered an attractive therapeutic target. Despite the pre-clinical appeal and intense pharmaceutical development, disappointing clinical trials suggest that its potential has not been fulfilled. This thesis examines the true complexity of this receptor system, with particular focus on its use of GPCR components and how they contribute to the paradigm of biased signalling.

Study I categorizes the therapeutic relevance of biased signalling at the IGF-1R. Our results identify small molecule Nutlin-3 as strategy that synergizes with MEK inhibition, by cotargeting the p53 and IGF-1R, without biased signal activation. Study II set out to define the role of the β-arrestin 2 isoform at the IGF-1R, and in doing so identifies the mechanism controlling a balanced versus biased receptor conformation. The β-arrestin isoforms antagonize each other’s function at the IGF-1R, imposing regulation on the receptor expression, signalling and crosstalk to p53. The position of β-arrestins, between an important mitogenic pathway and perhaps the ultimate tumour suppressor pathway, reveals potential for therapeutic gain. Study III develops strategies for targeting β-arrestin/GRK-biased agonism at the IGF-1R for cancer treatment, with focus on clinical applicability. This work provides the proof of concept for cross-targeting the IGF-1R through GRK 2 inhibition, and suggests clinical feasibility of such an approach by repurposing the widely used drug paroxetine. Acknowledging the clinical importance of biased signalling at the IGF-1R, Study IV aims to explore the utility of microRNAs as biomarkers to quantify signalling bias downstream of IGF-1R. MicroRNA array and IGF-1R mutation analysis identifies miR-106a as a candidate that can specify a β-arrestin biased IGF-1R signal that could be developed for patient stratification in anti-IGF-1R trials.

Altogether, our findings highlight the shortcomings of first line anti-IGF-1R strategies, and the overly simplistic models in use at the time. Armed with an appreciation of the true complexity, plasticity and interconnectivity of receptor systems, we have examined the therapeutic utility of the novel components GRK/β-arrestin, and identified targets that may hold potential in unlocking the true potential of anti-IGF-1R.