Dissecting the cellular and molecular mechanisms of Insulin-like growth factor binding protein-2 (IGFBP-2) action in a cancer cell model

Abstract

IGFBP2 is one of the most highly expressed genes in the signature of human tumour cells. Elevated expression has been observed in almost every major malignancy. Importantly, IGFBP2 is frequently overexpressed in advanced cancers and is suggested to be involved in the metastatic process. IGFBP-2 is expressed both in the serum and in tumour tissues, where tumour aggressiveness is directly proportional to IGFBP-2 levels; hence, it is recognised as a valuable prognostic biomarker for many of these cancers. These clinical findings are also supported by in vitro studies showing that high levels of IGFBP-2 promote the tumorigenic potential of cancer cells.

IGFBP-2 is a secreted protein which binds both IGFs with high affinity. Traditionally, IGFBP-2 was known mainly to modulate IGFs actions by regulating their bioavailability in the peri-/extracellular space; hence, these were referred to as its IGF-dependent actions. However, it is now becoming increasingly clear that, apart from its IGF-dependent actions, IGFBP-2 also exerts IGF-independent effects in the absence of IGFs or in the absence of IGF receptor signalling. These effects are shown to be mediated via other functional domains of IGFBP-2, such as HBD and RGD, which are distinct from its IGF-binding domains. In addition, more recently there has been evidence of intracellular functions and growing speculation about potential nuclear actions of IGFBP-2. However, to date, there has been no definitive evidence to demonstrate the molecular mechanisms of IGF-independent, nor the intracellular/intranuclear actions, of IGFBP-2.

Therefore, the aim of these PhD studies was to dissect the cellular and molecular mechanisms of IGFBP-2 actions using a cancer cell model.

The first results chapter (Chapter 3) investigates the overexpression of IGFBP2 in a neuroblastoma cell line. This provides novel insights into the activation of a pro-tumorigenic gene expression profile in response to IGFBP-2 and sheds light onto its intranuclear actions, while also demonstrating the molecular mechanisms behind the regulation of a target gene, VEGF, by IGFBP-2. The second results chapter (Chapter 4) describes the precise nuclear import mechanisms of IGFBP-2, identifying, for the first time, a functional nuclear localisation signal sequence, and its interaction with a nuclear import receptor. The final results chapter (Chapter 5) describes the development and characterisation of a novel non-IGF-binding IGFBP-2 mutant.

In conclusion, these studies contribute to our steadily expanding knowledge regarding the role of IGFBP-2 in tumorigenesis, and provide novel insights into the underlying molecular and cellular mechanisms of IGFBP-2 in cancer. These studies also provide a great set of non-IGF-binding IGFBP-2 mutants, which with further characterisation and functional studies, will allow us to acquire more definitive evidence on the IGF-independent actions of IGFBP-2, and in turn, build on our current understanding of these actions. Finally, these studies also raise many exciting questions and uncover other avenues for IGFBP-2 research. Such studies are imperative for the understanding and development of IGFBP-2 as a novel therapeutic target.