The biological role of the IGF-1 pool in breast cancer

The insulin-like growth factor-1 (IGF-1) is a polypeptide growth factor that is essential for normal body growth and development of several tissues. IGF-1 is implicated in the progression and risk of several malignancies, including breast cancer (BC). Alternative splicing of terminal exon 5 of the igf-1 gene results in multiple isoforms possessing distinct carboxy-terminal extensions, called the Ea-, Eb- and Ec-domains. The first chapter of the Thesis provides evidence for an evolutionary mechanism generating the diversity of igf-1 splicing and expression across species. Our study highlights how the igf-1 exon 5 originates from exonization of a Mammalian interspersed repetitive-b (MIR-b) element in mammals. The acquisition of exon 5 alters the splicing pattern of igf-1 in mammals by generating two new isoforms: IGF-1Eb and IGF-1Ec. The evolutionary analysis of mammalian IGF-1 domains showed that E-domains are subjected to a strong evolutionary constraint on the synonymous sites, and they are enriched in disorder-promoting amino acids (i.e. intrinsically disordered), suggesting an important and novel regulatory role for these domains, not previously described. In the second chapter, we highlighted that IGF-1 pro-hormones are not a simple inactive precursor of mature IGF-1, but are stable intermediates of their posttranslational processing. The IGF-1 pro-hormones can induce BC cell proliferation via the IGF-1 receptor, independently from the mature IGF-1 form. These results underline the importance of an accurate assessment of the presence of IGF-1 pro-hormones within the BC microenvironment. The third chapter describes the mechanisms, which control the IGF-1 pro-hormones biosynthesis. We demonstrated that N-linked glycosylation regulates the stability and secretion of IGF-1Ea pro-hormone, probably ensuring proper pro-hormone folding and favoring its passage through the secretory pathway. The alternative Eb- and Ec-domains lack N-terminal glycosylation sites hence IGF-1Eb and IGF-1Ec pro-hormones were insensitive to glycosylation status of the cells. Moreover, the Eb- and Ec-domains regulate the subcellular localizations of IGF-1Eb and IGF-1Ec pro-hormones, promoting their nuclear accumulation. Thus, disordered E-domains play an important role in the structure, regulation and functioning of IGF-1. The final chapter of the Thesis describes the data deriving from DIANA-5, and focuses on the effectiveness of modification in dietary change-associated with moderate physical activity in the prevention of BC recurrence, highlighting the importance of the lifestyle modification in the modulation of the circulating levels of IGF-1.