br Material and methods br Results and discussion br Discuss

Material and methods
Results and discussion
Discussion It is well established that the guanosine-rich DNA sequences within telomeric DNA repeats, and in the promoter regions of a number of oncogenes or proto-oncogenes such as c-myc [20,23], VEGF [28,48,49] and Bcl-2 [21,50], can readily assemble into G-quadruplex structures under physiological conditions. The potential roles of G-quadruplex structures in cellular physiology are broad. The telomeric G-quadruplexes are well known to act as a telomeric capping structure and inhibit telomerase activity. The G-quadruplexes in the gene promoters including c-myc, VEGF, and Bcl-2 promoters mainly function as a silencer element [20,22,49]. Therefore, molecular ligands or proteins that induce or unwind quadruplex formation are likely to affect gene expression. In the recent years, an increasing number of prokaryotic and enkaryotic proteins have been identified to bind to G-quadruplex structures with high affinity and selectivity, which perform various effects on G-quadruplex structures including quadruplex structure stabilization, unwinding, and cleaving. The effects of these proteins on G-quadruplexes involve transcriptional regulation, LY 235959 synthesis remodeling and DNA repair. Because G-quadruplexes have such a large number and many possible physiological roles, any mistakes in quadruplex recognition can lead to extensive replication errors, DNA damage or reorganization of chromosomes. Thus we speculate that the number of unknown proteins with quadruplex-binding specificity is still large. As a protein involved in cell growth and tumorigenesis, IGF-1 is quite possible to interact with the G-quadruplex structures in the telomere and oncogene promoters. Here, the higher selectivity of IGF-1 for the G-quadruplex structures over the single/double stranded DNAs has been confirmed, providing positive support for this speculation. However, we also found that the binding affinity of IGF-1 to G-quadruplexes is only in the micromolar range, which is much weaker than the binding affinity between most antibody-antigen and protein-DNA. We speculate that the interaction between IGF-1 and G-quadruplexes is only one possible mechanism of action. IGF-1 and its signaling pathway play a major role in normal growth and ageing, however the circulating levels of IGF-1 are known to reduce with advancing age. A number of studies have also addressed that lower circulating levels of IGF-1 are associated with shorter leukocyte telomere length [51,52]. And other studies also confirmed that a short leucocyte telomere length is associated with development of insulin resistance [53,54]. Since insulin resistance and telomere attrition could both reflect in some way the ageing process, whether there is a connection between telomere dynamics and insulin resistance in humans has become an issue of concern. In this paper, we proved that IGF-1 showed selective binding to the telomeric G-quadruplex structures. Although IGF-1 was unable to unfold or stabilize the telomeric G-quadruplex structures, their binding may still cause some specific physiological process associated with telomere maintenance and elongation. In addition to normal growth and ageing, IGF-1 has also been widely reported to be involved in the development and progression of various tumors. Many preclinical studies and animal models demonstrate that dysregulation of the IGF-1 signaling pathway could promote the transformation, growth and metastasis of cancer cells [55]. However, the mechanisms by which IGF-1 modulates cancer metastasis are still poorly understood. In recent studies, it was found that the expression levels of c-myc, VEGF and Bcl-2 genes are closely related to the activation of IGF-1 and IGF-1 receptor (IGF-1R), and high IGF-1 levels and IGF-1R activity are usually accompanied by an increase in the expression of these genes [[56], [57], [58], [59]]. The G-quadruplex structures in c-myc, VEGF and Bcl-2 genes have been shown to act as a gene transcription silencer, and the interaction between IGF-1 and these G-quadruplexes may result in altered levels of gene transcription levels. Then the interaction pathway between IGF-1 and G-quadruplex structures may be a novel therapeutic target.