Cyclic glycine-proline regulates IGF-1 homeostasis by altering the binding of IGFBP-3 to IGF-1 Jian Guan 1,2,3 , Peter Gluckman 1,3 , Panzao Yang 1,2 , Geoff Krissansen 4 , Xueying Sun 4 , Yongzhi Zhou 1,5 , Jingyuan Wen 5 , Gemma Phillips 6,7 , Paul R. Shorten 3,7,8 , Chris D. McMahon 3,7 , Graeme C. Wake 3,6,8 , Wendy H. K. Chan 1 , Mark F. Thomas 7 , April Ren 1 , Steve Moon 1 & Dong-Xu Liu 1 1 Liggins Institute, University of Auckland, 85 Park Road, Grafton, Auckland, New Zealand, 2 Centre for Brain Research, Faculty of Medicine and Health Sciences, University of Auckland, 85 Park Road, Grafton, Auckland, New Zealand, 3 Gravida National Centre for Growth and Development, University of Auckland, Auckland, New Zealand, 4 Department of Molecular Medicine and Pathology, Faculty of Medicine and Health Sciences, University of Auckland, 85 Park Road, Grafton, Auckland, New Zealand, 5 School of Pharmacy, Faculty of Medicine and Health Sciences, University of Auckland, 85 Park Road, Grafton, Auckland, New Zealand, 6 Institute of Natural and Mathematical Sciences, Massey University, Private Bag 102904, Auckland, New Zealand, 7 AgResearch Ltd, Ruakura Research Centre, Private Bag 3123, Hamilton 3240, New Zealand, 8 Riddet Institute, Massey University, Private Bag 11222, Palmerston North 4442, New Zealand. The homeostasis of insulin-like growth factor-1 (IGF-1) is essential for metabolism, development and survival. Insufficient IGF-1 is associated with poor recovery from wounds whereas excessive IGF-1 contributes to growth of tumours. We have shown that cyclic glycine-proline (cGP), a metabolite of IGF-1, can normalise IGF-1 function by showing its efficacy in improving the recovery from ischemic brain injury in rats and inhibiting the growth of lymphomic tumours in mice. Further investigation in cell culture suggested that cGP promoted the activity of IGF-1 when it was insufficient, but inhibited the activity of IGF-1 when it was excessive. Mathematical modelling revealed that the efficacy of cGP was a modulated IGF-1 effect via changing the binding of IGF-1 to its binding proteins, which dynamically regulates the balance between bioavailable and non-bioavailable IGF-1. Our data reveal a novel mechanism of auto-regulation of IGF-1, which has physiological and pathophysiological consequences and potential pharmacological utility. I nsulin-like growth factor 1 (IGF-1) plays an essential role in development 1 , metabolism and survival 2 . Its function is mediated through activating IGF-1 receptors for initiating downstream signalling pathways 3,4 . The homeostasis of IGF-1 function is dynamically regulated through reversible binding to circulating and tissue associated IGF-1 binding proteins (IGFBPs) 5 . Both insufficient and excessive amounts of IGF-1 are related to medical conditions, such as poor recovery from injuries 2 and tumorigenesis 6 , respectively. The unbound IGF-1 can be enzymatically metabolized to des-(1-3) IGF-1 (des-IGF-1) and glycine-proline- glutamate (GPE), which is cleaved from its N-terminal 7,8 . GPE is enzymatically unstable 9,10 and is rapidly metabolized into single amino acids and dipeptides including cyclic glycine-proline (cGP) 9 . Similar to IGF-1 11 , both GPE and cGP are neuroprotective in the treatment of hypoxic-ischemic (HI) brain injury in rats 10,12,13 . While the neuroprotective effect of IGF-1 is clearly mediated by IGF-1 receptors 11 , GPE does not interact with IGF-1 receptors 12–15 . The mode of action of GPE and cGP still remains unknown. The neuroprotective effects of GPE and cGP after intracerebroventricular (icv) administration are dose-dependent, suggesting receptor-mediated phar- macodynamics 12 . The neuroprotective effects of IGF-1 16 and GPE 17,18 also involve reduced vascular damage and improve vascular remodelling. This prompted us to investigate whether IGF-1-mediated vascular remodelling contributes to cGP associated neuroprotection in a rat model of unilateral ischemic brain injury. The investiga- tion led to the identification of cGP as a novel mechanism regulating the bioavailability of IGF-1. Results and Discussion Vascular protection of cGP was associated with IGF-1. Hypoxic ischemic (HI) injury to the brain was induced unilaterally in the hemisphere ipsilateral to the ligated carotid artery 17,19 . Compared to the contralateral side OPEN SUBJECT AREAS: TRANSLATIONAL RESEARCH APPLIED MATHEMATICS STEROID HORMONES Received 15 November 2013 Accepted 28 February 2014 Published 17 March 2014 Correspondence and requests for materials should be addressed to J.G. (j.guan@ auckland.ac.nz) SCIENTIFIC REPORTS | 4 : 4388 | DOI: 10.1038/srep04388 1