Chronic apelin analogue administration is more effective than established incretin therapies for alleviating metabolic dysfunction in diabetic db/db mice
Graphical abstract
Introduction
Type 2 diabetes is a highly prevalent and serious metabolic disease for which treatment regimens often fail to achieve the desired metabolic control, resulting in significant morbidity and mortality (WHO, 2016; CDC, 2011). Present antidiabetic drugs induce unwanted side-effects including hypoglycaemia, weight gain and increased cardiovascular risk (American Diabetes Association, 2019). There is therefore an urgent need for identification and therapeutic exploitation of new drug targets to offset the cost of this disease.
Adipose tissue plays a central role in lipid and glucose metabolism and is now considered a major endocrine organ (Scheja and Heeren, 2019). Apelin which activates the APJ receptor (membrane bound GPCR), is an adipocyte derived ‘adipokine’ that is secreted by both human and animal white adipocytes (Boucher et al., 2005). Native preproapelin is a 77 amino acid precursor, which is cleaved in vivo releasing C-terminally derived products including apelin-36, apelin-17, apelin-13 and apelin-12. One isoform (Pyr1)apelin-13 can also undergo an additional N-terminal post-translational modification.
Internal truncation along with the short half-life of these native apelin peptides impedes the exploitation of their potential beneficial effects (Murza et al., 2014; O'Harte et al., 2017). However, we have shown previously that structural modification of the N- and C-terminus of apelin-13 improves plasma stability and results in improved in vitro insulinotropic activity, and adipocyte glucose uptake, as well as enhanced glucose disposal and insulin release in high fat fed mice with diet induced obesity (DIO) (O'Harte et al., 2017). Apelin has been shown also to stimulate utilization of glucose in skeletal muscle (Dray et al., 2008; Parthsarathy et al., 2018), and mice deficient in apelin signalling displayed increased adiposity and elevated circulating free fatty acids (Yue et al., 2011). Investigations in our laboratory have also shown that DIO mice treated with the stable analogue pGlu(Lys8GluPAL)apelin-13 amide, exhibited decreased food intake and corresponding weight loss (Parthsarathy et al., 2018).
To further evaluate the potential therapeutic actions of modified apelin analogues, the present study has assessed their efficacy in a genetic form of degenerative diabetes induced by a defect in the leptin receptor gene (Leiter et al., 1981; Hummel et al., 1966). Thus, diabetic db/db mice exhibit a severe age-related form or diabetes with severe hyperglycaemia, insulinopenia and islet beta cell loss. The therapeutic potential of modified apelin analogues, apelin amide along with its acylated counterpart apelin FA, were compared with the well-established incretin therapies, exendin-4(1–39) and liraglutide.
Section snippets
Peptides and chemicals
Custom made apelin analogues were purchased from EZ Biolabs (Carmel, IN, USA) with a purity >95%. The purity and structural identity of peptides were checked by RP-HPLC and electrospray ionization mass spectrometry as described previously (Gault et al., 2011). Differences from native apelin-13 were introduced to confer stability and extend in vivo bioactivity by C-terminal amidation and/or internal acylation, as described previously (Green et al., 2004; Parthsarathy et al., 2018).
Animals
Mice
Effects of apelin analogues on body weight, food intake and metabolic markers
Twice daily administration of either apelin analogue or incretin peptides for 21 days failed to effect body weight in db/db mice (Fig. 1A and B). However, mice treated with exendin-4 showed a17% reduction in food intake, versus saline treated db/db controls (P < 0.001, Fig. 1C and D). In contrast, no change in food intake was observed in mice treated with either apelin amide or apelin FA (Fig. 1D).
Fig. 2A and B shows a significant reduction in non-fasting blood glucose in mice treated with
Discussion
The relative lack of insulin, defective beta cell function as well as insulin resistance makes T2DM a complicated metabolic disease, which is mostly associated with overweight or obesity (Sattar and Gill, 2014). Although no animal model is an exact match for human diabetes obese diabetic C57BL/KsJ db/db mice have been widely used as a therapeutic tool, due to similarities in pathogenesis to degenerative T2DM (Leiter et al., 1981; Cefalu, 2006).
Recent studies in our laboratory have demonstrated
Conclusion
In summary, we have demonstrated that both acylated and non-acylated apelin-13 peptide analogues have potential as antidiabetic agents. Further detailed studies to elucidate the molecular mechanisms and full spectrum of actions of apelin analogues at the APJ receptor will be required to move further towards this goal.
Author contributions
VP contributed to collection of data, analysis and writing of the manuscript. FOH, VP and PRF contributed to study design, analysis and writing of the manuscript. All authors approved the final version of the manuscript. FOH and PRF are named on patents filed by the Ulster University for exploitation of apelin related drugs and other peptide therapeutics.
Acknowledgements
This work was supported by Invest Northern Ireland, Proof of Concept, Phase III, [PoC518] and the Irish Endocrine Society [small grant 2016], Ireland.
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