Chronic apelin analogue administration is more effective than established incretin therapies for alleviating metabolic dysfunction in diabetic db/db mice

Highlights

• Apelin-13 peptide analogues improve glycaemic control in diabetic db/db mice.

• Antidiabetic actions of acylated apelin-13 compares favourably with liraglutide.

• Antidiabetic actions of apelin-13 amide has similar efficacy with exendin-4.

• Apelin-13 analogues are as effective for metabolic control as incretin mimetics.

• Apelin-13 amide lower plasma triglycerides more than incretin mimetics.

Abstract

Stable apelin-13 peptide analogues have shown promising acute antidiabetic effects in mice with diet-induced obesity diabetes. Here the efficacy of (pGlu)apelin-13 amide (apelin amide) and the acylated analogue (pGlu)(Lys⁸GluPAL)apelin-13 amide (apelin FA), were examined following chronic administration in db/db mice, a genetic model of degenerative diabetes. Groups of 9-week old male db/db mice (n = 8) received twice daily injections (09:00 and 17:00 h; i.p.) or saline vehicle, apelin amide, apelin FA, or the established incretin therapies, exendin-4(1–39) or liraglutide, all at 25 nmol/kg body weight for 21 days. Control C57BL/6J mice were given saline twice daily. No changes in body weight or food intake were observed with either apelin or liraglutide treatments, but exendin-4 showed a reduction in cumulative food intake (p < 0.01) compared with saline-treated db/db mice. Apelin analogues and incretin mimetics induced sustained improvements of glycaemia (p < 0.05 to p < 0.001, from day 9–21), lowered HbA_1c at 21 days (p < 0.05) and raised plasma insulin concentrations. The treatments also improved OGTT and ipGTT with enhanced insulin responses compared with saline-treated control db/db mice (p < 0.05 to p < 0.001). Apelin amide was superior to incretin mimetics in lowering plasma triglycerides by 34% (p < 0.05). Apelin analogues unlike both incretin mimetics reduced pancreatic α-cell area (p < 0.05 to p < 0.01) and all peptide treatments enhanced pancreatic insulin content (p < 0.05 to p < 0.01). In conclusion, longer-term administration of apelin-13 analogues, induced similar and in some respects more effective metabolic improvements than incretin mimetics in db/db mice, providing a viable alternative approach for counteracting metabolic dysfunction for mild and more degenerative forms of the disease.

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.