Growth hormone secretagogue receptor‐1a mediates ghrelin's effects on attenuating tumour‐induced loss of muscle strength but not muscle mass

Abstract Background Ghrelin may ameliorate cancer cachexia (CC) by preventing anorexia, muscle, and fat loss. However, the mechanisms mediating these effects are not fully understood. This study characterizes the pathways involved in muscle mass and strength loss in the Lewis lung carcinoma (LLC)‐induced cachexia model, and the effects of ghrelin in mice with or without its only known receptor: the growth hormone secretagogue receptor‐1a ((GHSR‐1a), Ghsr+/+ and Ghsr−/−). Methods Five to 7‐month‐old male C57BL/6J Ghsr+/+ and Ghsr−/− mice were inoculated with 1 × 106 heat‐killed (HK) or live LLC cells (tumour implantation, TI). When tumours were palpable (7 days after TI), tumour‐bearing mice were injected with vehicle (T + V) or ghrelin twice/day for 14 days (T + G, 0.8 mg/kg), while HK‐treated mice were given vehicle (HK + V). Body weight and grip strength were evaluated before TI and at termination (21 days after TI). Hindlimb muscles were collected for analysis. Results Less pronounced body weight (BW) loss (87.70 ± 0.98% vs. 83.92 ± 1.23%, percentage of baseline BW in tumour‐bearing Ghsr+/+ vs. Ghsr−/−, P = 0.008), and lower upregulation of ubiquitin‐proteasome system (UPS, MuRF1/Trim63, 5.71 ± 1.53‐fold vs. 9.22 ± 1.94‐fold‐change from Ghsr+/+ HK + V in tumour‐bearing Ghsr+/+ vs. Ghsr‐/‐, P = 0.036) and autophagy markers (Becn1, Atg5, Atg7, tumour‐bearing Ghsr+/+ < Ghsr−/−, all P < 0.02) were found in T + V Ghsr+/+ vs. Ghsr−/− mice. Ghrelin attenuated LLC‐induced UPS marker upregulation in both genotypes, [Trim63 was decreased from 5.71 ± 1.53‐fold to 1.96 ± 0.47‐fold in Ghsr+/+ (T + V vs. T + G: P = 0.032) and 9.22 ± 1.94‐fold to 4.72 ± 1.06‐fold in Ghsr−/− (T + V vs. T + G: P = 0.008)]. Only in Ghsr+/+ mice ghrelin ameliorated LLC‐induced grip strength loss [improved from 89.24 ± 3.48% to 97.80 ± 2.31% of baseline (T + V vs. T + G: P = 0.042)], mitophagy markers [Bnip3 was decreased from 2.28 ± 0.56 to 1.38 ± 0.14‐fold (T + V vs. T + G: P ≤ 0.05)], and impaired mitochondrial respiration [State 3u improved from 698.23 ± 73.96 to 934.37 ± 95.21 pmol/min (T + V vs. T + G: P ≤ 0.05)], whereas these markers were not improved by ghrelin Ghsr−/−. Compared with Ghsr+/+, Ghsr−/− tumour‐bearing mice also showed decreased response to ghrelin in BW [T + G‐treated Ghsr+/+ vs. Ghsr −/−: 91.75 ± 1.05% vs. 86.18 ± 1.13% of baseline BW, P < 0.001)], gastrocnemius (T + G‐treated Ghsr+/+ vs. Ghsr−/−: 96.9 ± 2.08% vs. 88.15 ± 1.78% of Ghsr+/+ HK + V, P < 0.001) and quadriceps muscle mass (T + G‐treated Ghsr+/+ vs. Ghsr−/−: 96.12 ± 2.31% vs. 88.36 ± 1.94% of Ghsr+/+ HK + V, P = 0.01), and gastrocnemius type IIA (T + G‐treated Ghsr+/+ vs. Ghsr−/−: 1250.49 ± 31.72 vs. 1017.62 ± 70.99 μm2, P = 0.027) and IIB fibre cross‐sectional area (T + G‐treated Ghsr+/+ vs. Ghsr−/−: 2496.48 ± 116.88 vs. 2183.04 ± 103.43 μm2, P = 0.024). Conclusions Growth hormone secretagogue receptor‐1a mediates ghrelin's effects on attenuating LLC‐induced weakness but not muscle mass loss by modulating the autophagy‐lysosome pathway, mitophagy, and mitochondrial respiration

EXT418, a novel long‐acting ghrelin, mitigates Lewis lung carcinoma induced cachexia in mice

Abstract Background Ghrelin is a potential therapy for cachexia due to its orexigenic properties and anabolic effects on muscle and fat. However, its clinical use is limited by the short half‐life of active (acylated) ghrelin (~11 min in humans). EXT418 is a novel long‐acting, constitutively active ghrelin analog created by covalently linking it to a vitamin D derivative. Here, we evaluated the effects and mechanisms of action of EXT418 on Lewis lung carcinoma (LLC)‐induced cachexia in mice. Methods Male C57BL/6J mice (5‐ to 7‐month‐old) were implanted with 1 × 106 heat‐killed (HK) or live LLC cells. When the tumour was palpable, mice were injected with vehicle (T + V) or EXT418 daily (T + 418 Daily, 0.25 mg/kg/day) or every other day (T + 418 EOD, 0.5 mg/kg/EOD) for up to 14 days, whereas HK‐treated mice were given vehicle (HK + V). Subsets of T + 418 Daily or EOD‐treated mice were pair‐fed to the T + V group. Body composition and grip strength were evaluated before tumour implantation and at the end of the experiment. Molecular markers were probed in muscles upon termination. Results In tumour‐bearing mice, administration of EXT418 daily or EOD partially prevented weight loss (T + V vs. T + 418 Daily, P = 0.030; and vs. T + 418 EOD, P = 0.020). Similar effects were observed in whole body fat and lean body mass. Grip strength in tumour‐bearing mice was improved by EXT418 daily (P = 0.010) or EOD (P = 0.008) administration compared with vehicle‐treated mice. These effects of EXT418 on weight and grip strength were partially independent of food intake. EXT418 daily administration also improved type IIA (P = 0.015), IIB (P = 0.037) and IIX (P = 0.050) fibre cross‐sectional area (CSA) in tibialis anterior (TA) and EXT418 EOD improved CSA of IIB fibres in red gastrocnemius (GAS; P = 0.005). In skeletal muscles, tumour‐induced increases in atrogenes Fbxo32 and Trim63 were ameliorated by EXT418 treatments (TA and GAS/plantaris, PL), which were independent of food intake. EXT418 administration decreased expression of the mitophagy marker Bnip3 (GAS/PL; P ≤ 0.010). Similar effects of EXT418 EOD were observed in p62 (GAS/PL; P = 0.039). In addition, EXT418 treatments ameliorated the tumour‐induced elevation in muscle Il6 transcript levels (TA and GAS/PL), independently of food intake. Il‐6 transcript levels in adipose tissue and circulating IL‐10 were elevated in response to the tumour but these increases were not significant with EXT418 administration. Tumour mass was not altered by EXT418. Conclusions EXT418 mitigates LLC‐induced cachexia by attenuating skeletal muscle inflammation, proteolysis, and mitophagy, without affecting tumour mass and partially independent of food intake