Effects of Bioconverted Guava Leaf (Psidium guajava L.) Extract on Skeletal Muscle Damage by Regulation of Ubiquitin–Proteasome System and Apoptosis in Type 2 Diabetic Mice

Skeletal muscle atrophy is one of the serious complications of diabetes, which increases the risk of frailty, falls, and mortality. However, interventions for muscle atrophy are limited, and research is needed regarding the treatment of muscle wasting. Recently, the bioconversion of natural products by lactic acid bacteria has been highlighted as a possibility to improve the bioavailability of active ingredients. This process also produces metabolites, which are key signaling mediators for a variety of physiological functions. This study investigated the effect of bioconverted guava leaf (<i>Psidium guajava</i> L., GL) by <i>Lactobacillus plantarum</i> on hyperglycemia-induced skeletal muscle atrophy in type 2 diabetes mellites (T2DM) mice. Diabetes was induced by a high-fat diet with a two-time streptozotocin (STZ) injection (60 mg/kg BW) in male C57BL/6J mice. After diabetes was induced (a fasting blood glucose level (FBG) ≥ 300 mg/dL), the mice were administered with GL (100 mg/kg/day) or bioconverted GL (FGL) (50 mg/kg/day) by oral gavage for 14 weeks. FGL contains different substances such as hydroxyl-isocaproic acid and hydroxyl-isovaleric acid compared to GLE itself, which have potential to prevent muscle degradation in T2DM mice. GL and FGL supplementation reduced the FBG level in T2DM mice. In addition, GL and FGL supplementation enhanced muscle strength, the skeletal muscle cross-sectional area, and ameliorated ubiquitin-proteasome system (UPS)-related pathways in T2DM mice. On the other hand, GLE supplementation ameliorated glucose tolerance demonstrated by oral glucose tolerance test and enhanced insulin signaling pathway. In addition, only FGL supplementation attenuated skeletal muscle inflammation and apoptosis with an improved mammalian target of the rapamycin (mTOR)-autophagy-related pathway. Although administered at a half dose of GLE, FGL demonstrated greater efficacy in regulating the expression of these molecular markers. The result suggests that even GL itself has anti-diabetic effects, and the functionality would be enhanced by the bioconversion of GL with <i>L. Plantarum</i>, which has an additive or/and a synergistic effect. Taken together, FGL could be used as a potential nutraceutical to attenuate muscle degradation by the inhibition of inflammation, the UPS, and the apoptosis pathway.