Naphthalene-Type Glycosides from Rumex obtusifolius Roots and Their Protective Effects Against Muscle Atrophy in C2C12 Myotubes

Background/Objectives: Rumex obtusifolius L. (Polygonaceae) has been traditionally used to treat various disorders, including hepatic and gastrointestinal diseases. However, the phytochemical constituents of its roots and their potential protective effects against skeletal muscle atrophy remain poorly understood. This study aimed to isolate and characterize bioactive constituents from R. obtusifolius roots and evaluate their protective effects against dexamethasone (DEX)-induced muscle atrophy in C2C12 myotubes. Methods: LC–MS-guided phytochemical investigation of the ethanol extract of R. obtusifolius roots, followed by successive column chromatography and HPLC purification, resulted in the isolation of four naphthalene-type glycosides. Their structures were elucidated using 1D and 2D NMR spectroscopy, HR-ESIMS, and chemical transformation. The protective effects of compounds 1 and 4 against dexamethasone (DEX)-induced muscle atrophy were evaluated by assessing myotube morphology, myogenic and atrophy-related protein expression, and PI3K/Akt/mTOR signaling. Results: A new naphthalene malonylglucoside, nepodin-8-O-β-D-(6′-O-malonyl)-glucopyranoside (1), together with three known glycosides (2–4), was identified. Among the isolated compounds, compound 1 significantly attenuated DEX-induced muscle atrophy in a concentration-dependent manner by increasing myotube diameter and improving myotube morphology. It restored the expression of the myogenic markers MyoD and myogenin while suppressing the atrophy-related proteins MuRF1 and MAFBX. Furthermore, compound 1 reversed DEX-induced suppression of the PI3K/Akt/mTOR signaling pathway, indicating recovery of anabolic signaling. Conclusions: This study reports a new naphthalene malonylglucoside (1) from R. obtusifolius roots and demonstrates that compound 1 protects against DEX-induced skeletal muscle atrophy through restoration of myogenic differentiation and activation of the PI3K/Akt/mTOR pathway. These findings suggest that compound 1 is a promising natural lead compound for the development of therapeutics targeting muscle wasting disorders.