RAB18-dependent recruitment of OPTN initiates selective lipophagy in osteoblasts

Selective lipophagy requires cargo recognition and recruitment of autophagy receptors to lipid droplets (LDs), yet the molecular mechanisms that couple LDs to the core autophagy machinery remain poorly defined. Here, we identified the small GTPase RAB18 (RAB18, member RAS oncogene family) as an upstream initiator of lipophagy that directly recruited the macroautophagy/autophagy receptor OPTN (optineurin) to LDs in osteoblasts. Lipid stress induced RAB18 activation and its localization to LDs, where RAB18 engaged OPTN enabling OPTN-LC3 bridging and lysosomal degradation of LDs. Loss of either RAB18 or OPTN impaired lipophagic flux, resulting in lipid accumulation and defective osteogenic differentiation, whereas OPTN overexpression partially rescued RAB18 deficiency, supporting a hierarchical RAB18-OPTN pathway. Thus, while OPTN acted as a critical effector downstream of RAB18, it did not feed back to promote RAB18 recruitment. In vivo, perturbation of this axis compromised bone regeneration under hyperlipidemic conditions. Together, these findings establish RAB18-dependent recruitment of OPTN as a molecular mechanism for selective lipophagy and reveal lipophagy as a critical metabolic adaptation that sustains osteoblast function during lipid stress.Abbreviations: AAV: adeno-associated virus; ALP: alkaline phosphatase; Baf A1: bafilomycin A₁; CC domain 1: coiled-coil domain 1; CCK-8: cell counting Kit-8 kit; Co-IP: co-immunoprecipitation; ER: endoplasmic reticulum; HE: hematoxylin and eosin; IF: immunofluorescence; IHC: immunohistochemistry; KD: knockdown; LDs: lipid droplets; Micro-CT: microscopic computerized tomography; OE: overexpression; OPTN: optineurin; qRT-PCR: quantitative real-time polymerase chain reaction; RAB18: RAB18, member RAS oncogene family; ROI: region of interest; SD: standard deviations; TBK1: TANK binding kinase 1; TEM: transmission electron microscopy; WB: western blot.