Differential Impact of Olive Leaf Extract and Its Secoiridoid Components, Oleuropein Aglycone and Oleacin, on Adipogenic Differentiation and Proliferation of Bone Marrow Mesenchymal Stem Cells

Background/Objectives: Bone marrow adipose tissue (BMAT) serves multiple physiological roles but accumulates with age, compromising skeletal health. This expansion is largely driven by an adipogenic drift of bone marrow mesenchymal stromal cells (BMSCs), shifting attention toward stromal cell fate regulation as a target to preserve bone marrow homeostasis. Preventing adipogenic commitment may be as relevant as directly inducing osteogenesis for maintaining a bone-permissive marrow microenvironment. Here, we investigated whether olive leaf extract (OLE) and its purified secoiridoid components, oleacin (OC) and oleuropein aglycone (OA), modulate the adipogenic differentiation and proliferative capacity of human BMSCs. Methods: Human BMSCs were induced to undergo adipogenic differentiation and treated with OLE, OC, or OA. Intracellular lipid accumulation and the expression of key adipogenic regulators were assessed. Proliferative capacity was evaluated under both maintenance and adipogenic conditions. Results: Under adipogenic conditions, OLE markedly reduced intracellular lipid accumulation and induced a coordinated downregulation of PPAR gamma, PLIN1, FABP4, ADIPOQ, LEP and the adipogenesis-associated miR-422a. In contrast, OC and OA exerted more selective and specific effects on biomarkers, indicating the partial and complementary modulation of adipogenic programs. Notably, OLE also increased BMSC proliferation under both maintenance and adipogenic conditions, suggesting the preservation of a less committed stromal cell pool. Although the relative contribution of enhanced proliferation versus the direct inhibition of adipogenic pathways cannot be fully disentangled, the combined molecular and functional data support a dual action of OLE on stromal cell fate. Conclusions: OLE limits adipogenic commitment while maintaining stromal cell proliferative competence, processes that are critically involved in BMAT expansion and bone marrow dysfunction. OC and OA contribute to OLE bioactivity deserving further investigation, particularly in combination, as potential modulators of BMAT expansion.