Abstract Body: INTRODUCTION: Aortic stenosis (AS) is the most prevalent valvulopathy in elderly adults, with increased incidence in chronic kidney disease (CKD). Elevated serum phosphate, due to FGF23-Klotho axis dysregulation and impaired excretion, has been identified as a potential pathogenic link between both conditions, along with disturbances in the renin-angiotensin system. Aortic valve interstitial cells (VIC) play a central role in AS pathogenesis; high inorganic phosphate (HP) promotes their osteogenic trans-differentiation via the type III phosphate transporter (Pit-1), triggering mitochondrial dysfunction, NADPH oxidase activation, and ROS generation. Pathways such as PI3K/Akt, MAPK, and TGF-β/Smad have been implicated in HP-driven osteogenesis and fibrosis. OBJECTIVE: To elucidate the pathways activated in the aortic valve (AV) by the HP levels occurring during CKD and its potential role in activating mineralocorticoid receptor (MR) signaling. METHODS: VIC were isolated from the AV of wild type or transgenic rats overexpressing the human (h)-MR. Cells were treated with HP levels (3.2 mM CaCl₂ and 2.6 mM NaH₂PO₄) alone or in combination with aldosterone 10^-8M in the presence of MR antagonist (MRA) finerenone (10^-6 M), or TLR4 inhibitor (TAK-242 10 µM). Control or CKD male rats, generated by 5/6 nephrectomy and having HP diet, were used for ex vivo experiments. WB, ELISA, IHQ and qPCR were used. RESULTS: HP levels trigger ligand-independent transcription and signaling of the MR leading to VIC activation and fibrosis. Transgenic VIC overexpressing h-MR confirmed that elevated MR levels promote VIC activation and fibrotic responses. In parallel, HP increases Toll-like receptor 4 (TLR4) expression and signaling leading to the induction of fibrosis, osteogenesis and inflammation. These effects were intensified by co-treatment of VIC with aldosterone and HP because of the TLR4 signaling potentiation. In vivo, rats suffering from CKD showed significant extracellular matrix (ECM) alterations that were largely reversed by MRA. Exposure of VIC to serum from CKD rats disrupted the expression of key ECM-related and pro-fibrotic genes, including FGF-23 and biglycan, in an MR-dependent manner. CONCLUSIONS: In vitro studies revealed for the first time that HP levels trigger ligand-independent activation of the MR signaling, likely through the increase of oxidative stress and ROS generation. This links phosphate overload to MR-driven fibrosis and ECM remodeling in VIC.