GPR81 Regulates MCT1 Membrane Translocation Through a PKA-Dependent Signaling Pathway in Rat Podocytes

Podocytes and their foot processes form a functional layer of the glomerular filtration barrier. Due to their unique morphology and function, podocytes employ distinct nutrient pathways to maintain the bioenergetic balance, with lactate being one of several available energy substrates. Enhanced lactate intake modulates the redox state of the cell by increasing mitochondrial respiration and reactive oxygen species production. Monocarboxylate transporter 1 (MCT1) is the primary lactate transporter, and alterations in its surface expression may contribute to the regulation of lactate uptake in podocytes. Beyond its metabolic role, lactate also acts as a crucial signaling molecule by binding to G-protein-coupled receptor 81 (GPR81), mediating a wide range of physiological effects through the inhibition of protein kinase A (PKA). The present study investigated novel regulatory mechanisms of MCT1 internalization, which depend on GPR81 signaling and PKA activity in primary rat podocytes, through the biotinylation assay. Surprisingly, both PKA inhibition (with H89 and PKI 14-22) and activation (with 8-bromo-cAMP and H₂O₂) increased MCT1 internalization. GPR81 was also found to regulate MCT1 membrane trafficking, likely through the modulation of PKA activity but also potentially through PKA-independent mechanism. Additionally, general dynamic changes in endocytic activity were detected under the present conditions with pHrodo-dextran fluorescence analysis. These results suggest that the modulation of PKA activity and GPR81 signaling may regulate lactate transport via MCT1, thereby ensuring its proper metabolic function in podocytes.