An Acidic Tubulointerstitial Microenvironment Delays Cell-Cycle Progression in Proximal Tubule Cells and Fibroblasts and Synergizes With Lactate to Drive Phenotypic Alterations.

BACKGROUND

Chronic kidney disease (CKD) is characterized by progressive tubulointerstitial inflammation and fibrosis, particularly affecting the proximal tubule. Both proximal tubular epithelial cells and interstitial fibroblasts contribute to CKD progression through cell-cycle arrest and phenotypic transitions. The aim of the present study was to address whether lactic acidosis contributes to these processes.

METHODS

Human proximal tubular cells (HK-2 cells) and fibroblasts (CCDSk cells) were exposed to lactic acid, hydrochloric acid, or sodium lactate. Transcriptomic changes were assessed by RNA sequencing followed by GO term enrichment as well as canonical pathway and upstream-regulator analyses. Predicted effects on the cell cycle were validated experimentally using BrdU incorporation and digital microscopy-based cell-cycle analysis. Protein expression of IL-6, c-FOS, phospho-JAK1 and JAK2, STAT3, SRF, and p38 was assessed by Western blot.

RESULTS

Transcriptomic analysis revealed synergistic effects of lactate and acidification on gene expression, which were associated with cell type-specific phenotypic changes. Biological assays confirmed these predictions: HK-2 cells underwent a sustained G1 arrest with reduced DNA synthesis, while CCDSk cells displayed only a transient arrest resolving after 48 h. Predictions further indicated that lactic acidosis amplifies these effects, driving fibrotic reprogramming in HK-2 cells and promoting a phenotypic switch of CCDSk cells toward an inflammatory state.

CONCLUSIONS

Our findings suggest that extracellular lactic acidosis primes tubular cells for fibrotic remodeling and CCDSk cells for inflammatory activation, and may therefore represent an initiating microenvironmental trigger for maladaptive changes in CKD progression.