Acidosis-induced p38-kinase activation triggers an IL-6-mediated crosstalk of renal proximal tubule cells with fibroblasts leading to their inflammatory response.

BACKGROUND

Local interstitial acidosis in chronic kidney disease (CKD) induces inflammatory responses and dedifferentiation of proximal tubule cells (PTCs), disrupting cellular crosstalk through cytokine and COX-2 metabolite secretion. This promotes a switch to an inflammatory fibroblast phenotype, further exacerbating inflammation and PTC dedifferentiation. p38-signaling and downstream transcription factors, including P-CREB and c-fos, contribute to these responses. This study investigates the impact of acidosis on inflammatory responses in PTCs and fibroblasts, focusing on cellular crosstalk and the role of p38-signaling.

METHODS

HK-2 (human PTCs) and CCD-1092Sk (human fibroblasts) were exposed to acidic or control media in mono- and coculture for 30 min, 3 h, or 48 h. Protein expression of IL-6, phosphorylated (P-) and total CREB, P- and total SRF, c-fos, and P- and total p38 was analyzed by western blot. IL-6 secretion was measured using ELISA. The impact of p38 and IL-6 receptor activity was assessed by pharmacological intervention.

RESULTS

In coculture, acidosis initially caused a transient decrease in IL-6 secretion but significantly increased IL-6 levels after 48 h. Acidosis induced intracellular IL-6 expression in HK-2 cells within 3 h independent of culture conditions, with sustained IL-6 protein increase after 48 h only in coculture. Acidosis also enhanced P-CREB and c-fos expression in coculture during the first 3 h. Regardless of culture conditions, acidosis increased IL-6, c-fos, and P-SRF expression in CCDSK cells after 48 h. P-CREB and COX-2 expression were elevated in CCDSK in coculture. Acidosis-mediated effects on IL-6, P-CREB, and P-SRF expression were p38-dependent in both cell lines. Finally, we assessed the pH-dependency of IL-6 action and found that IL-6 addition increased COX-2 expression via the IL-6 receptor in acidic but not control media. Thus, acidosis enhances IL-6 secretion and potentiates its receptor-mediated biological effects.

CONCLUSION

This study identifies IL-6 as a key mediator of tubule-fibroblast crosstalk in an acidic milieu, promoting inflammatory processes. Acidosis induces IL-6 expression, secretion, and biological effects, with p38 kinase as a crucial mediator. If validated in vivo, these findings could enhance understanding of CKD and support early interventions.