Acidosis Activates the Nrf2 Pathway in Renal Proximal Tubule-Derived Cells through a Crosstalk with Renal Fibroblasts

Crosstalk with renal proximal tubule cells drives acidosis-induced inflammatory response and dedifferentiation of fibroblasts via p38-singaling

BACKGROUND

Tubulointerstitial kidney disease associated microenvironmental dysregulation, like acidification, inflammation and fibrosis, affects tubule cells and fibroblasts. Micromilieu homeostasis influences intracellular signaling and intercellular crosstalk. Cell-cell communication in turn modulates the interstitial microenvironment. We assessed the impact of acidosis on inflammatory and fibrotic responses in proximal tubule cells and fibroblasts as a function of cellular crosstalk. Furthermore, cellular signaling pathways involved were identified.

METHODS

HK-2 (human proximal tubule) and CCD-1092Sk (human fibroblasts), in mono and coculture, were exposed to acidic or control media for 3 or 48 h. Protein expression of inflammation markers (TNF, TGF-ß and COX-2), dedifferentiation markers (N-cadherin, vinculin, ß-catenin and vimentin), fibrosis markers (collagen III and fibronectin) and phospho- as well as total MAPK levels were determined by western blot. Secreted collagen III and fibronectin were measured by ELISA. The impact of MAPK activation was assessed by pharmacological intervention. In addition, necrosis, apoptosis and epithelial permeability were determined.

RESULTS

Independent of culture conditions, acidosis caused a decrease of COX-2, vimentin and fibronectin expression in proximal tubule cells. Only in monoculture, ß-Catenin expression decreased and collagen III expression increased in tubule cells during acidosis. By contrast, in coculture collagen III protein expression of tubule cells was reduced. In fibroblasts acidosis led to an increase of TNF, COX-2, vimentin, vinculin, N-cadherin protein expression and a decrease of TGF-ß expression exclusively in coculture. In monoculture, expression of COX-2 and fibronectin was reduced. Collagen III expression of fibroblasts was reduced by acidosis independent of culture conditions. In coculture, acidosis enhanced phosphorylation of ERK1/2, JNK1/2 and p38 transiently in proximal tubule cells. In fibroblasts, acidosis enhanced phosphorylation of p38 in a sustained and very strong manner. ERK1/2 and JNK1/2 were not affected in fibroblasts. Inhibition of JNK1/2 and p38 under coculture conditions reduced acidosis-induced changes in fibroblasts significantly.

CONCLUSIONS

Our data show that the crosstalk between proximal tubule cells and fibroblasts is crucial for acidosis-induced dedifferentiation of fibroblasts into an inflammatory phenotype. This dedifferentiation is at least in part mediated by p38 and JNK1/2. Thus, cell-cell communication is essential for the pathophysiological impact of tubulointerstitial acidosis.

The synergism of cytosolic acidosis and reduced NAD+/NADH ratio is responsible for lactic acidosis-induced vascular smooth muscle cell impairment in sepsis

BACKGROUND

During sepsis, serve vascular dysfunctions lead to life-threatening multiple organ failure, due to vascular smooth muscle cells (VSMC) impairments, resulting in vasoplegia, hypotension and hypoperfusion. In addition, septic patients have an altered cell metabolism that leads to lactic acidosis. Septic patients suffering from lactic acidosis have a high risk of mortality. In addition, septic survivors are at risk of secondary vascular disease. The underlying mechanisms of whether and how lactic acidosis leads to the changes in VSMCs is not well understood. The aim of this study was to comprehensively investigate the effect of lactic acidosis on VSMCs and additionally compare the effects with those induced by pure acidosis and sodium lactate.

METHODS

Primary human aortic smooth muscle cells (HAoSMCs) were treated for 48 h with lactic acidosis (LA_pH 6.8), hydrochloric acid (HCl_pH 6.8), sodium lactate (Na+-lactate_pH 7.4) and the respective controls (ctrl._pH 7.4; hyperosmolarity control: mannitol_pH 7.4) and comparatively analyzed for changes in (i) transcriptome, (ii) energy metabolism, and (iii) phenotype.

RESULTS

Both types of acidosis led to comparable and sustained intracellular acidification without affecting cell viability. RNA sequencing and detailed transcriptome analysis revealed more significant changes for lactic acidosis than for hydrochloric acidosis, with lactate being almost ineffective, suggesting qualitative and quantitative synergism of acidosis and lactate. Bioinformatic predictions in energy metabolism and phenotype were confirmed experimentally. Lactic acidosis resulted in strong inhibition of glycolysis, glutaminolysis, and altered mitochondrial respiration which reduced cellular ATP content, likely due to increased TXNIP expression and altered NAD+/NADH ratio. Hydrochloric acidosis induced significantly smaller effects without changing the NAD+/NADH ratio, with the ATP content remaining constant. These metabolic changes led to osteo-/chondrogenic/senescent transdifferentiation of VSMCs, with the effect being more pronounced in lactic acidosis than in pure acidosis.

CONCLUSIONS

Overall, lactic acidosis exerted a much stronger effect on energy metabolism than pure acidosis, whereas lactate had almost no effect, reflecting the qualitative and quantitative synergism of acidosis and lactate. As a consequence, lactic acidosis may lead to acute functional impairments of VSMC, sustained perturbations of the transcriptome and cellular dedifferentiation. Moreover, these effects may contribute to the acute and prolonged vascular pathomechanisms in septic patients.