Question: 

Extracellular acidosis is a major characteristic of many solid growing tumours and leads to altered intracellular signalling and tumour phenotype. Reactive oxygen species (ROS) may be part of the signalling cascade. For this reason the impact of acidosis on ROS formation and secondary on the activation of ERK1/2, p38 and CREB in rat R-3327 prostate cancer cells (AT1) was analysed.

Methods: 

Cells were exposed either to a control (pH 7.4) or an acidic extracellular environment (pH 6.6) for 3 h. Production of ROS was monitored using a derivative of reduced fluorescein and CRE (cAMP response element) promoter activity was analysed by reporter gene assays. Phosphorylation of ERK1/2, p38 and CREB was determined with phospho-specific antibodies.

Results: 

In AT1 cells extracellular acidosis led to an increase in ROS production and ERK1/2- as well as p38 phosphorylation. ROS like H₂O₂ activated ERK1/2 and p38 without changing their expression levels, while scavenging of ROS using tiron diminished the acidosis-induced phosphorylation. These findings indicate that ROS formation is critical for MAP kinase activation by extracellular acidosis. MAP kinase activation in turn leads to an activation of other signalling pathways for instance the cAMP-dependent pathway. Experiments showed that acidosis increased CREB phosphorylation and CRE-mediated gene expression. Inhibition of p38, but not of ERK1/2, abolished acidosis-induced phosphorylation of CREB, a transcription factor known to regulate expression of genes related to malignant phenotypes.

Conclusion: 

The results reveal a complex network linking extracellular acidosis to gene expression in which ROS, p38 and CREB seem to play an important role. (Supported by the Deutsche Krebshilfe and the BMBF ProNet-T3)