Modelling tumour acidity and invasion.

The intracellular pH (pHi) of mammalian cells is tightly regulated by the concerted action of a number of different pumps in the plasma membrane. Despite the acidic extracellular environment (pHe 6.8-7.0) of some tumours, the pHi of solid tumours is neutral or slightly alkaline compared to normal tissues (pHi 7.0-7.4). This gives rise to a reversed pH gradient across the cell membrane between tumours and normal tissue, which has been implicated in many aspects of tumour progression. One such area is tumour invasion: the incubation of tumour cells at low pH has been shown to induce more aggressive invasive behaviour in vitro. In this paper we use mathematical models to investigate whether altered proteolytic activity at low pH is responsible for the stimulation of a more metastatic phenotype. We examined the effect of culture pH on the secretion and activity of two different classes of proteinases: the metalloproteinases (MMPs), and the cysteine proteinases (such as cathepsin B). The modelling predicts that, in addition to metalloproteinase activity, the pH-induced peripheral redistribution of cathepsin B could be a major factor involved in the acquisition of a more metastatic phenotype in malignant cells at low pHe.