BREAST-CANCER CELLS HAVE A HIGH-CAPACITY TO ACIDIFY EXTRACELLULAR MILIEU BY A DUAL MECHANISM

The extracellular pH in malignant tumors is known to be lower than in normal tissues and may therefore facilitate extracellular activation o f secreted lysosomal cathepsins, We have tested the capability of huma n mammary cells (continuous cell lines and primary culture) to acidify their extracellular environment, using two techniques, By measuring p H changes through alterations of phenolsulfone phthaleine absorbance, we found that the more aggressive MDA-MB-231 human breast cancer cells were more active in acidifying a non-buffered balanced salt solution than the estrogen receptor positive MCF7 and ZR75 cell lines and than normal mammary epithelial cells in primary culture. Metastatic breast cancer cells from pleural effusions were up to 200-fold more active in acidifying their extracellular milieu than non-malignant mammary cell s cultured in the same conditions, strongly suggesting that this diffe rence also occurs in vivo, The use of inhibitors in the presence or ab sence of glucose showed that both lactate and an ATP-driven proton pum p sharing some characteristics of the vacuolar H+ pump were involved, Bafilomycin A1, a specific inhibitor of the vacuolar (V-type) ATP-H+ p ump inhibited part of the acidification by MCF7 cells, but not by MDA- MB-231 cells, We also used microelectrodes to measure extracellular pH , in close contact to the MCF7 breast cancer cells, The pH at the free surface of MCF7 cells was lower by 0.33 +/- 0.14 unit than that of th e surrounding medium, while insertion of the microelectrode tip beneat h the attached surface of the cells showed a greater lowering of pH fr om 0.3 to 1.7 pH unit as long as cell attachment on the substrate prev ented H+ diffusion, We conclude that breast carcinoma cells have a hig her capacity for acidifying their extracellular milieu than normal mam mary cells, and that both a plasma membrane H+-ATPase, and lactic acid production are involved in this acidification. It is therefore possib le that the aspartyl and cysteinyl procathepsins secreted in excess by tumor cells may be activated extracellularly in vivo close to the bas ement membrane.