Protodynamic intracellular acidification by cis-urocanic acid promotes apoptosis of melanoma cells in vitro and in vivo.

ABSTRACT

The extracellular tumor microenvironment is acidified, whereas the intracellular pH of tumor and stromal cells is neutral. cis-Urocanic acid (cis-UCA), an endogenous compound of the skin, can acidify the cytosol by transporting protons into the cells. This phenomenon, termed the protodynamic concept, was studied here in human cancer cells. cis-UCA dose-dependently reduced the number of viable human melanoma, cervical carcinoma, and fibrosarcoma cells at weakly acidic extracellular pH. The intracellular pH decreased by up to 0.5 pH units in a concentration-dependent manner with 0.3-30 m cis-UCA at extracellular pH 6.5 but not at pH 7.4. Under the same conditions, 30 mM cis-UCA induced annexin-V binding and activation of caspase-3 in A2058 melanoma cells as signs of apoptotic cell death. Finally, growth of human melanoma xenografts in SCID mice was suppressed by 60% following intratumoral injection of cis-UCA. Accordingly, the percentage of tumor necrosis and active caspase-3-immunopositive cells increased, whereas proliferation activity decreased. These results identify cis-UCA as an anticancer agent inhibiting melanoma growth by immediate intracellular acidification followed by apoptotic cell death in vivo.