Facilitated internalization of neocarzinostatin and its lipophilic polymer conjugate, SMANCS, into cytosol in acidic pH.

The effects of environmental pH on the binding and cytotoxicity of the antitumor proteins neocarzinostatin (NCS) and SMANCS [copoly(styrene-maleic acid)-conjugated NCS] to cultured cells were studied by using their fluorescent-labeled derivatives (F-drugs). At 37 degrees C the binding of these drugs to HeLa cells was pH dependent: The amount of cell-bound drugs increased with an increase in the acidity of the medium. The pH-dependent change in the binding of the drugs was not as evident at 0 degree C. The cytotoxic action of these drugs was much more rapid at acidic pH compared with that at neutral or slightly alkaline pH. Furthermore, F-drugs could be utilized to probe the microenvironmental pH in Meth-A cells, in which the drug was located by the ratio of fluorescent intensities at 450 and 490 nm. The environment of the cell-bound F-drugs became acidic with incubation time at 37 degrees C but not at 0 degree C. Inasmuch as these drugs directly attack DNA, these results suggest that NCS and SMANCS are translocated across the membrane of acidic vesicles into the cytosol after endocytotic uptake. This hypothesis is also supported by the finding that NH4Cl and chloroquine protected HeLa cells against the cytotoxicity of the drugs. Data also showed that the hydrophobic polyanion conjugate SMANCS had a much greater cell binding (10 times) and more rapid internalization compared with NCS. Taken together, our results show that acidic pH of tumor tissue is preferable for effective binding and internalization into cytosol for NCS and SMANCS.