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Identification of an Anionic Form of Glutathione Transferase Present in Many Human Tumors and Human Tumor Cell Lines¹

Laboratory of Cancer Pharmacology, Dana-Farber Cancer Institute, and the Department of Medicine, Harvard Medical School, Boston, Massachusetts 02115

Glutathione transferase (GST) activity and GST isoenzyme composition have been determined for 24 human neoplasms and 6 human tumor cell lines. Substantial activity (40–1010 milliunits/mg protein) was identified in all tumor specimens examined and three of the tumor cell lines. Three tumor cell lines, the human small cell carcinoma line SW2-10S, the Burkitt's lymphoma derived cell line Raji, and the human breast carcinoma cell line MCF-7, contained minimal GST activity. Although the small size of the tumor samples precluded isoenzyme analysis by substrate specificities, analysis of GST activity following sample separation by isoelectric focusing indicated that the predominant (comprising at least 70% of the 1-chloro-2,4-dinitrobenzene-conjugating activity) GST isoenzyme in each of these primary tumor (17 of 17) and tumor cell line (3 of 3) extracts was anionic (isoelectric point, 4.5–4.8). In three tumor samples, adenocarcinomas of the lung, colon, and stomach, analysis by isoelectric focusing identified minor but detectable (10–20% of total) cationic GST.

The anionic form of GST has been purified to homogeneity from three primary human tumors: a malignant melanoma; a mesothelioma; and a breast carcinoma. GST from these tumors consists of two subunits each of M_r 25,200. On Western blot analysis, antibodies raised against the anionic GST purified from mesothelioma detect protein of M_r 25,000 in extracts of both normal kidney and tumors containing anionic GST activity but not in extracts of human liver that did not contain detectable anionic activity. The amino acid compositions of these proteins were quite similar to that previously described for GST- and the amino-terminal amino acid sequences for these tumor-derived isoenzymes are identical to one another and to that previously described for GST- from human placenta.

GST is a major enzymatic activity in many human malignancies, comprising as much as 3% of the cytosolic protein of some tumors. Anionic GST is the predominant form of glutathione transferase activity in many human tumors and human tumor cell lines. In selected tumor samples the predominant anionic GST isoenzyme has been identified as a member of the pi class of this enzyme family. In addition, at least 3 of 17 tumor samples contained lesser but detectable amounts of cationic GST, probably of the alpha class. By conjugating glutathione with electrophilic anticancer drugs, the substantial levels of GST in human tumors may have a role in the innate or acquired resistance of these neoplasms to anticancer therapy.

¹ This work was supported by USPHS Grant CA 19589.

² Recipient of a fellowship award from the Pharmaceutical Manufacturers Association Foundation of America.

³ Recipient of an American Society of Clinical Oncology Young Investigator Award. Present address: Department of Medical Oncology, Yale University School of Medicine, New Haven, CT 06510.

⁴ Recipient of a Leukemia Society of America Scholar Award.

Received 7/10/86. Revised 6/30/87. Revised 10/16/87. Accepted 11/ 3/87.

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