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Cytostatic and Cytotoxic Properties of Pyronin Y: Relation to Mitochondrial Localization of the Dye and Its Interaction with RNA¹

Sloan-Kettering Institute for Cancer Research, Walker Laboratory, Rye, New York 10580

Pyronin Y (PY) is an intercalating cationic dye that shows specificity towards RNA. In viable cells this dye also accumulates in mitochondria. The cytostatic and cytotoxic effects of PY on L1210 and Chinese hamster ovary cells were studied in relation to its intracellular localization and compared with the affinity of PY to bind to double-stranded DNA and RNA and its propensity to condense single-stranded DNA and RNA. Antitumor properties of PY were tested on L1210 leukemia and Sarcoma 180 ascites in mice.

At a concentration of 1.7 to 3.3 µM, PY was localized almost exclusively in mitochondria of cultured cells, similar to another mitochondrial probe, rhodamine 123. At that concentration PY was not toxic but suppressed cell growth, arresting cells in G₁. At a concentration of 6.7 to 33.0 µM, PY was also localized in nucleoli and uniformly in cytoplasm, bound to the RNase-sensitive material therein. At that high concentration PY induced cell arrest in G₂ and S and was cytotoxic. The dye exhibited a propensity to bind and condense (precipitate) single-stranded nucleic acids, and condensation could be measured by the appearance of light-scattering products. Among a variety of natural and synthetic nucleic acids the most sensitive were the RNA polymer, polyriboadenylate, and the copolymer, polyriboadenylate and polyriboguanylate, which underwent condensation at a PY concentration of 6.6 to 10.0 µM. Natural and synthetic DNA polymers were resistant to condensation.

The data suggest that the cytostatic (G₂ and S arrest) and cytotoxic (inability to exclude trypan blue, loss of clonogenicity) effects of PY seen at 6.7 to 33.0 µM concentration may be a consequence of the dye binding to RNA. PY may intercalate to double-stranded RNA and/or cause the specific condensation of single-stranded RNA; the polyadenylated sections of mRNA appear to be the most sensitive cellular targets to undergo condensation.

PY showed antitumor properties extending survival of L1210 leukemic mice by 50% and slowing growth of Sarcoma 180 ascites tumor. The possibility that certain antitumor drugs, generally believed to act via intercalation to DNA, may exert chemotherapeutic effects via their interactions with RNA is discussed.

¹ Supported by Grants CA 28704 and 23296 from the National Cancer Institute.

² To whom requests for reprints should be addressed, at Sloan-Kettering Institute for Cancer Research, Walker Laboratory, 145 Boston Post Road, Rye, NY 10580.

Received 4/14/86. Revised 7/17/86. Accepted 7/24/86.

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