Mammalian ribonucleotide reductase is rate limiting for the synthesis of DNA. The active enzyme is composed of two dissimilar components called R1 and R2, encoded by different genes. The 3′ untranslated regions (3′ UTRs) of R1 and R2 messages contain sequences that are important in regulating gene expression through changes in message stability. We have constructed expression plasmids containing the R1 or R2 mRNA 3′ UTRs, and we show that transfection of these plasmids into highly malignant mouse 10T½ cells significantly suppresses the tumorigenic properties of these cells in syngeneic mice when compared with cells transfected with the same plasmid lacking R1 or R2 3′ UTR sequences or when compared with cells transfected with the same plasmid expressing a heterologous sequence as a control. Furthermore, cells expressing the R2 3′ UTR exhibit significantly reduced potential to disseminate to the lungs of syngeneic animals in experimental metastasis assays. The tumor-suppressive effects of the mouse R1 and R2 3′ UTRs were not confined to mouse cells, because human HeLa cells transfected with expression plasmids containing either R1 or R2 3′ UTRs were also significantly less tumorigenic in assays using BALB/c nu/nu mice. These studies demonstrate that the untranslated regions of ribonucleotide reductase mRNAs can function as modifiers of tumor cell development and for the more complex process of tumor dissemination. We propose that these malignancy-suppressive effects are mediated through RNA interactions with cellular components involved in growth regulation through mechanisms of posttranscriptional control of gene expression. In addition, these observations emphasize the enormous potential of untranslated RNA to act directly as modifiers of biological characteristics relevant to mechanisms of malignancy.

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This work was supported by the National Cancer Institute of Canada and the Natural Sciences and Engineering Research Council of Canada (to J. A. W.) and by a postdoctoral fellowship from the Medical Research Council of Canada (to H. F.). J. A. W. is a Terry Fox Senior Scientist of the National Cancer Institute of Canada.

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