Tumor suppression by the prohibitin gene 3'untranslated region RNA in human breast cancer

Prohibitin is a candidate tumor suppressor gene located on human chromosome 17q21, a region of frequent loss of heterozygosity in breast cancers. We showed previously that microinjection of RNA encoded by the prohibitin gene 3'untranslated region (3'UTR) blocks the G(1)-S transition causing cell cycle arrest in several human cancer cell lines, including MCF7. Two allelic forms (C versus T) of the prohibitin 3'UTR exist, and carriers of the less common variant (Tallele) with a family history of breast cancer exhibited an increased risk of breast cancer. In the present study, we examined the tumor suppressor activity of the prohibitin 3'UTR in human breast cancer cells. Stable clones of MCF7 cells expressing either the C allele or the T allele RNA under the control of the cytomegalovirus promoter were isolated and compared with empty vector clones. Clones expressing the C allele RNA (UTR/C) exhibited significant suppression of growth in cell proliferation assays, inhibition of colony formation in soft agar assays, and suppression of xenograft tumor growth when implanted on nude mice, compared with either T allele expressing or empty vector clones. Immunohistochemical analyses with Ki67 staining confirmed a significant reduction in proliferation of UTR/C tumors. Thus, the C allele of prohibitin 3'UTR produces a functional RNA, whereas a single nucleotide polymorphism creates a null allele (T allele) of which the RNA product has lost activity. Our data demonstrate for the first time that an RNA molecule functions as a tumor suppressor in human breast cancer.