We reported the presence in human cells of a noncoding mitochondrial RNA that contains an inverted repeat (IR) of 815 nucleotides (nt) covalently linked to the 5 end of the mitochondrial 16S RNA (16S mtrRNA). The transcript contains a stem-loop structure and is expressed in human proliferating cells but not in resting cells. Here, we demonstrate that, in addition to this transcript, normal human proliferating cells in culture express 2 antisense mitochondrial transcripts. These transcripts also contain stem-loop structures but strikingly they are down-regulated in tumor cell lines and tumor cells present in 17 different tumor types. The differential expression of these transcripts distinguishes normal from tumor cells and might contribute a unique vision on cancer biology and diagnostics.differential expression in cancer ͉ RNAs with stem-loop structures R ecently, we described a novel human mitochondrial transcript of 2,374 nt that contains a long inverted repeat (IR) linked to the 5Ј end of the 16S mitochondrial rRNA (16S mtrRNA) (1, 2), which we designated noncoding mitochondrial RNA or ncmtRNA (2). The IR generates a stem-loop structure with an 820-bp doublestranded region and a 40-nt loop (2). In situ hybridization (ISH) showed that the ncmtRNA is overexpressed in several tumor cell lines but not in nondividing cells, suggesting that the ncmtRNA may play a role in cell proliferation (2).Because the results described before were obtained using tumor cell lines (2), we asked whether the ncmtRNA (from now on, sense ncmtRNA, SncmtRNA) is expressed in normal proliferating cells. Here we show that ISH of human umbilical vein endothelial cells (HUVEC), foreskin keratinocytes (HFK) (3), and human tonsil endothelial cells (HUTEC) (4) also express the SncmtRNA. However, and in striking contrast with tumor cell lines, ISH of normal proliferating cells revealed expression of antisense transcripts. These molecules were identified as 2 unique transcripts containing IRs linked to the 5Ј region of the antisense 16S mtRNA transcribed from the L-strand of the mtDNA (1, 2). We named these transcripts antisense ncmtRNA-1 (ASncmtRNA-1) and antisense ncmtRNA-2 (ASncmtRNA-2). Finally, we show that the antisense transcripts are also expressed in proliferating cells present in normal human tissues but are down-regulated in cells present in human tumors of different types and patients.

Background: Down-regulation of the antisense mitochondrial ncRNAs (ASncmtRNAs) is a general vulnerability of cancer cells.Results: Knocking down the ASncmtRNAs induces apoptosis, down-regulation of survivin, and generation of microRNAs.Conclusion: Induction of apoptosis in cancer cells is potentiated by down-regulation of survivin.Significance: The ASncmtRNAs represent a new potential therapeutic target for cancer.

We reported that knockdown of the antisense noncoding mitochondrial RNAs (ASncmtRNAs) induces apoptotic death of several human tumor cell lines, but not normal cells, suggesting this approach for selective therapy against different types of cancer. In order to translate these results to a preclinical scenario, we characterized the murine noncoding mitochondrial RNAs (ncmtRNAs) and performed in vivo knockdown in syngeneic murine melanoma models. Mouse ncmtRNAs display structures similar to the human counterparts, including long double-stranded regions arising from the presence of inverted repeats. Knockdown of ASncmtRNAs with specific antisense oligonucleotides (ASO) reduces murine melanoma B16F10 cell proliferation and induces apoptosis in vitro through downregulation of pro-survival and metastasis markers, particularly survivin. For in vivo studies, subcutaneous B16F10 melanoma tumors in C57BL/6 mice were treated systemically with specific and control antisense oligonucleotides (ASO). For metastasis studies, tumors were resected, followed by systemic administration of ASOs and the presence of metastatic nodules in lungs and liver was assessed. Treatment with specific ASO inhibited tumor growth and metastasis after primary tumor resection. In a metastasis-only assay, mice inoculated intravenously with cells and treated with the same ASO displayed reduced number and size of melanoma nodules in the lungs, compared to controls. Our results suggest that ASncmtRNAs could be potent targets for melanoma therapy. To our knowledge, the ASncmtRNAs are the first potential non-nuclear targets for melanoma therapy.

The ubiquitous localization of these mitochondrial transcripts in the nucleus suggests that they are new players in the mitochondrial-nuclear communication pathway or retrograde signaling. Down regulation of the ASncmtRNAs seems to be an important step on neoplastic transformation and cancer progression.