RB1 adenovirus

Triple-negative breast cancer (TNBC) includes basal-like and claudin-like subtypes, for which no specific treatments are currently available. Although the retinoblastoma tumor suppressor gene (RB1) is frequently deleted along with TP53 in TNBC, the gene cannot be directly targeted. Therefore, there is great interest in identifying vulnerabilities downstream of RB1 that could be exploited therapeutically. Here, researchers determined that combined inactivation of mouse Rb and p53 induces claudin-like TNBC with amplification of Met, Birc2/3-Mmp13-Yap1, and Pvt1-Myc in different mammary epithelial cells. Gene set enrichment analysis revealed elevated expression of the mitochondrial protein translation (MPT) gene pathway in Rb/p53-deficient tumors compared with tumors lacking p53 alone. Thus, bioinformatic, functional, and biochemical analyses revealed that the RB1-E2F complex binds to the MPT gene promoter, thereby regulating transcription and controlling MPT. In addition, screening of drugs approved by the U.S. Food and Drug Administration (FDA) revealed that the MPT antagonist tigecycline (TIG) is a potent inhibitor of the proliferation of Rb/p53-deficient tumor cells. TIG preferentially inhibits the proliferation of RB1-deficient TNBC cells, targets the tumor bulk and tumor stem cells, and significantly inhibits the growth of xenograft tumors. TIG also synergizes with sulfasalazine in culture and xenograft experiments. These findings suggest that RB1 deficiency promotes cancer cell proliferation in part by enhancing mitochondrial function and identify TIG as a clinically approved drug for the treatment of RB1-deficient TNBC.

Indeed, coinfection with Ad-E2F1 and Ad-BCL2 significantly reduced apoptosis. qRT-PCR under these conditions revealed robust induction of multiple MPT genes in the RB1+ line (MDA-MB-231) as well as the RB1- TNBC lines BT549 and HCC1937 (Figure 1A). Importantly, E2F1 stimulated MPT gene expression to the same extent that it induced the known E2F1-regulated cell cycle gene and the pro-apoptotic gene BBC3 (also known as PUMA). Three days after transduction, Ad-E2F1 significantly induced COX II in four different BC lines (RB1+: MDA-MB-231 and MCF7; RB1-: BT549 and HCC1937) compared to Ad-GFP controls, regardless of coinfection with Ad-BCL2 (Figure 1B). Finally, the researchers investigated whether overexpression of RB1 would repress the MPT gene. The RB1-deficient BT549 cell line was transduced with a recombinant adenovirus encoding wild-type human RB1, and Ad-GFP was used as a control. Two days after transduction, the expression of the MPT gene, as well as known E2F-regulated cell cycle and apoptosis genes, was significantly repressed by about 2-fold (Figure 1C). Therefore, the MPT gene is induced by E2F1 and repressed by pRb with the same efficiency as a bona fide E2F1-regulated gene.

Figure 1. E2F1 and RB1 control MPT gene expression in BC cells. (Jones R A, et al., 2016)