Search

The hypoxia-inducible factor (HIF) pathway is essential for cell survival under low oxygen and plays an important role in tumor cell homeostasis. We investigated the function of miR-210, the most prominent microRNA upregulated by hypoxia and a direct transcriptional target of HIFs. miR-210 expression was elevated in multiple cancer types and correlated with metastasis of breast and melanoma tumors. miR-210 overexpression in cancer cell lines bypassed hypoxia-induced cell cycle arrest and partially reversed the hypoxic gene expression signature. We identified MNT, a known MYC antagonist, as a miR-210 target. MNT mRNA contains multiple miR-210 binding sites in the 3' UTR and its knockdown phenocopied miR-210 overexpression. Furthermore, loss of MYC abolished miR-210-mediated override of hypoxia-induced cell cycle arrest. Comparison of miR-210 and MYC overexpression with MNT knockdown signatures also indicated that miR-210 triggered a "MYC-like" transcriptional response. Thus, miR-210 influences the hypoxia response in tumor cells through targeting a key transcriptional repressor of the MYC-MAX network.

We have recently reported a connection between the expression of the Werner syndrome gene (WRN), whose loss of function has been implicated in a human progeroid syndrome (WS), and the Myc oncoprotein. Myc overexpression directly elevates trancription of the WRN gene, whose presence is required to avoid senescence during Myc proliferative stimuli. Here we discuss several hypotheses to explain why WRN might be required to support oncogenic proliferation in light of the known function of WRN protein and Myc in genomic instability and transcriptional modulation. In addition, we address the apparent paradox of why patients with WS, lacking WRN function, have increased incidence of certain cancers.

The synthesis of tRNA and 5S rRNA by RNA polymerase (pol) III is cell cycle regulated in higher organisms. Overexpression of pol III products is a general feature of transformed cells. These observations may be explained by the fact that a pol III-specific transcription factor, TFIIIB, is strongly regulated by the tumour suppressors RB and p53, as well as the proto-oncogene product c-Myc. RB and p53 repress TFIIIB, but this restraint can be lost in tumours through a variety of mechanisms. In contrast, c-Myc binds and activates TFIIIB, causing potent induction of pol III transcription. Using chromatin immunoprecipitation and RNA interference, we show that c-Myc interacts with tRNA and 5S rRNA genes in transformed cervical cells, stimulating their expression. Availability of pol III products may be an important determinant of a cell's capacity to grow. The ability to regulate pol III output may therefore be integral to the growth control functions of RB, p53 and c-Myc.