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CITED4 Inhibits Hypoxia-Activated Transcription in Cancer Cells, and Its Cytoplasmic Location in Breast Cancer Is Associated with Elevated Expression of Tumor Cell Hypoxia-Inducible Factor 1

¹ Nuffield Department Clinical Laboratory Sciences, John Radcliffe Hospital and ² Department of Cardiovascular Medicine, Wellcome Trust Centre for Human Genetics, Henry Wellcome Building of Genomic Medicine, University of Oxford; and ³ Cancer Research UK Molecular Oncology Laboratory, Weatherall Institute of Molecular Medicine, John Radcliffe Hospital, Oxford, United Kingdom

The interaction of hypoxia-inducible factor 1 and the CH1 domain of the transcriptional coactivator p300/CBP is necessary for the expression of hypoxia responsive genes and tumor angiogenesis. The transcription factor CITED2 binds p300/CBP at the CH1 domain and functions as a negative regulator of hypoxia signaling by competing with hypoxia-inducible factor 1. CITED4, a recently identified member of the CITED family, binds p300/CBP via the CH1 domain and functions as a coactivator for transcription factor AP-2. Here, we show that CITED4 blocks the binding of hypoxia-inducible factor 1 to p300 in vitro and inhibits hypoxia-inducible factor-1 transactivation and hypoxia-mediated reporter gene activation. These studies suggest that CITED4 might function as an inhibitor of hypoxia-inducible factor 1. To explore the function of CITED4 in breast cancer, we determined its expression in normal, in situ and invasive breast cancers. We also correlated its expression in 286 invasive breast tumors with clinicopathological, hypoxia markers and survival. In contrast to the nuclear localization of CITED4 in normal breast tissue, breast tumors were characterized by cytoplasmic and nuclear localization. Nuclear CITED4 expression was significantly inversely associated with tumor hypoxia-inducible factor 1 (P < 0.05), tumor size (P = 0.03), tumor grade (P = 0.0001), and Chalkley vessel count (P = 0.04). CITED4 showed no significant correlation with patient age (P = 0.45), estrogen receptor (P = 0.11), or epidermal growth factor receptor (P = 0.48). These results show that breast cancer development is characterized by either nuclear loss or cytoplasmic translocation of CITED4, with consequent loss of hypoxia-inducible factor-1 transcriptional antagonist activity. This may be an important mechanism by which tumors enhance hypoxia-inducible factor expression and result in an aggressive phenotype.

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