The central theme of our research is to understand the role of hypoxia-inducible factor (HIF) in tumor initiation and progression at the molecular and cellular levels. The overall goals are to identify the novel hypoxia-dependent therapeutic vulnerabilities and ultimately to translate knowledge to breast cancer therapy.

Epigenetic regulation of HIF in human cancers.

We are currently studying how HIF is activated by epigenetic regulators and their functional significance in cancer progression and metastasis.

1. The epigenetic reader ZMYND8 is a primary HIF coactivator in breast cancer (Chen Y et al., JCI 2018).

We found that ZMYND8 is induced by HIF-1 and HIF-2 in breast cancer cells and also upregulated in human breast tumors, and is correlated with poor survival of breast cancer patients. Genetic deletion of ZMYND8 decreases breast cancer cell colony formation, migration, and invasion in vitro, and inhibits breast tumor growth and metastasis to the lungs in mice. The ZMYND8’s oncogenic effect in breast cancer requires HIF-1 and HIF-2. We further showed that ZMYND8 interacts with HIF-1α and HIF-2α and enhances elongation of the global HIF-induced oncogenic genes by increasing recruitment of BRD4 and subsequent release of paused RNA polymerase II in breast cancer cells. ZMYND8 acetylation at lysine 1007 and 1034 by p300 is required for HIF activation and breast cancer progression and metastasis. These findings uncover a primary epigenetic mechanism of HIF activation and HIF-mediated breast cancer progression, and discover a possible molecular target for the diagnosis and treatment of breast cancer (Figure 1).


Figure 1. The molecular mechanism underlying ZMYND8-mediated HIF activation and breast cancer progression and metastasis

2. The methyltransferases G9a/GLP are the negative HIF-1 coregulators in GBM (Bao L et al., NAR 2018)

We found that the lysine methyltransferases G9a and GLP directly bound to the α subunit of HIF-1 (HIF-1α) and catalyzed mono- and di-methylation of HIF-1α at lysine (K) 674 in vitro and in vivo. K674 methylation suppressed HIF-1 transcriptional activity and expression of its downstream target genes PTGS1, NDNF, SLC6A3, and Linc01132 in human glioblastoma U251MG cells. Inhibition of HIF-1 by K674 methylation is due to reduced HIF-1α transactivation domain function but not increased HIF-1α protein degradation or impaired binding of HIF-1 to hypoxia response elements. K674 methylation significantly decreased HIF-1-dependent migration of U251MG cells under hypoxia. Importantly, we found that G9a was downregulated by hypoxia in glioblastoma, which was inversely correlated with PTGS1 expression and survival of patients with glioblastoma. Therefore, our findings uncover a hypoxia-induced negative feedback mechanism that maintains high activity of HIF-1 and cell mobility in human glioblastoma (Figure 2).

Figure 2. The molecular mechanism underlying G9a/GLP-mediated HIF-1 repression in GBM cells.

HIF and non-coding RNAs in human cancers

We are studying hypoxia-induced non-coding RNAs in human breast tumors and their roles in breast cancer progresion. Recently we identified a hitherto uncharacterized hypoxia-induced long non-coding RNA (lncRNA) RAB11B-AS1 in breast cancer cells (Niu Y et al., Cancer Res 2020). RAB11B-AS1 is a natural lncRNA upregulated in human breast cancer and its expression is induced by hypoxia-inducible factor 2 (HIF-2), but not HIF-1, in response to hypoxia. RAB11B-AS1 enhanced the expression of angiogenic factors including VEGFA and ANGPTL4 in hypoxic breast cancer cells by increasing recruitment of RNA polymerase II. In line with increased angiogenic factors, conditioned media from RAB11B-AS1-overexpressing breast cancer cells promoted tube formation of human umbilical vein endothelial cells in vitro. Gain- and loss-of-function studies revealed that RAB11B-AS1 increased breast cancer cell migration and invasion in vitro and promoted tumor angiogenesis and breast cancer distant metastasis without affecting primary tumor growth in mice. Taken together, these findings uncover a fundamental mechanism of hypoxia-induced tumor angiogenesis and breast cancer metastasis (Figure 3). 

Figure 3. The Role of Hypoxia-Induced RAB11B-AS1 in Breast Cancer Metastasis

HIF and cancer metabolism

Hypoxia-inducible factors (HIFs) mediate metabolic reprogramming in response to hypoxia. We previously dissected a reciprocal regulation between HIF-1 and PKM2 in cancer cells. PKM2 is hydroxylated by PHD3 and promotes HIF-1 transactivation and reprograms glucose metabolism in cancer cells (Figure 4). 

Recently, we found that hypoxia upregulates mRNA and protein levels of the branched-chain amino acid (BCAA) transporter LAT1 and the BCAA metabolic enzyme BCAT1, but not their paralogs LAT2-4 and BCAT2, in human glioblastoma (GBM) cell lines as well as primary GBM cells. Hypoxia-induced LAT1 protein upregulation is mediated by both HIF-1 and HIF-2 in GBM cells. Although both HIF-1alpha and HIF-2alpha directly bind to the hypoxia response element at the first intron of the human BCAT1 gene, HIF-1alpha is exclusively responsible for hypoxia-induced BCAT1 expression in GBM cells. Knockout of HIF-1alpha and HIF-2alpha significantly reduces glutamate labeling from BCAAs in GBM cells under hypoxia, which provides functional evidence for HIFs-mediated reprogramming of BCAA metabolism. Genetic or pharmacological inhibition of BCAT1 inhibits GBM cell growth under hypoxia. Together, these findings uncover a previously unrecognized HIFs-dependent metabolic pathway that increases GBM cell growth under conditions of hypoxic stress. (Zhang B, Chen Y, et al. Cell Mol Life Sci 2020).

Figure 4. The molecular mechanism underlying PKM2-mediated HIF activation in cancer cells