A SUMOylation-dependent HIF-1\u03b1/CLDN6 negative feedback mitigates hypoxia-induced breast cancer metastasis

Yiyang Jia,Da Qi,Yantong Guo,Yuan Dong,Yingying Liang,Qiu Jin,Wenhong Xu,Xiaoli Zhang,Chengshi Quan,Peiye Song,Huinan Qu,Xinqi Wang

doi:10.1186/s13046-020-01547-5

Abstract

BackgroundWe have previously described CLDN6 as a tumor suppressor gene in breast cancer. Here, a new finding is that CLDN6 was upregulated under hypoxia, a commonly recognized factor that promotes tumor metastasis. In this study, we aim to explain this confusing finding and delineate the role of CLDN6 in the breast cancer metastasis induced by hypoxia.MethodsRNAi and ChIP assays were used to confirm that CLDN6 is transcriptional regulated by HIF-1α. mRNA seq and KEGG analysis were performed to define the downstream pathways of CLDN6. The roles of the CLDN6/SENP1/HIF-1α signaling on tumor metastasis were evaluated by function experiments and clinical samples. Finally, the possible transcription factor of SENP1 was suspected and then validated by ChIP assay.ResultsWe demonstrated a previously unrecognized negative feedback loop exists between CLDN6 and HIF-1α. CLDN6 was transcriptionally up-regulated by HIF-1α under hypoxia. On the other hand, in cytoplasm CLDN6 combines and retains β-catenin, a transcription factor of SENP1, causing β-catenin degradation and preventing its nuclear translocation. This process reduced SENP1 expression and prevented the deSUMOylation of HIF-1α, ultimately leading to HIF-1α degradation and breast cancer metastasis suppression.ConclusionsOur data provide a molecular mechanistic insight indicating that CLDN6 loss may lead to elevated HIF-1α-driven breast cancer metastasis in a SUMOylation-dependent manner.

Highlights

We have previously described CLDN6 as a tumor suppressor gene in breast cancer
Because HIF-1α is widely known as the key transcription factor that mediates adaptive cellular hypoxia responses (Fig. 1a), we speculated that the hypoxia-induced increase in CLDN6 was mediated by HIF-1α
These results suggested that HIF-1α upregulates CLDN6 expression under hypoxia

Summary

Introduction

A new finding is that CLDN6 was upregulated under hypoxia, a commonly recognized factor that promotes tumor metastasis. We aim to explain this confusing finding and delineate the role of CLDN6 in the breast cancer metastasis induced by hypoxia. HIF-1α is hydroxylated by prolyl hydroxylase domain enzymes (PHDs) and targeted by the ubiquitin ligase component von Hippel-Lindau (VHL) for degradation [4]. Hypoxia decreases the hydroxylation activity of PHDs and results in HIF-1α protein stabilization and translocation to the nucleus; here, HIF-1α dimerizes with HIF-1β and binds to hypoxia response elements (HREs; 5′-A/ GCGTG-3′) in the genome, leading to the transcriptional activation of hundreds of genes including VEGF, TWIST, SNAIL and GLUT, promoting multiple steps within the metastatic cascade [5]. SUMOylation, the conjugation of small ubiquitin-related modifier protein (SUMO) to a target protein, has been considered to play an essential regulatory role in HIF-1α protein stability [10,11,12]

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