PTBP1-mediated regulation of AXL mRNA stability plays a role in lung tumorigenesis

Chun-Yu Cho,Shankung Lin,Jong-Ding Lay,Jhy-Shrian Huang,Shih-Ying Chung,Ya-Yu Yang,Shih-Sheng Jiang,Gi-Ming Lai,Yen-Lin Chen,Li-Chun Cheng,Shuang-En Chuang,Tsu-Hsiang Kuo

doi:10.1038/s41598-019-53097-2

Abstract

AXL is expressed in many types of cancer and promotes cancer cell survival, metastasis and drug resistance. Here, we focus on identifying modulators that regulate AXL at the mRNA level. We have previously observed that the AXL promoter activity is inversely correlated with the AXL expression levels, suggesting that post-transcriptional mechanisms exist that down-regulate the expression of AXL mRNA. Here we show that the RNA binding protein PTBP1 (polypyrimidine tract-binding protein) directly targets the 5′-UTR of AXL mRNA in vitro and in vivo. Moreover, we also demonstrate that PTBP1, but not PTBP2, inhibits the expression of AXL mRNA and the RNA recognition motif 1 (RRM1) of PTBP1 is crucial for this interaction. To clarify how PTBP1 regulates AXL expression at the mRNA level, we found that, while the transcription rate of AXL was not significantly different, PTBP1 decreased the stability of AXL mRNA. In addition, over-expression of AXL may counteract the PTBP1-mediated apoptosis. Knock-down of PTBP1 expression could enhance tumor growth in animal models. Finally, PTBP1 was found to be negatively correlated with AXL expression in lung tumor tissues in Oncomine datasets and in tissue micro-array (TMA) analysis. In conclusion, we have identified a molecular mechanism of AXL expression regulation by PTBP1 through controlling the AXL mRNA stability. These findings may represent new thoughts alternative to current approaches that directly inhibit AXL signaling and may eventually help to develop novel therapeutics to avoid cancer metastasis and drug resistance.

Highlights

Lung cancer accounts for about 10–20% of total cancer deaths
While the AXL promoter activity was inversely correlated with the AXL mRNA levels in both lung cancer cell lines and breast cancer cell lines as determined by reporter assays (Fig. 1a,b and Supplementary Fig. 1), the AXL mRNA levels were positively correlated with its 5′-UTR reporter activity (Fig. 1c) but not its 3′-untranslated region (3′-UTR) reporter activity in our recent study[32]
These results indicate that expression of AXL mRNA may be regulated by certain factors through its 5′-UTR

Summary

Introduction

Lung cancer accounts for about 10–20% of total cancer deaths. Non-small cell lung cancer (NSCLC) causes 1.4 million cancer deaths worldwide every year. PTBP1 was found to induce p19 mRNA expression via promoter regulation and inhibit cell proliferation[26] These findings suggest that PTBP1 may play different roles in the nucleus vs cytoplasm. Studies focusing on its RNA splicing regulatory role in the nucleus have found that PTBP1 may promote a malignant phenotype in cancer cells[17,29,30,31]. Our tissue microarray (TMA) data and the correlation analysis of Oncomine datasets both showed that PTBP1 was negatively correlated with AXL expression in lung tumor tissues Together, these results suggest that, as an alternative to direct inhibitors of AXL, elucidation of the mechanisms underlying how AXL is regulated at the mRNA level may help develop novel AXL-targeting approaches for cancer treatment in the future

Methods

Results

Conclusion