Groulx Jean-François, Boudjadi Salah, Beaulieu Jean-François
Laboratory of Intestinal Physiopathology, Department of Anatomy and Cell Biology, Faculty of Medicine and Health Sciences, Université de Sherbrooke, Sherbrooke, QC J1H 5N4, Canada.
Laboratory of Pathology, Cancer Molecular Pathology Section, National Cancer Institute, Bethesda, MD 20892, USA.
Cancers (Basel). 2018 Feb 3;10(2):42. doi: 10.3390/cancers10020042.
The α6 integrin subunit () pre-mRNA undergoes alternative splicing to form two splicing variants, named ITGA6A and ITGA6B. In primary human colorectal cancer cells, the levels of both ITGA6 and β4 integrin subunit (ITGB4) subunits of the α6β4 integrin are increased. We previously found that the upregulation of ITGA6 is a direct consequence of the increase of the pro-proliferative ITGA6A variant. However, the mechanisms that control ITGA6 expression and splicing into the ITGA6A variant over ITGA6B in colorectal cancer cells remain poorly understood. Here, we show that the promoter activity of the gene is regulated by MYC. Pharmacological inhibition of MYC activity with the MYC inhibitor (MYCi) 10058-F4 or knockdown of MYC expression by short hairpin RNA (shRNA) both lead to a decrease in ITGA6 and ITGA6A levels in colorectal cancer cells, while overexpression of MYC enhances promoter activity. We also found that MYC inhibition decreases the epithelial splicing regulatory protein 2 (ESRP2) splicing factor at both the mRNA and protein levels. Chromatin immunoprecipitation revealed that the proximal promoter sequences of and were occupied by MYC and actively transcribed in colorectal cancer cells. Furthermore, expression studies in primary colorectal tumors and corresponding resection margins confirmed that the up-regulation of the subunit can be correlated with the increase in and . Taken together, our results demonstrate that the proto-oncogene MYC can regulate the promoter activation and splicing of the integrin gene through ESRP2 to favor the production of the pro-proliferative ITGA6A variant in colorectal cancer cells.
α6整合素亚基（）前体mRNA进行可变剪接，形成两种剪接变体，命名为ITGA6A和ITGA6B。在原发性人结肠直肠癌细胞中，α6β4整合素的ITGA6和β4整合素亚基（ITGB4）的水平均升高。我们之前发现ITGA6的上调是促增殖性ITGA6A变体增加的直接结果。然而，在结肠直肠癌细胞中，控制ITGA6表达并将其剪接为ITGA6A变体而非ITGA6B变体的机制仍知之甚少。在此，我们表明基因的启动子活性受MYC调控。用MYC抑制剂（MYCi）10058 - F4对MYC活性进行药理抑制或通过短发夹RNA（shRNA）敲低MYC表达，均导致结肠直肠癌细胞中ITGA6和ITGA6A水平降低，而MYC的过表达增强了启动子活性。我们还发现，MYC抑制在mRNA和蛋白质水平上均降低了上皮剪接调节蛋白2（ESRP2）剪接因子。染色质免疫沉淀显示，和的近端启动子序列被MYC占据，并在结肠直肠癌细胞中活跃转录。此外，对原发性结肠直肠肿瘤及其相应手术切缘的表达研究证实，亚基的上调与和的增加相关。综上所述，我们的结果表明，原癌基因MYC可通过ESRP2调节整合素基因的启动子激活和剪接，从而有利于结肠直肠癌细胞中促增殖性ITGA6A变体的产生。
