Department of Medicine, University of California , Irvine, California.
Department of Veterans Affairs Medical Center , Long Beach, California.
Am J Physiol Gastrointest Liver Physiol. 2019 Jan 1;316(1):G55-G63. doi: 10.1152/ajpgi.00259.2018. Epub 2018 Oct 4.
Vitamin C is an antioxidant and acts as a cofactor for many enzymatic reactions. Humans obtain vitamin C from dietary sources via intestinal absorption, a process that involves the sodium-dependent vitamin C transporters-1 and -2 (SVCT1 and SVCT2). Enterotoxigenic Escherichia coli (ETEC) infection impacts intestinal absorption/secretory functions, but nothing is known about its effect on ascorbic acid (AA) uptake. Here we demonstrate that infection of Caco-2 cells with ETEC led to a significant inhibition in intestinal AA uptake. This inhibition was associated with a marked reduction in hSVCT1 and hSVCT2 protein, mRNA, and heterogeneous nuclear RNA (hnRNA) expression levels as well as significant inhibition in the activity of both the SLC23A1 and SLC23A2 promoters. Similarly, exposure of mice to ETEC led to a significant inhibition in intestinal AA uptake and reduction in mSVCT1 and mSVCT2 protein, mRNA, and hnRNA expression levels. Inhibition was caused by the action of heat labile enterotoxin (LT), since infecting Caco-2 cells with LT-deficient ETEC (ΔLT) failed to impact AA uptake. Because LT activates adenylate cyclase, we also examined the effect of dibutyryl-cAMP in AA uptake by Caco-2 cells and observed a significant inhibition. Furthermore, treating the cells with celastrol, a specific NF-κB inhibitor, significantly blocked the inhibition of AA uptake caused by ETEC infection. Together, these data demonstrate that ETEC infection impairs intestinal AA uptake through a cAMP-dependent NF-κB-mediated pathway that regulates both SLC23A1 and SLC23A2 transcription. NEW & NOTEWORTHY Our findings demonstrate that heat-labile enterotoxin produced by enterotoxigenic Escherichia coli inhibits AA uptake in intestinal epithelial cells and mouse intestine. This effect is mediated through transcriptional repression of SLC23A1 (SVCT1) and SLC23A2 (SVCT2) via a cAMP-dependent NF-κB signaling pathway.
维生素 C 是一种抗氧化剂，作为许多酶反应的辅助因子。人类通过肠道吸收从饮食中获得维生素 C，这一过程涉及到钠离子依赖的维生素 C 转运体-1 和 -2（SVCT1 和 SVCT2）。肠产毒性大肠杆菌（ETEC）感染会影响肠道吸收/分泌功能，但目前尚不清楚其对抗坏血酸（AA）摄取的影响。在这里，我们证明 ETEC 感染 Caco-2 细胞会导致肠道 AA 摄取显著抑制。这种抑制与 hSVCT1 和 hSVCT2 蛋白、mRNA 和异质核 RNA（hnRNA）表达水平的明显降低以及 SLC23A1 和 SLC23A2 启动子活性的显著抑制有关。同样，暴露于 ETEC 的小鼠也导致肠道 AA 摄取显著抑制和 mSVCT1 和 mSVCT2 蛋白、mRNA 和 hnRNA 表达水平降低。抑制作用是由不耐热肠毒素（LT）引起的，因为用 LT 缺失的 ETEC（ΔLT）感染 Caco-2 细胞不会影响 AA 摄取。由于 LT 激活腺苷酸环化酶，我们还研究了二丁酰环磷酸腺苷（cAMP）对 Caco-2 细胞 AA 摄取的影响，观察到明显的抑制作用。此外，用特定的 NF-κB 抑制剂槲皮素处理细胞，显著阻断了 ETEC 感染引起的 AA 摄取抑制。综上所述，这些数据表明，ETEC 感染通过依赖 cAMP 的 NF-κB 介导途径损害肠道 AA 摄取，该途径调节 SLC23A1（SVCT1）和 SLC23A2（SVCT2）的转录。
