Fraser G M, Portnoy M, Bleich M, Ecke D, Niv Y, Greger R, Schwartz B
Department of Gastroenterology, Rabin Medical Center (Beilinson Campus) and Sackler School of Medicine,
Pflugers Arch. 1997 Nov;434(6):801-8. doi: 10.1007/s004240050468.
Glucocorticoids, such as dexamethasone, induce amiloride-sensitive Na+ conductances in rat distal colon epithelium. The activity of these conductances diminishes from the surface to the base of the crypt whereas cAMP-stimulated Cl- secretion decreases from the crypt base to the surface. These gradients are likely to be perturbed during carcinogenesis. We therefore determined the magnitude of Na+ and Cl- conductances in colonocytes isolated from normal and carcinogen-treated rats. Colon carcinogenesis was induced by injection of dimethylhydrazine (DMH) (18 mg/kg) for 5 weeks. Before sacrifice animals were treated for 3 days with dexamethasone. Colonocyte populations from the surface to the crypt base (C1-C5) were harvested from the distal colon by a Ca2+-chelating procedure. The activity of Na+ conductances was determined by uptake of 22Na+ by surface and crypt colonocyte populations and by membrane vesicles in the presence and absence of 10 microM amiloride. In control rats Na+ conductance was highest in surface colonocytes and absent in the crypt base. As early as 2 weeks after initiation of DMH treatment amiloride-inhibited Na+ uptake was virtually absent in the upper crypt. Transcriptional assessment of the alpha-, beta- and gamma-subunits that constitute the epithelial Na+ channel revealed that DMH treatment reduces the expression of beta-subunit mRNA. We then examined 36Cl- efflux from isolated colonocytes of normal and carcinogen-treated rats in response to forskolin (0.01 mM). Forskolin induced a marked rise in cAMP in lower crypt cells concomitant with a significant stimulation of 36Cl- efflux. Intracellular cAMP increased in upper crypt cells in response to forskolin without an increase in 36Cl- efflux. By contrast, upper crypt colonocytes from DMH-treated rats showed forskolin-stimulated efflux beginning 4 weeks after initiation of treatment. We conclude that induction of Na+ conductances by glucocorticoids is inhibited during the early stages of chemical carcinogenesis due to lack of induction of the beta-subunit of the channel. By contrast, Cl- transport is stimulated both in surface and lower crypt cell compartments during different stages of chemical carcinogenesis.
糖皮质激素，如地塞米松，可诱导大鼠远端结肠上皮细胞产生对氨氯吡咪敏感的钠离子电导。这些电导的活性从隐窝表面向基部逐渐降低，而环磷酸腺苷（cAMP）刺激的氯离子分泌则从隐窝基部向表面减少。在致癌过程中，这些梯度可能会受到干扰。因此，我们测定了从正常大鼠和经致癌物处理的大鼠分离出的结肠细胞中钠离子和氯离子电导的大小。通过注射二甲基肼（DMH）（18毫克/千克）持续5周诱导结肠癌发生。在处死动物前，用地塞米松处理3天。通过钙离子螯合程序从远端结肠收集从表面到隐窝基部的结肠细胞群体（C1 - C5）。通过表面和隐窝结肠细胞群体以及膜囊泡在存在和不存在10微摩尔氨氯吡咪的情况下对22Na+的摄取来测定钠离子电导的活性。在对照大鼠中，钠离子电导在表面结肠细胞中最高，在隐窝基部不存在。早在DMH处理开始后2周，氨氯吡咪抑制的钠离子摄取在上部隐窝中几乎不存在。对构成上皮钠离子通道的α、β和γ亚基的转录评估显示，DMH处理降低了β亚基mRNA的表达。然后我们检测了正常大鼠和经致癌物处理的大鼠分离出的结肠细胞对福斯高林（0.01毫摩尔）的36Cl-流出情况。福斯高林诱导下部隐窝细胞中的cAMP显著升高，同时伴随36Cl-流出的显著刺激。上部隐窝细胞中，福斯高林刺激后细胞内cAMP增加，但36Cl-流出没有增加。相比之下，DMH处理大鼠的上部隐窝结肠细胞在处理开始4周后显示出福斯高林刺激的流出。我们得出结论，在化学致癌的早期阶段，由于通道β亚基缺乏诱导，糖皮质激素诱导的钠离子电导受到抑制。相比之下，在化学致癌的不同阶段，表面和下部隐窝细胞区室中的氯离子转运均受到刺激。
