Department of Pediatrics, University of Arizona Health Sciences Center, Tucson, AZ, USA.
Gastroenterology. 2013 Sep;145(3):613-24. doi: 10.1053/j.gastro.2013.06.002. Epub 2013 Jun 5.
BACKGROUND & AIMS: Dysregulated Ca(2+) homeostasis likely contributes to the etiology of inflammatory bowel disease-associated loss of bone mineral density. Experimental colitis leads to decreased expression of Klotho, a protein that supports renal Ca(2+) reabsorption by stabilizing the transient receptor potential vanilloid 5 (TRPV5) channel on the apical membrane of distal tubule epithelial cells.
Colitis was induced in mice via administration of 2,4,6-trinitrobenzenesulfonic acid (TNBS) or transfer of CD4(+)interleukin-10(-/-) and CD4(+), CD45RB(hi) T cells. We investigated changes in bone metabolism, renal processing of Ca(2+), and expression of TRPV5.
Mice with colitis had normal serum levels of Ca(2+) and parathormone. Computed tomography analysis showed a decreased density of cortical and trabecular bone, and there was biochemical evidence for reduced bone formation and increased bone resorption. Increased fractional urinary excretion of Ca(2+) was accompanied by reduced levels of TRPV5 protein in distal convoluted tubules, with a concomitant increase in TRPV5 sialylation. In mouse renal intermedullary collecting duct epithelial (mIMCD3) cells transduced with TRPV5 adenovirus, the inflammatory cytokines tumor necrosis factor, interferon-γ, and interleukin-1β reduced levels of TRPV5 on the cell surface, leading to its degradation. Cytomix induced interaction between TRPV5 and UBR4 (Ubiquitin recoginition 4), an E3 ubiquitin ligase; knockdown of UBR4 with small interfering RNAs prevented cytomix-induced degradation of TRPV5. The effects of cytokines on TRPV5 were not observed in cells stably transfected with membrane-bound Klotho; TRPV5 expression was preserved when colitis was induced with TNBS in transgenic mice that overexpressed Klotho or in mice with T-cell transfer colitis injected with soluble recombinant Klotho.
After induction of colitis in mice via TNBS administration or T-cell transfer, tumor necrosis factor and interferon-γ reduced the expression and activity of Klotho, which otherwise would protect TRPV5 from hypersialylation and cytokine-induced TRPV5 endocytosis, UBR4-dependent ubiquitination, degradation, and urinary wasting of Ca(2+).
钙稳态失调可能导致炎症性肠病相关的骨密度降低。实验性结肠炎导致 Klotho 表达下调，Klotho 是一种蛋白，可通过稳定瞬时受体电位香草酸 5（TRPV5）通道稳定远端肾小管上皮细胞的顶端膜，从而支持肾脏对 Ca2+的重吸收。
通过给予 2,4,6-三硝基苯磺酸（TNBS）或转移 CD4+白细胞介素-10（-/-）和 CD4+、CD45RB（hi）T 细胞，在小鼠中诱导结肠炎。我们研究了骨代谢、肾脏 Ca2+处理和 TRPV5 表达的变化。
结肠炎小鼠的血清 Ca2+和甲状旁腺激素水平正常。计算机断层扫描分析显示皮质和小梁骨密度降低，生化证据表明骨形成减少和骨吸收增加。尿 Ca2+排泄分数增加伴有远曲小管 TRPV5 蛋白水平降低，同时 TRPV5 唾液酸化增加。在转染 TRPV5 腺病毒的小鼠肾髓质集合管上皮（mIMCD3）细胞中，炎性细胞因子肿瘤坏死因子、干扰素-γ和白细胞介素-1β降低细胞表面 TRPV5 水平，导致其降解。细胞因子诱导 TRPV5 与 E3 泛素连接酶 UBR4 之间相互作用；用小干扰 RNA 敲低 UBR4 可防止细胞因子诱导的 TRPV5 降解。在稳定转染膜结合型 Klotho 的细胞中未观察到细胞因子对 TRPV5 的影响；在过表达 Klotho 的 TNBS 诱导的结肠炎转基因小鼠或注射可溶性重组 Klotho 的 T 细胞转移结肠炎小鼠中，Klotho 可防止 TRPV5 过度唾液酸化和细胞因子诱导的 TRPV5 内吞作用、UBR4 依赖性泛素化、降解和尿钙丢失。
在通过 TNBS 给药或 T 细胞转移诱导小鼠结肠炎后，肿瘤坏死因子和干扰素-γ降低了 Klotho 的表达和活性，否则 Klotho 将保护 TRPV5 免受过度唾液酸化和细胞因子诱导的 TRPV5 内吞作用、UBR4 依赖性泛素化、降解和尿钙丢失。
