Uvarov Anton V, Mesaeli Nasrin
Institute of Cardiovascular Sciences, St. Boniface General Hospital Research Centre, Winnipeg, Canada.
Biochim Biophys Acta. 2008 Jun;1783(6):1237-47. doi: 10.1016/j.bbamcr.2008.03.004. Epub 2008 Mar 20.
Calreticulin is a lectin chaperone essential for intracellular calcium homeostasis. Deletion of calreticulin gene compromises the overall quality control within the endoplasmic reticulum (ER) leading to activation of the unfolded protein response. However, the ER structure of calreticulin deficient cells (crt-/-) is not altered due to accumulation of misfolded proteins. Therefore, the aim of this study was to determine whether the ubiquitin-proteasome pathway is activated in crt-/- cells as a compensatory mechanism for cell survival. Here we show a significant increase in the expression of genes involved in ER associated degradation and activation of the ubiquitin-proteasome system in crt-/- cells. We also demonstrated that the ubiquitination of two proteins processed in ER, connexin 43 and A1AT NHK (alpha1-antitrypsin mutant) are increased in crt-/- cells. Furthermore, we showed that the increased proteasome activity in the crt-/- cells could be rescued upon re-introduction of calreticulin or calsequestrin (a muscle calcium binding protein). We also illustrated that increased cytosolic Ca2+ enhances the proteasome activity. Interestingly, suppression of calnexin function using siRNA further elevated the proteasome activity in crt-/- cells. This is the first report to show that loss of calreticulin function enhances the ubiquitin-proteasome activity which could function as a compensatory mechanism for cell survival.
钙网蛋白是一种对细胞内钙稳态至关重要的凝集素伴侣蛋白。钙网蛋白基因的缺失会损害内质网（ER）内的整体质量控制，导致未折叠蛋白反应的激活。然而，由于错误折叠蛋白的积累，钙网蛋白缺陷细胞（crt-/-）的内质网结构并未改变。因此，本研究的目的是确定泛素-蛋白酶体途径是否在crt-/-细胞中被激活，作为细胞存活的一种补偿机制。在此我们表明，在crt-/-细胞中，参与内质网相关降解和泛素-蛋白酶体系统激活的基因表达显著增加。我们还证明，在内质网中加工的两种蛋白，连接蛋白43和A1AT NHK（α1-抗胰蛋白酶突变体）的泛素化在crt-/-细胞中增加。此外，我们表明，在重新引入钙网蛋白或肌集钙蛋白（一种肌肉钙结合蛋白）后，crt-/-细胞中增加的蛋白酶体活性可以恢复。我们还表明，细胞溶质Ca2+的增加会增强蛋白酶体活性。有趣的是，使用小干扰RNA抑制钙连蛋白功能进一步提高了crt-/-细胞中的蛋白酶体活性。这是第一份表明钙网蛋白功能丧失会增强泛素-蛋白酶体活性的报告，泛素-蛋白酶体活性可作为细胞存活的一种补偿机制。
