Division of Pulmonary, Allergy, and Critical Care Medicine, University of Pittsburgh School of Medicine, Pittsburgh, PA, USA.
J Cell Mol Med. 2011 Apr;15(4):957-69. doi: 10.1111/j.1582-4934.2010.01063.x.
Our previous studies have demonstrated increased expression of insulin-like growth factor binding protein-5 (IGFBP-5) in fibrotic tissues and IGFBP-5 induction of extracellular matrix (ECM) components. The mechanism resulting in increased IGFBP-5 in the extracellular milieu of fibrotic fibroblasts is unknown. Since Caveolin-1 (Cav-1) has been implicated to play a role in membrane trafficking and signal transduction in tissue fibrosis, we examined the effect of Cav-1 on IGFBP-5 internalization, trafficking and secretion. We demonstrated that IGFBP-5 localized to lipid rafts in human lung fibroblasts and bound Cav-1. Cav-1 was detected in the nucleus in IGFBP-5-expressing fibroblasts, within aggregates enriched with IGFBP-5, suggesting a coordinate trafficking of IGFBP-5 and Cav-1 from the plasma membrane to the nucleus. This trafficking was dependent on Cav-1 as fibroblasts from Cav-1 null mice had increased extracellular IGFBP-5, and as fibroblasts in which Cav-1 was silenced or lipid raft structure was disrupted through cholesterol depletion also had defective IGFBP-5 internalization. Restoration of Cav-1 function through administration of Cav-1 scaffolding peptide dramatically increased IGFBP-5 uptake. Finally, we demonstrated that IGFBP-5 in the ECM protects fibronectin from proteolytic degradation. Taken together, our findings identify a novel role for Cav-1 in the internalization and nuclear trafficking of IGFBP-5. Decreased Cav-1 expression in fibrotic diseases likely leads to increased deposition of IGFBP-5 in the ECM with subsequent reduction in ECM degradation, thus identifying a mechanism by which reduced Cav-1 and increased IGFBP-5 concomitantly contribute to the perpetuation of fibrosis.
我们之前的研究表明，胰岛素样生长因子结合蛋白-5（IGFBP-5）在纤维化组织中的表达增加，并且 IGFBP-5 诱导细胞外基质（ECM）成分的产生。导致纤维化成纤维细胞细胞外基质中 IGFBP-5 增加的机制尚不清楚。由于 Cav-1（Cav-1）已被认为在组织纤维化的膜转运和信号转导中发挥作用，因此我们研究了 Cav-1 对 IGFBP-5 内化、转运和分泌的影响。我们证明 IGFBP-5 定位于人肺成纤维细胞中的脂筏，并与 Cav-1 结合。在表达 IGFBP-5 的成纤维细胞中，Cav-1 被检测到在核内，与富含 IGFBP-5 的聚集体内，这表明 IGFBP-5 和 Cav-1 从质膜到核内的协调转运。这种转运依赖于 Cav-1，因为 Cav-1 缺失的成纤维细胞的细胞外 IGFBP-5 增加，并且沉默 Cav-1 或通过胆固醇耗竭破坏脂筏结构的成纤维细胞也存在 IGFBP-5 内化缺陷。通过给予 Cav-1 支架肽来恢复 Cav-1 功能，可显著增加 IGFBP-5 的摄取。最后，我们证明 ECM 中的 IGFBP-5 可保护纤连蛋白免受蛋白水解降解。总之，我们的研究结果确定了 Cav-1 在 IGFBP-5 的内化和核内转运中的新作用。纤维化疾病中 Cav-1 表达的减少可能导致 ECM 中 IGFBP-5 的沉积增加，随后 ECM 降解减少，从而确定了 Cav-1 减少和 IGFBP-5 增加同时促进纤维化持续存在的机制。
