Yan Qi, Weaver Matt, Perdue Nikole, Sage E Helene
Hope Heart Program, Benaroya Research Institute at Virginia Mason, Seattle, Washington 98104-2046, USA.
J Cell Physiol. 2005 Apr;203(1):286-94. doi: 10.1002/jcp.20226.
The matricellular glycoprotein, secreted protein acidic and rich in cysteine (SPARC), has complex biological activities and is important for lens epithelial cell function and regulation of cataract formation. To understand how SPARC influences lens epithelial cell activity and homeostasis, we have studied the subcellular distribution of SPARC in murine lens epithelial cells in vitro. We demonstrate that endogenous SPARC is located in the cytoplasm of either quiescent or dividing lens epithelial cells in culture. However, cytoplasmic SPARC was translocated into the nuclei of immortalized lens epithelial cells upon a significant reduction of intracellular SPARC in these cells. Recombinant human (rh) SPARC added to the culture media was quickly and efficiently internalized into the cytosol of SPARC-null lens epithelial cells. Moreover, cytoplasmic rhSPARC was also translocated into the nucleus after exogenous rhSPARC was removed from the culture media. The translocation of SPARC into the nucleus was therefore triggered by the reduction of SPARC protein normally available to the cells. A mouse SPARC-EGFP chimeric fusion protein (70 kDa) was expressed in lens epithelial cells and 293-EBNA cells, and was observed both in the cytoplasm and culture medium, but not in the nucleus. SPARC does not appear to have a strong nuclear localization sequence. Alternatively, SPARC might pass through the nuclear pore complex by passive diffusion. SPARC therefore functions not only as an extracellular protein but also potentially as an intracellular protein to influence cellular activities and homeostasis.
基质细胞糖蛋白，即富含半胱氨酸的酸性分泌蛋白（SPARC），具有复杂的生物学活性，对晶状体上皮细胞功能及白内障形成的调节至关重要。为了解SPARC如何影响晶状体上皮细胞活性和内环境稳定，我们在体外研究了SPARC在小鼠晶状体上皮细胞中的亚细胞分布。我们证明内源性SPARC位于培养的静止或分裂的晶状体上皮细胞的细胞质中。然而，当这些细胞内的SPARC显著减少时，细胞质中的SPARC会转位到永生化晶状体上皮细胞的细胞核中。添加到培养基中的重组人（rh）SPARC迅速且有效地内化到SPARC缺失的晶状体上皮细胞的细胞质中。此外，从培养基中去除外源性rhSPARC后，细胞质中的rhSPARC也会转位到细胞核中。因此，SPARC向细胞核的转位是由细胞中正常可用的SPARC蛋白减少触发的。小鼠SPARC-EGFP嵌合融合蛋白（70 kDa）在晶状体上皮细胞和293-EBNA细胞中表达，在细胞质和培养基中均有观察到，但在细胞核中未观察到。SPARC似乎没有很强的核定位序列。或者，SPARC可能通过被动扩散穿过核孔复合体。因此，SPARC不仅作为一种细胞外蛋白发挥作用，还可能作为一种细胞内蛋白影响细胞活性和内环境稳定。
