title: Matricellular protein SPARC is translocated to the nuclei of immortalized murine lens epithelial cells.
abstract: 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.
authors.affiliation: Hope Heart Program, Benaroya Research Institute at Virginia Mason, Seattle, Washington 98104-2046, USA. qyan@hopeheart.org
authors.forename: Qi
authors.initials: Q
authors.lastname: Yan
authors.forename: Matt
authors.initials: M
authors.lastname: Weaver
authors.forename: Nikole
authors.initials: N
authors.lastname: Perdue
authors.forename: E Helene
authors.initials: EH
authors.lastname: Sage
mesh_terms: D021581:Active Transport, Cell Nucleus / Q000502:physiology; D000818:Animals; D002461:Cell Line, Transformed; D002467:Cell Nucleus / Q000378:metabolism*; D003593:Cytoplasm / Q000378:metabolism; D004815:Epidermal Growth Factor / Q000235:genetics / Q000378:metabolism; D004847:Epithelial Cells / Q000166:cytology / Q000378:metabolism / Q000502:physiology*; D006706:Homeostasis / Q000502:physiology; D007908:Lens, Crystalline / Q000166:cytology / Q000378:metabolism*; D051379:Mice; D008810:Mice, Inbred C57BL; D008817:Mice, Mutant Strains; D015676:Osteonectin / Q000235:genetics / Q000378:metabolism*; D011994:Recombinant Proteins / Q000235:genetics / Q000493:pharmacokinetics
chemical_list: D015676:Osteonectin; D011994:Recombinant Proteins; D004815:Epidermal Growth Factor