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.

SPARC from Olfactory Ensheathing Cells Stimulates Schwann Cells to Promote Neurite Outgrowth and Enhances Spinal Cord Repair

“…With no other recorded secretion deficit, this could reflect a specific decline in SPARC secretion with age. Alternatively, SPARC may instead be actively internalized by senescent LP-OECs, which become flattened and adherent, similar to changes in SPARC secretion reported in aging lens epithelia (Yan et al, 2005).…”

“…SPARC distribution in the lamina propria coincides with laminin (Fig. 2 F), indicating that SPARC is ideally placed to interact with and regulate the activity of laminin-1, as it can in other tissues (Rempel et al, 2001;Sweetwyne et al, 2004;Yan et al, 2005). LP-OEC expression of SPARC declines in adult LP OECs, which might reflect a shift in OEC state to preferentially stabilize mature axons because, in the adult OE, there is a lower frequency of axon growth (Farbman, 1990).…”

“…directly compare its bioactivity in our outgrowth assays with its activity in other assays (Bradshaw et al, 2000;Sweetwyne et al, 2004;Wang et al, 2005;Yan et al, 2005). We next tested whether the outgrowth promoting-activity of SPARC could be inhibited using functionblocking antibodies specific to both mouse and human SPARC.…”

Genome-wide expression analysis of therapy-resistant tumors reveals SPARC as a novel target for cancer therapy

“…For the detection of SPARC in serum samples of animal xenografts of MIP/SP and MIP/Zeo cells, 250 μg of total protein from each sample was incubated with mouse anti-human SPARC antibody (Haematologic Technologies Inc.), which has no cross-reactivity with mouse SPARC (unpublished data and ref. 45), overnight at 4°C and immunoprecipitated with Protein G Sepharose beads (Sigma-Aldrich) at 4°C for 4 hours and washed 5 times with lysis buffer. The beads were boiled in Laemmli buffer and centrifuged at 12,000 g for 10 seconds.…”

Retention of the Matricellular Protein SPARC in the Endoplasmic Reticulum of Chondrocytes from Patients with Pseudoachondroplasia

“…As a typical matricellular protein, however, SPARC is not a resident structural ECM protein in that it is not generally found extracellularly in nonpathological tissues (Brekken and Sage 2001). In contrast, exogenous or endogenously produced SPARC has been reported to be translocated to the nuclei of endothelial and lens epithelial cells in vitro (Gooden et al 1999;Yan et al 2005a). The association of SPARC with mutated forms of COMP in PSACH chondrocytes suggests a new intracellular function for SPARC in the trafficking of COMP.…”