Activation of SPARC expression in reactive stroma associated with human epithelial ovarian cancer

Microarray analysis of gene expression mirrors the biology of an ovarian cancer model

We have previously described an ovarian cancer model based on four independent spontaneously immortalized epithelial ovarian cancer cell lines (TOV-21G, TOV-81D, TOV-112D and OV-90) from patients who were never exposed to chemotherapy or radiation therapy. These cell lines are particularly interesting since they retain characteristics of the original epithelial ovarian cancers (EOC) from which they were derived. Here we report the characterization of this model system using high-density DNA microarrays in order to assess gene expression. Expression profiles were generated from total RNAs extracted from the four EOC cell lines. For comparison, expression profiling is also provided for a primary culture of normal ovarian surface epithelium (NOV-31) and a fresh EOC sample (TOV-578G). Comparison of expression profiles revealed patterns of expression that distinguish NOV-31 from that of all tumor derived samples. The expression pattern of TOV-81D, an EOC cell line that was derived from a patient with indolent disease, most closely resembles NOV-31 while profiles of samples derived from patients with more aggressive disease (TOV-21G, OV-90, TOV-112D and TOV-578G) showed more divergent patterns of expression. The microarray analysis (http://genome.mcgill.ca) results confirm the usefulness of an ovarian cancer model based on the characterization of these EOC cell lines.

SPARC (secreted protein acidic and rich in cysteine) induces apoptosis in ovarian cancer cells

Secreted protein acidic and rich in cysteine (SPARC) is an extracellular Ca(2+)-binding matricellular glycoprotein that associates with cell populations undergoing migration, morphogenesis, and differentiation. Studies on endothelial cells have established that its principal functions in vitro are counteradhesion and antiproliferation. The mechanism(s) underlying these antitumor effects is unknown. In this study, we showed that SPARC expression in ovarian cancer cells is inversely correlated with the degree of malignancy. The immunohistochemical data presented here confirmed the importance of diminished SPARC expression in ovarian cancer development. Treating human ovarian surface epithelial cells and ovarian cancer cells with SPARC revealed that as SPARC inhibits the proliferation of both normal and cancer cells, it induces apoptosis only in cancer cells. This observation indicates that down-regulation of SPARC is essential for ovarian carcinogenesis as cancer cells become sensitized to the apoptotic activity of SPARC during malignant transformation. We also showed here the first direct evidence that putative SPARC receptors are present on ovarian epithelial cells. Their levels are higher in human ovarian surface epithelial cells than cancer cells. Binding of SPARC to its receptor is likely to trigger tissue-specific signaling pathways that mediate its tumor suppressing functions. Decrease in ligand-receptor interaction by the down-regulation of SPARC and/or its receptor is essential for ovarian carcinogenesis.

Association of SPARC (osteonectin, BM-40) with extracellular and intracellular components of the ciliated surface ectoderm of Xenopus embryos

SPARC (Secreted Protein, Acidic, Rich in Cysteine) was detected by immunohistochemistry in the sensorial layer of the bilayered embryonic epidermis of Xenopus laevis during neurulation, when a subset of the sensorial cells are selected to differentiate into ciliated cell precursors. After the ciliated cells had intercalated into the outer layer and had undergone ciliogenesis, intense SPARC immunostaining was associated with the cilia and remained associated with the cilia throughout their persistence on the epidermis. Circumferential SPARC immunostaining was also detected at the interface between surface epithelial cells. Animal cap explants indicated that the embryonic activation of SPARC expression in the dorsal ectoderm does not require signaling from factors secreted by the underlying mesoderm. Immunoelectron microscopy revealed that SPARC is intimately associated with the 9 + 2 microtubule arrays of cilia. Our data indicate that SPARC plays a role in the development and function of the surface ciliated epidermis of Xenopus embryos. We propose that the counter-adhesive activity of SPARC facilitates the intercalation of ciliary cell precursors to the surface epithelial layer, where its Ca(2+)-binding abilities promote cell-cell adhesion. Based on its association with ciliary microtubule arrays, we also propose that intracellular SPARC may play a role in regulating ciliary beat frequency and polarity.