Expression of SPARC is correlated with altered morphologies in transfected F9 embryonal carcinoma cells

SPARC (secreted protein, acidic and rich in cysteine) is a Ca(2+)-binding glycoprotein that has recently been identified as a member of a group of proteins that exert antispreading effects on various cultured cells. In addition, SPARC is induced during the later stages of F9 stem cell differentiation to parietal endoderm (PE). When treated with retinoic acid and dibutyryl cAMP, F9 cells differentiate into PE and SPARC mRNA is increased approximately 20-fold. To determine whether the chronic overexpression or inhibition of expression of SPARC would affect the morphology, attachment, or differentiation of F9 cells, we transfected undifferentiated F9 cells with cDNA encoding SPARC or anti-sense SPARC and cloned lines that expressed either elevated or reduced levels of SPARC protein. The transfected F9 cells displayed altered morphologies in culture: cells of four overexpressing lines appeared clumped and rounded, whereas those of three underexpressing lines were spread and flat, in comparison to controls. Moreover, the morphological differences persisted during differentiation of the lines to PE. The altered morphology was not due to an increased expression of collagenases and did not affect the ability of the cells to attach and adhere to tissue culture plastic. The altered phenotype of the transfected F9 cells appeared to be directly related to the level of extracellular SPARC. Since overexpression of SPARC induced rounding and aggregation of F9 cells in culture, we propose that SPARC facilitates modulation of cell-cell or cell-substrate interactions in vivo.