Cell cycle-dependent nuclear location of the matricellular protein SPARC: Association with the nuclear matrix

Secreted protein acidic and rich in cysteine (SPARC) is a matricellular pro tein that inhibits cellular adhesion and proliferation. In this study, we r eport the detection of SPARC in the interphase nuclei of embryonic chicken cells in vivo. Differential partitioning of SPARC was also noted in the cyt oplasm these cells during discrete stages of M-phase: cells in metaphase an d anaphase exhibited strong cytoplasmic immunoreactivity, whereas cells in telophase were devoid of labeling. Immunocytochemical analysis of embryonic chicken cells in vitro likewise showed the presence of SPARC in the nucleu s. Furthermore, elution of soluble proteins and DNA from these cells indica ted that SPARC might be a component of the nuclear matrix. We subsequently examined cultured bovine aortic endothelial cells, which initially appeared to express SPARC only in the cytoplasm. However, after elution of soluble proteins and chromatin, we also detected SPARC in the nuclear matrix of the se cells. Embryonic chicken cells incubated with recombinant SPARC were see n to take up the protein and to translocate it to the nucleus progressively over a period of 17 h. These observations provide new information about SP ARC, generally recognized as a secreted glycoprotein that mediates interact ions between cells and components of the extracellular matrix. The evidence presented in this study indicates that SPARC might subserve analogous func tions in the nuclear matrix. J. Cell. Biochem. 74:152-167, 1999. (C)1999 Wi ley-Liss, Inc.