Expression and structure of human SPARC transcripts: SPARC mRNA is expressed by human cells involved in extracellular matrix production and some of these cells show an unusual expression pattern.

A mouse SPARC cDNA clone was used to elucidate the expression of SPARC mRNA in normal diploid human cells as well as in tumor cells. Among 40 cell lines examined, 19 showed expression. The mRNA transcribed by the majority of the expressors are 2.1 kb with a trace amount of 3 kb. However, three cell types, undifferentiated basal keratinocytes, their differentiated derivatives, and breast adenocarcinoma cells, showed an expression pattern distinct from the typical one, having abundant 3-kb mRNA but no detectable 2.1-kb mRNA. The mRNA was translated and the product secreted. This expression pattern was not observed before in human cells and was not found in tumor cells of keratinocytes, squamous carcinoma cells, or many other adenocarcinoma cells. We showed by Northern hybridization that the SPARC-expressing melanocytic melanoma cell lines produced laminin, a component of extracellular matrix. Other cell types expressing the SPARC mRNA were also reported to synthesize extracellular matrix components. Thus, our results indicate an association between SPARC gene expression and production of extracellular matrix. However, the opposite is not true since non-SPARC-producers may or may not produce extracellular matrix. For example, A431 cell line, which does not express SPARC mRNA, is known to produce extracellular matrix components while the normal diploid melanocytes and undifferentiated embryonal carcinoma cells, which do not express SPARC mRNA, do not produce extracellular matrix component.