Expression of CD44 is repressed in neuroblastoma cells.

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Abstract
We have used cDNA subtractive cloning to identify a group of human genes that are expressed in diverse differentiated derivatives of neural crest origin but not in neuroblastoma cell lines. One of these genes was identified as CD44, which encodes an integral membrane glycoprotein that serves as the principal receptor for hyaluronate and participates in specific cell-cell and cell-extracellular matrix interactions. The repression of CD44 expression in neuroblastoma cell lines might be relevant to their high metastatic potential. We have cloned full-length cDNAs corresponding to CD44 trancscripts and identified a novel splice variant of CD44 lacking 31 amino acids of the extracellular domain. As a first step toward analysis of CD44 downregulation in neuroblastoma cells, we have mapped the CD44 RNA initiation site and analyzed the structure of the upstream regulatory region. We constructed a series of plasmids containing different amounts of CD44 upstream regulatory region linked to the bacterial chloramphenicol acetyltransferase gene and then analyzed their ability to promote transcription in neuroblastoma and melanoma cells. We found that a DNA segment including about 150 bp of the CD44 upstream region and the 5' end of the gene itself was sufficient to induce substantial transcription of the chloramphenicol acetyltransferase gene in both neuroblastoma and melanoma cells. Several upstream cis-acting elements contribute to the downregulation of CD44 in neuroblastoma cells, the most prominent being a 120-bp DNA fragment located 450 bp upstream to the RNA initiation site. Our data suggest that multiple factors might be involved in downregulation of CD44 in neuroblastoma cells.