Regulatory sequences for the transcription of the laminin B2 gene in astrocytes.
Astrocytes synthesize only the B2 chain of laminin and that this chain is sufficient to stimulate neurite outgrowth. In this study, we have examined laminin B1 and B2 promoter constructs in various cell types in order to understand the transcriptional regulation of laminin B2 gene in astrocytes. Comparison of nuclear factor binding by Southwestern analysis with the highly active B2 promoter fragment revealed different patterns of nuclear factor binding. In HepG2 cells, two proteins of 105 and 98 kDa were identified while, in primary astrocytes, human U251 and rat C6 glioma cells, a greater number of nuclear proteins ranging from 43 to 212 kDa were detected. The laminin B1 promoter construct was inactive in transient transfection experiments in astrocytes yet active in the HepG2 hepatoma cells which synthesize both the B1 and B2 chains. In contrast, the laminin B2 promoter construct was active in both astrocytes and HepG2 cells. These results are consistent with the lack of laminin B1 mRNA expression in astrocytes and suggest that the differential regulation of the laminin B1 and B2 gene is controlled at the transcriptional level. Delineation of the 5'-flanking regions responsible for basal levels of B2 laminin promoter activity revealed a silencer-like segment between -830 and -224 which reduced promoter activity. Deletion analysis further revealed that B2 laminin promoter possesses a highly active short promoter (-94 to +106) and basal transcriptional activity resides within -61 to +106. DNase 1 footprinting, gel-shift competition assays and site-directed mutagenesis of a highly active short promoter revealed that this region contained binding sites for cell-type nuclear factors. The shortest construct containing only residues -21 to +106 was inactive in HepG2 and U251 glioma cells. In primary astrocytes, however, this construct showed a high level of transcriptional activity. Deletion of 47 bp (+59 to +106) in 5'-UTR completely blocked promoter activity in astrocytes confirming that this downstream region is important for transcriptional activity in primary astrocytes. Together, these results suggest that astrocytes may utilize mutually exclusive transcription factors and regulatory sequences, in addition to common factors in the control of the laminin B2 promoter.
Kedar
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Laboratory of Molecular Biology, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD 20892, USA. vishramk@qimr.edu.au
Freese
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E
Hempel
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FG
D000818:Animals; D001253:Astrocytes / Q000378:metabolism*; D001483:Base Sequence; D002478:Cells, Cultured; D007797:Laminin / Q000235:genetics*; D008969:Molecular Sequence Data; D011401:Promoter Regions, Genetic / Q000235:genetics; D051381:Rats; D017207:Rats, Sprague-Dawley; D014157:Transcription Factors / Q000235:genetics*
D007797:Laminin; D014157:Transcription Factors