Murine Laminin α3A and α3B Isoform Chains Are Generated by Usage of Two Promoters and Alternative Splicing

We already identified two distinct laminin α3A and α3B chain isoforms which differ in their amino-terminal ends and display different tissue-specific expression patterns. In this study we have investigated whether these two different isoforms are products of the same laminin α3 (lama3) gene and transcribed from one or two separate promoters. Genomic clones were isolated that encompass the sequences upstream to the 5′ ends of both the α3A and the α3B cDNAs. Sequence analysis of the region upstream to the α3A open reading frame revealed the presence of a TATA box and potential binding sites for responsive elements. By primer extension analysis, the transcription start site of the α3B mRNA isoform was defined. The sequences upstream to the α3B mRNA transcription start site do not contain a TATA box near the transcription initiation sites, but AP-1, AP-2, and Sp1 consensus binding site sequences were identified. The genomic regions located immediately upstream of the α3A and α3B transcription start sites were shown to possess promoter activities in transfection experiments. In the promoter regions, response elements for the acute phase reactant signal and NF-interleukin 6 were found, and their possible relevance in the context of inflammation and wound healing is discussed. Our results demonstrate that thelama3 gene produces the two polypeptides by alternative splicing and contains two promoters, which regulate the production of the two isoforms α3A and α3B.