Murine Laminin Alpha3A and Alpha3B Isoform Chains Are Generated by Usage of Two Promoters and Alternative Splicing

We already identified two distinct laminin alpha3A and alpha3B 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 alpha3 (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 alpha3A and the alpha3B cDNAs. Sequence analysis of the region upstream to the alpha3A 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 alpha3B mRNA isoform was defined. The sequences upstream to the alpha3B 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 alpha3A and alpha3B 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 the lama3 gene produces the two polypeptides by alternative splicing and contains two promoters, which regulate the production of the two isoforms alpha3A and alpha3B.