Sp1-mediated transactivation of LamC1 promoter and coordinated expression of laminin-gamma1 and Sp1 in human hepatocellular carcinomas.

The laminin-gamma1 chain is present in most basement membranes and is involved in various physiological and pathological processes, including carcinogenesis in the liver. We have investigated the role of the transcription factor Sp1 in the activation of the LamC1 gene, which encodes laminin-gamma1, both in hepatocytes and in human hepatocellular carcinomas. DNAse I hypersensitive sites were mapped in the murine LamC1 promoter using early hepatocyte primary cultures in which LamC1 becomes activated. Three hypersensitive sites were found in enhancer-like elements that contain GC-rich regions. Gel-shift analyses showed that specific complexes were resolved using GC-containing oligonucleotides and Faza 567 hepatoma cells, which constitutively express laminin-gamma1 at a high level. Increased GC-binding activity was observed using nuclear extracts from early hepatocyte cultures versus normal liver. Sp1 overexpression in normal hepatocytes transfected with an Sp1 expression vector induced a marked increased of laminin-gamma1 mRNA content and co-transfection of promoter fragments in Drosophila melanogaster SL2 cells demonstrated that Sp1 transactivates LamC1. In human hepatocellular carcinomas, Sp1 and laminin-gamma1 mRNA were simultaneously expressed at high levels, and gel-shift experiments demonstrated a higher GC-binding activity to Sp1 compared with control livers. In situ hybridization indicated that cells exhibiting a high content of laminin-gamma1 mRNA were also strongly positive for Sp1 mRNA, including both cancer cells at the invasion front and stromal cells. These results show that Sp1 is involved in the activation of LamC1 that occurs in human hepatocellular carcinomas.