Kazue Hattori

The multifunctional actions of transforming growth factor beta 1 (TGF-beta 1) indicate that it has a pivotal control function in many physiological and pathological processes. An important property of TGF-beta 1 is its ability to activate its own mRNA expression and thereby increase its own secretion. Two distinct regions of the promoter of the TGF-beta 1 gene are responsive to autoregulation: one 5' to the upstream transcriptional start site and another located between the two major start sites. In both promoter regions, autoinduction is mediated by binding of the AP-1 (Jun-Fos) complex. An important contribution to this positive regulation is the autoactivation of c-jun transcription by AP-1. Cotransfection of antisense c-jun or antisense c-fos expression vectors prevents TGF-beta 1 autoinduction. These results demonstrate that both components of the AP-1 complex are required for TGF-beta 1 autoinduction. Induction of jun expression by TGF-beta 1, as well as jun autoinduction, may amplify the action of TGF-beta 1 during normal development and oncogenesis.

The JUN protooncogene encodes a protein that is functionally and biochemically identical to the transcription factor AP-1 (activator protein 1). To understand the structure and regulation of this important gene, a genomic clone of human JUN was isolated and its primary structure and transcription pattern were determined. Most surprisingly, the sequence of the genomic clone was found to be contiguous with the sequence of the JUN cDNA, suggesting that it lacks introns. RNase protection experiments confirm that JUN is an intronless gene that yields several transcripts due to 5' and 3' heterogeneities. Transfection experiments show that the cloned gene is functional, as it encodes a trans-acting factor that stimulates transcription of AP-1-dependent reporter gene. In situ hybridization was used to map JUN to chromosomal region 1p31-32. Interestingly, this region is frequently deleted in neuroblastomas, suggesting that elimination of AP-1 may play an important role in the pathogenesis of this disease.

The low basal expression of Fos and the rapid and effective turn-off of serum induced Fos transcription is due to autoregulation. Fos and Jun/AP-1 protein cooperate in the repression mechanism. Overexpressions of Fos and Jun decrease basal and induced transcription from Fos-CAT constructs and from the endogenous gene in NIH3T3 cells. The introduction into cells of either antisense Fos or antisense Jun sequences leads to elevated basal Fos promoter activity. Gel retardation experiments with synthetic oligonucleotides define two target sequences in the Fos promoter which bind Fos-Jun/AP-1 (centering at about -296 and -60). In vivo competition with these oligonucleotides relieves repression.