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p53 is a tumor suppressor protein that induces apoptosis at least in part through its ability to act as a sequence-specific transactivator. This work reports that intron 1 of the mouse Fas death receptor gene contains a p53-responsive element (p53RE) that matches the p53 consensus sequence and that is located between nucleotides ؉1704 and ؉1723 from the transcription initiation site. This element is specifically bound by p53 and functions as a p53-dependent enhancer in mammalian or in yeast reporter gene assays. Contrary to bax, another known pro-apoptotic p53-target gene, both mouse and human FAS p53REs are still activated by the discriminatory p53 mutants Pro-175 and Ala-143, a class of mutants unable to induce apoptosis. We propose that p53-dependent up-regulation of Fas does not induce apoptosis per se but sensitizes the cell to other proapoptotic signal(s). The functional conservation of p53-dependent Fas up-regulation argues strongly in favor of its biological importance and suggests that murine models may be used to study further the in vivo role of Fas in the p53 response.

Fibrosis is an unregulated tissue repair process whose predominant characteristics are the proliferation of myofibroblasts and an excessive deposition of extracellular matrix. Transforming growth factor (TGF)-beta1 is considered as one of the most fibrogenic cytokines. However, the molecular mechanisms involved in its profibrotic role are not fully understood. Here, we addressed the role of TGF-beta1 on cell proliferation and intracellular signal transduction in a pig model of skin fibrosis induced by gamma-irradiation. Primary myofibroblasts were isolated from the fibrotic tissue and their response to TGF-beta1 was compared to that of normal skin fibroblasts. The present results show that the differentiation of myofibroblasts involves a lack of TGF-beta1 growth inhibition and an impaired TGF-beta1 signaling. Receptor activity and Smad2/4 or Smad3/4 complex formation were similar in both cell types after TGF-beta1 treatment. However, the translocation of Smad3 protein into the nucleus was reduced in myofibroblasts as compared to that in fibroblasts, as well as its binding to target DNA sequences and the activation of the Smad binding elements found in the PAI-1. Interestingly, Smad2 was translocated similarly to the nucleus in both cell types suggesting that this protein may function normally in myofibroblasts. We propose that uncoupling of antiproliferative and profibrotic actions of TGF-beta1 in fibrosis may occur through differential regulation of the activities of Smad2 and Smad3 transcription factors.

In order to facilitate the process of epitope-tagging of yeast proteins, we have constructed two Saccharomyces cerevisiae-Escherichia coli shuttle vectors that allow fusion of a sequence encoding an epitope of the human c-myc protein at the 3' end of any gene. An example of the use of this technique is presented.

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Two new yeast genes, named MBR1 and MBR3, were isolated as multicopy suppressors of the growth defect of a strain lacking the HAP2 transcriptional activator. Both genes when overexpressed can also suppress the growth defect of hap3 and hap4 null mutants. However, overexpression of MBR1 cannot substitute for the HAP2/3/4 complex in activation of the CYC1 gene. Nucleotide sequencing of MBR1 and MBR3 revealed that these two genes encode serine-rich, hydrophilic proteins with regions of significant homology. The functional importance of one of these conserved regions was shown by mutagenesis. Disruption of MBR1 leads to a partial growth defect on glycerol medium. Disruption of MBR3 has no major effect but the double disruptant shows a synthetic phenotype suggesting that the MBR1 and MBR3 gene products participate in common function.

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