Various p53 mutant proteins differently regulate the Ras circuit to induce a cancer-related gene signature.

Concomitant expression of mutant p53 and oncogenic Ras, leading to cellular transformation, is well documented. However, the mechanisms by which the various mutant p53 categories cooperate with Ras remain largely obscure. From this study we suggest that different mutant p53 categories cooperate with H-Ras in different ways to induce a unique expression pattern of a cancer-related gene signature (CGS). The DNA-contact p53 mutants (p53(R248Q) and p53(R273H)) exhibited the highest level of CGS expression by cooperating with NFκB. Furthermore, the Zn(+2) region conformational p53 mutants (p53(R175H) and p53(H179R)) induced the CGS by elevating H-Ras activity. This elevation in H-Ras activity stemmed from a perturbed function of the p53 transcription target gene, BTG2. By contrast, the L3 loop region conformational mutant (p53(G245S)) did not affect CGS expression. Our findings were further corroborated in human tumor-derived cell lines expressing Ras and the aforementioned mutated p53 proteins. These data might assist in future tailor-made therapy targeting the mutant p53-Ras axis in cancer.

Chromatin regulators as capacitors of interspecies variations in gene expression.

Gene expression varies widely between closely related species and strains, yet the genetic basis of most differences is still unknown. Several studies suggested that chromatin regulators have a key role in generating expression diversity, predicting a reduction in the interspecies differences on deletion of genes that influence chromatin structure or modifications. To examine this, we compared the genome-wide expression profiles of two closely related yeast species following the individual deletions of eight chromatin regulators and one transcription factor. In all cases, regulator deletions increased, rather than decreased, the expression differences between the species, revealing hidden genetic variability that was masked in the wild-type backgrounds. This effect was not observed for individual deletions of 11 enzymes involved in central metabolic pathways. The buffered variations were associated with trans differences, as revealed by allele-specific profiling of the interspecific hybrids. Our results support the idea that regulatory proteins serve as capacitors that buffer gene expression against hidden genetic variability.

MdmX is required for p53 interaction with and full induction of the Mdm2 promoter after cellular stress.

The activity of the tumor suppressor p53 is tightly controlled by its main negative regulator, Mdm2, which inhibits p53's transcriptional activity and targets it for degradation via the proteasome pathway. The closely related Mdm2 homolog, MdmX, is also considered to be a general inhibitor of transactivation by p53, through binding to the p53 activation domain. We show here that, unexpectedly, upon DNA damage and ribosomal stress, MdmX plays a positive role in p53-mediated activation of the Mdm2 gene, but not of numerous other p53 target genes including p21. Downregulation of MdmX results in lower levels of mature and nascent Mdm2 transcripts following cellular stress. This correlates with a longer p53 half-life following DNA damage. In vitro, Mdm2 inhibits the binding of p53 to DNA to a much greater extent than does MdmX, although MdmX does not stimulate p53 interaction with Mdm2 promoter DNA. Strikingly, however, MdmX is required for optimal p53 binding to the Mdm2 promoter in vivo. Thus, we have described a new mechanism by which MdmX can suppress p53, which is through transcriptional activation of p53's principal negative regulator, Mdm2.