PTEN tumor suppressor is frequently mutated in human cancers and is a negative regulator of PI3'K/PKB/Akt-dependent cellular survival. Investigation of the human genomic PTEN locus revealed a p53 binding element directly upstream of the PTEN gene. Deletion and mutation analyses showed that this element is necessary for inducible transactivation of PTEN by p53. A p53-independent element controlling constitutive expression of PTEN was also identified. In contrast to p53 mutant cell lines, induction of p53 in primary and tumor cell lines with wild-type p53 increased PTEN mRNA levels. PTEN was required for p53-mediated apoptosis in immortalized mouse embryonic fibroblasts. Our results reveal a unique role for p53 in regulation of cellular survival and an interesting connection in tumor suppressor signaling.

The p53 tumor suppressor exerts anti-proliferative effects in response to various types of stress including DNA damage and abnormal proliferative signals. Tight regulation of p53 is essential for maintaining normal cell growth and this occurs primarily through posttranslational modifications of p53. Here, we describe Pirh2, a gene regulated by p53 that encodes a RING-H2 domain-containing protein with intrinsic ubiquitin-protein ligase activity. Pirh2 physically interacts with p53 and promotes ubiquitination of p53 independently of Mdm2. Expression of Pirh2 decreases the level of p53 protein and abrogation of endogenous Pirh2 expression increases the level of p53. Furthermore, Pirh2 represses p53 functions including p53-dependent transactivation and growth inhibition. We propose that Pirh2 is involved in the negative regulation of p53 function through physical interaction and ubiquitin-mediated proteolysis. Hence, Pirh2, like Mdm2, participates in an autoregulatory feedback loop that controls p53 function.

Normal somatic cells have a finite life span [1] and lose telomeric DNA, present at the ends of chromosomes, each time they divide as a function of age in vivo or in culture [2-4]. In contrast, many cancer cells and cell lines established from tumours maintain their telomere length by activation of an RNA-protein complex called telomerase, an enzyme originally discovered in Tetrahymena [5], that synthesizes telomeric repeats [6-8]. These findings have led to the formation of the 'telomere hypothesis', which proposes that critical shortening of telomeric DNA due to the end-replication problem [9] is the signal for the initiation of cellular senescence [10,11]. In yeast, the EST2 gene product, the catalytic subunit of telomerase, is essential for telomere maintenance in vivo [12-14]. The recent cloning of the cDNA encoding the catalytic subunit of human telomerase (hTERT) [15,16] makes it possible to test the telomere hypothesis. In this study, we expressed hTERT in normal human diploid fibroblasts, which lack telomerase activity, to determine whether telomerase activity could be reconstituted leading to extension of replicative life span. Our results show that retroviral-mediated expression of hTERT resulted in functional telomerase activity in normal aging human cells. Moreover, reconstitution of telomerase activity in vivo led to an increase in the length of telomeric DNA and to extension of cellular life span. These findings provide direct evidence in support of the telomere hypothesis, indicating that telomere length is one factor that can determine the replicative life span of human cells.