The type 2C phosphatase Wip1: an oncogenic regulator of tumor suppressor and DNA damage response pathways

Using mice to examine p53 functions in cancer, aging, and longevity

The p53 tumor suppressor is a multifaceted transcription factor that responds to a diverse array of stresses that include DNA damage and aberrant oncogene signaling. On activation, p53 prevents the emergence of cancer cells by initiating cell cycle arrest, senescence (terminal cell cycle arrest), or apoptosis. Although its role in assuring longevity by suppressing cancer is well established, recent studies obtained largely from genetically engineered mouse models suggest that p53 may regulate longevity and aging. In some contexts, it appears that altered p53 activity may enhance longevity, and in others, it appears to suppress longevity and accelerate aging phenotypes. Here, we discuss how genetically engineered mouse models have been used to explore antiproliferative functions of p53 in cancer suppression and how mouse models with altered aging phenotypes have shed light on how p53 might influence the aging process.

Posttranslational modification of p53: cooperative integrators of function

The p53 protein is modified by as many as 50 individual posttranslational modifications. Many of these occur in response to genotoxic or nongenotoxic stresses and show interdependence, such that one or more modifications can nucleate subsequent events. This interdependent nature suggests a pathway that operates through multiple cooperative events as opposed to distinct functions for individual, isolated modifications. This concept, supported by recent investigations, which provide exquisite detail as to how various modifications mediate precise protein-protein interactions in a cooperative manner, may explain why knockin mice expressing p53 proteins substituted at one or just a few sites of modification typically show only subtle effects on p53 function. The present article focuses on recent, exciting progress and develops the idea that the impact of modification on p53 function is achieved through collective and integrated events.

PPM1D430, a novel alternative splicing variant of the human PPM1D, can dephosphorylate p53 and exhibits specific tissue expression

PPM1D is a PPM1 type protein phosphatase and is induced in response to DNA damage. PPM1D-deficient mice show defects in spermatogenesis and lymphoid cell functions but the mechanisms underlying these phenotypes remain unknown. In our current study, we identify and characterize an alternative splicing variant (denoted PPM1D430) of human PPM1D at both the mRNA and protein level. PPM1D430 comprises the common 420 residues of the known PPM1D protein (PPM1D605) and contains a stretch of PPM1D430-specific 10 amino acids. Semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) analysis revealed that PPM1D430 mRNA is also induced in response to the genotoxic stress in a p53-dependent manner. In vitro phosphatase analysis and PPM1D430-specific RNA interference analysis further indicated that PPM1D430 can dephosphorylate Ser15 of human p53 both in vitro and in vivo. On the other hand, expression profiling of this gene by RT-PCR analysis of a human tissue cDNA panel revealed that PPM1D430 is expressed exclusively in testes and in leucocytes whereas PPM1D605 is ubiquitous. In addition, PPM1D430 shows a different subcellular localization pattern and protein stability when compared with PPM1D605 under some conditions. Our current findings thus suggest that PPM1D430 may exert specific functions in immune response and/or spermatogenesis.