Cross-talk between Akt, p53 and Mdm2: possible implications for the regulation of apoptosis.

The p53 tumor suppressor protein and the Akt/PKB kinase play important roles in the transduction of pro-apoptotic and anti-apoptotic signals, respectively. We provide evidence that conflicting signals transduced by Akt and p53 are integrated via negative feedback between the two pathways. On the one hand, the combination of ionizing radiation and survival factor deprivation, which leads to rapid apoptosis of IL-3 dependent DA-1 cells, entails a caspase- and p53-dependent destruction of Akt. This destruction of Akt is not a secondary consequence of apoptosis, since it is not seen when the same cells are triggered to undergo apoptosis under different conditions. On the other hand upon serum stimulation, when Akt becomes active and enhances cell survival, phosphorylation occurs at an Akt consensus site (serine 166) within the Mdm2 protein, a key regulator of p53 function. Taken together, our findings suggest that depending on the balance of signals, p53-dependent downregulation of Akt may promote an irreversible commitment to apoptotic cell death, whereas effective recruitment of Akt by appropriate survival signals may lead to activation of Mdm2, inactivation of p53, and eventually inhibition of p53-dependent apoptosis.

Regulation of p53: intricate loops and delicate balances.

The p53 tumor suppressor protein provides a major anti-cancer defense mechanism, as underscored by the fact that the p53 gene is the most frequent target for genetic alterations in human cancer. Recent work has led to the realization that p53 lies at the hub of a very complex network of signaling pathways, which integrate a variety of intracellular and extracellular inputs. Part of this network consists of an array of autoregulatory feedback loops, where p53 exhibits very intricate interactions with other proteins known to play important roles in the determination of cell fate. We discuss two such loops, one involving the beta catenin protein and the other centering on the Akt/protein kinase B. In both cases, the central module is the interplay between p53 and the murine double minute 2 (Mdm2) protein, which inactivates p53 and targets it for rapid proteolysis. Whereas deregulated beta catenin can lead to Mdm2 inactivation and p53 accumulation, active p53 can promote the degradation and downregulation of beta catenin. Similarly, Akt can block p53 activation by potentiating Mdm2, whereas activated p53 can tune down Akt in several different ways. In each case, the actual output of the loop is determined by the delicate balance between the opposing effects of its different components. Often, this balance is dictated by additional signaling processes that occur simultaneously within the same cell. Genetic alterations characteristic of cancer are capable of severely distorting this balance, thereby overriding the tumor suppressor effects of p53 in a manner that facilitates neoplastic conversion.

Regulation of p53: intricate loops and delicate balances.

The p53 tumor suppressor protein provides a major anti-cancer defense mechanism, as underscored by the fact that the p53 gene is the most frequent target for genetic alterations in human cancer. Recent work has led to the realization that p53 lies at the hub of a very complex network of signaling pathways that integrate a variety of intracellular and extracellular inputs. Part of this network consists of an array of autoregulatory feedback loops, where p53 exhibits very intricate interactions with other proteins known to play important roles in the determination of cell fate. We discuss two such loops, one involving the beta-catenin protein and the other centering on the Akt/PKB protein kinase. In both cases, the central module is the interplay between p53 and the Mdm2 protein, which inactivates p53 and targets it for rapid proteolysis. Whereas deregulated beta-catenin can lead to Mdm2 inactivation and p53 accumulation, active p53 can promote the degradation and down-regulation of beta-catenin. Similarly, Akt can block p53 activation by potentiating Mdm2, whereas activated p53 can tune down Akt in several different ways. In each case, the actual output of the loop is determined by the delicate balance between the opposing effects of its different components. Often, this balance is dictated by additional signaling processes that occur simultaneously within the same cell. Genetic alterations characteristic of cancer are capable of severely distorting this balance, thereby overriding the tumor suppressor effects of p53 in a manner that facilitates neoplastic conversion.

Expression of XPG protein in human normal and tumor tissues.

XPG (Xeroderma pigmentosum group G complementing factor) is a protein associated with DNA repair and transcription. Point mutations in ERCC5, the gene coding for XPG, cause the cancer-prone disorder xeroderma pigmentosum (XP) while truncation mutations give rise to individuals with the combined clinical features of XP and Cockayne syndrome. Polymorphisms of ERCC5 or alterations in XPG mRNA expression were also associated to an increase risk of different cancers types and to prognosis of cancer patients. However, the expression of XPG protein in different normal or tumor human tissues is not well known. In the present work, we have validated an immunohistochemistry (IHC) assay for detection of expression levels of XPG protein in FFPE human tissue samples. We have also tested this IHC assay in different normal and tumor human tissues. On a microarray containing 28 normal cores, positive staining was observed in 60% of the samples. The highest staining was detected in adrenal gland, breast, colon, heart, kidney, thyroid and tongue. In tumors, positive staining was observed in 9 of 10 breast cancer samples and in all 5 ovarian cancer and 5 sarcomas samples. Subcellular localization was predominantly nuclear. The use of this validated methodology would help to interpret the role of XPG in tumorogenesis and its use as a possible prognostic or predictive factor.