The tumor suppressor and apoptotic transcription factor p53 has been linked to Alzheimer disease in numerous ways, but perhaps the most direct link is revealed in the June 7 Journal of Neuroscience. Frederic Checler and colleagues report that APP intracellular domains (AICDs) activate the p53 promoter and increase p53 mRNA and protein activity. The findings help explain observations of increased p53 in Alzheimer disease (AD) brain and suggest that p53-driven apoptosis may play a significant role in AD pathology.
First author Cristine Alves da Costa and colleagues at Checler’s lab at Nice-Sophia-Antipolis University, Valbonne, France, together with Peter St. George-Hyslop and colleagues at the University of Toronto and Nadège Girardot at the Pitié-Salpêtrière Hospital, Paris, used immunoreactivity and luciferase reporter assays to measure levels of p53 and activation of the p53 promoter and p53 target genes. Their first hint that APP processing might be tied to p53 came from studies of presenilin (PS)-negative fibroblasts. The authors found that p53 levels are significantly lower in cells devoid of both PS1 and PS2 than in wild-type. The luciferase p53 promoter assay confirmed that the loss of the transcription factor was due, at least partly, to poor transcription of the p53 gene.
Next, Alves da Costa and colleagues found that γ-secretase inhibitors DFK167 and L685458 also lowered p53 levels, supporting the idea that γ-secretase-dependent signaling may be linked to activation of p53. Of course, the intramembrane protease has many substrates, including APP and Notch, but when the authors transfected cells with APP intracellular domains (AICDs), they found that p53 activity and transactivation of p53 target genes were increased. In this experiment the authors used AICD50 and AICD59, cleavage products of γ and ε cleavage of APP, respectively. They also found that transfecting fibroblasts with the AICDs led to an increase in staurosporine-stimulated caspase-3 activity. Because the caspase is a key regulator of apoptosis and is activated in response to p53, the findings outline a direct signaling pathway from APP to p53 to activation of apoptosis. In support of this, the authors found that PS2-induced caspase-3 activation failed in the absence of p53 and that p53 levels were reduced almost 50 percent in APP-negative fibroblasts.
But how might the intracellular domains of APP stimulate the p53 promoter? Growing evidence links the intracellular domain of APP to transcriptional activation with several AICD partners, including the proteins Fe65 and Tip60 that form a trimeric complex with AICD (see ARF related news story), suspected of playing a role. Indeed, when Alves da Costa and colleagues co-transfected these two proteins with AICD59, the APP intracellular domain was stabilized (as previously reported—see Kimberly et al., 2001), and caspase-3 activity increased by about threefold. The combined evidence suggests that AICD can activate p53 transcription and subsequently activate caspase-3, though what lies directly downstream of AICD in this signaling pathway remains to be determined.
Most of this work was done in cultured fibroblasts, but whether these relationships might hold up in neurons is unclear. However, there are indications that they might. The authors did find higher levels of immunoreactive p53 in both sporadic and familial AD brain samples, confirming previous similar reports (see, for example, de la Monte et al., 1997). More specifically, they found that the number of neurons staining positive for the transcription factor was twofold higher in AD and FAD samples compared to controls. The increase in p53-positive neurons in idiopathic AD samples is difficult to link to APP processing, because as the authors state, sporadic AD is not generally regarded as being due to altered γ-secretase activity. Instead, the authors suggest that p53 increases in sporadic AD may result from poor degradation of AICDs. In fact, they report that insulin-degrading enzyme (IDE) levels are lower in AD compared to both FAD and normal brain tissue; in addition to Aβ, IDE has also been shown to degrade AICD (see Edbauer et al., 2002).
In the FAD samples it may be easier to reconcile the increased p53 levels with changes in APP processing because when Alves da Costa and colleagues transfected cells with FAD PS1 mutants, they noticed that p53 activity was increased between two- and fourfold. It should be noted, however, that there is increasing evidence that many FAD PS mutations result in loss of γ-secretase function (see Bentahir et al., 2006 and related ARF Forum Discussion debating whether such mutations result in gain or loss of function). So how could loss of PS1 activity result in higher p53? The answer to that conundrum might come from studying the relationship between the two presenilins and p53 signaling. The authors found that the two PS isoforms have opposite effects on p53. Overexpression of PS1 decreased p53 expression and activity, while overexpression of PS2 increased p53. Interestingly, p53 may suppress expression of PS1 (see Roperch et al., 1998), suggesting that there is some cross-talk between the two presenilins. That cross-talk “could ultimately control the levels of γ-secretase-mediated AICD formation,” write the authors. In this regard, the increase in p53 elicited by PS1 mutations could be due to a compensatory increase in PS2 activity, which would fit with the loss-of-function theory for PS1 FAD mutations.
In a final caveat, the authors hammer another nail in the coffin of any potential γ-secretase-linked AD therapy. “The present report additionally suggests that chronic treatment with γ-secretase inhibitors could affect p53 drastically and, potentially therefore, ultimately lead to tumorigenicity,” the authors write.—Tom Fagan