The DNA repair enzyme O6-methylguanine-DNA methyltransferase (MGMT) is commonly overexpressed in cancers and is implicated in the development of chemoresistance. The use of drugs inhibiting MGMT has been hindered by their haematologic toxicity and inefficiency. As a different strategy to inhibit MGMT we investigated cellular regulators of MGMT expression in multiple cancers. Here we show a significant correlation between Wnt signalling and MGMT expression in cancers with different origin and confirm the findings by bioinformatic analysis and immunofluorescence. We demonstrate Wnt-dependent MGMT gene expression and cellular co-localization between active β-catenin and MGMT. Pharmacological or genetic inhibition of Wnt activity downregulates MGMT expression and restores chemosensitivity of DNA-alkylating drugs in mouse models. These findings have potential therapeutic implications for chemoresistant cancers, especially of brain tumours where the use of temozolomide is frequently used in treatment.

Medulloblastomas are the most common malignant brain tumors in children. They express high levels of COX-2 and produce PGE₂, which stimulates tumor cell proliferation. Human cytomegalovirus (HCMV) is prevalent in the human population and encodes proteins that provide immune evasion strategies and promote oncogenic transformation and oncomodulation. In particular, HCMV induces COX-2 expression; STAT3 phosphorylation; production of PGE₂, vascular endothelial growth factor, and IL-6; and tumor formation in vivo. Here, we show that a large proportion of primary medulloblastomas and medulloblastoma cell lines are infected with HCMV and that COX-2 expression, along with PGE₂ levels, in tumors is directly modulated by the virus. Our analysis indicated that both HCMV immediate-early proteins and late proteins are expressed in the majority of primary medulloblastomas. Remarkably, all of the human medulloblastoma cell lines that we analyzed contained HCMV DNA and RNA and expressed HCMV proteins at various levels in vitro. When engrafted into immunocompromised mice, human medulloblastoma cells induced expression of HCMV proteins. HCMV and COX-2 expression correlated in primary tumors, cell lines, and medulloblastoma xenografts. The antiviral drug valganciclovir and the specific COX-2 inhibitor celecoxib prevented HCMV replication in vitro and inhibited PGE₂ production and reduced medulloblastoma tumor cell growth both in vitro and in vivo. Ganciclovir did not affect the growth of HCMV-negative tumor cell lines. These findings imply an important role for HCMV in medulloblastoma and suggest HCMV as a novel therapeutic target for this tumor.

The objective of this article is to amalgamate previous results into a speculative synthesis that sheds light on the causes of secondary brain damage following either global/forebrain or focal ischemia. The hypothesis is based on the well-founded assumption that the pathophysiology of the brain damage incurred by global or forebrain ischemia is different from that of focal ischemia. In the former, the ischemia is usually dense and of brief duration and, provided that reperfusion is adequate, cell damage is conspicuously delayed, mostly affecting selectively vulnerable neurons. In contrast, focal ischemia is either long-lasting or permanent, and it is usually less severe, particularly in the perifocal penumbral regions. The lesion is typically pan-necrotic ("infarction"), initially affecting the focus supplied by the occluded artery, later invading the penumbra zone. Available results allow a restatement of the calcium hypothesis of cell death. In global or forebrain ischemia, calcium influx through channels gated by voltage or glutamate receptors is envisaged to trigger reactions that limit the survival of neurons during reperfusion, leading to secondary neuronal death after hours or days of survival. It can be hypothesized that the initial insult leads to a sustained alteration of membrane calcium handling, resulting in slow, gradual calcium overload of mitochondria. Alternatively, a sustained perturbation of the intracellular signal transduction pathway leads to changes in transcription or translation, bereaving the cells of heat shock and stress proteins, of trophic factors, or of enzymes required for survival. However, with the possible exception of the gerbil, neither microvascular failure nor primary mitochondrial dysfunction is believed to be involved. In focal ischemia, similar reactions are probably triggered by calcium influx, whether this is sustained (the focus) or intermittent (the penumbra). However, these play a minor role in cell death since they are overridden by reactions producing mediators of rapidly developing secondary damage, affecting either microvessels or mitochondria. Very probably, some of these mediators are free radicals, or nitric oxide, or other reactive metabolites, emanating from lipid hydrolysis and arachidonic acid metabolism. During continuous ischemia, or during recirculation following 1–3 h of ischemia, these mediators activate adhesion molecules in endothelial cells or polymorphonuclear leucocytes, or oxidize key proteins. The result is either failure of micro-circulation ("capillary plugging"), or sustained mitochondrial failure. Since calcium influx is an initial event, agents reducing presynaptic depolarization and calcium entry through glutamate receptor-gated and other calcium channels have predictably a narrow therapeutic window; however, since spin trapping agents of the nitrone class act many hours after the induction of focal ischemia, their therapeutic window is potentially very wide. This may be because expression of mRNAs for adhesion molecules and their synthesis are relatively slow processes, and because the nitrones act on events that involve adhesion of leukocytes to the endothelial cells, with plugging of capillaries and postcapillary venules, and on the ensuing inflammatory response.