Mutual regulation of hypoxia-inducible factor and mammalian target of rapamycin as a function of oxygen availability.

The mammalian target of rapamycin (mTOR) regulates cellular growth and proliferation, mainly by controlling cellular translation. Most tumors show constitutive activation of the mTOR pathway. In hypoxia, mTOR is inactivated, which is believed to be part of the program of the cell to maintain energy homeostasis. However, certain proteins are believed to be preferentially translated during hypoxia via 5' terminal oligopyrimidine tract mechanisms with controversial discussion about the involvement of the mTOR-dependent ribosomal protein S6 (rpS6). The hypoxia-inducible transcription factor (HIF) is the master regulator of hypoxic adaptation and itself strongly implicated in tumor growth. HIF is translationally regulated by mTOR. The regulatory features and the involvement of molecular oxygen itself in this regulation of HIF by mTOR are poorly understood. mTOR inhibition leads to profound attenuation of HIFalpha protein in the majority of primary and cancer cells studied. Under severe hypoxia, no influence of mTOR inhibitors was observed; thus, stimulation of HIFalpha by mTOR may only be relevant under mild hypoxia or even normoxia. HIF expression and phosphorylated rpS6 negatively correlate in experimental tumors. In cell culture, prolonged hypoxia abolishes rpS6 phosphorylation, which seems to be partly independent of the upstream p70S6 kinase. We show that hypoxic repression of rpS6 is largely dependent on HIF, implicating a negative feedback loop, which may influence cellular translational rates and metabolic homeostasis. These data implicate that the hypoxic microenvironment renders tumor cells resistant to mTOR inhibition, at least concerning hypoxic gene activation, which would add to the difficulties of other established therapeutic strategies in hypoxic cancer tissues.

Treatment of cytomegalovirus disease with valganciclovir in renal transplant recipients: a single center experience.

Recent data suggest valganciclovir (VGC) to be as effective as ganciclovir for cytomegalovirus (CMV) prophylaxis. The objective of this study was to analyze the effect of oral valganciclovir in renal transplant patients with symptomatic CMV infection. Twenty-one patients with symptomatic CMV infection received VGC in doses adjusted to renal function until resolution of CMV antigenemia. The patients were followed for a mean of 5.5 months. During therapy, CMV antigenemia dropped in all patients from pretreatment positive levels of 5.2 +/- 3.7 to negative values of 0.25 +/- 0.2 positive cells/10,000 PBMC (P<0.001). After cessation of therapy, none of patients developed relapse of CMV antigenemia/symptoms within the follow-up. VGC therapy was well tolerated in all patients and no major adverse effects occurred. This pilot trial showed VGC to be safe and highly effective in antiviral therapy after renal transplantation. However, subsequent multicenter clinical trials for treatment of CMV disease are necessary.