Novel Insights into the Biological Effects of the Isoprenoid Derivative N6-Isopentenyladenosine: Involvement of the Metabolic Sensor Ampk in Angiogenesis Inhibition

Abstract

N6-isopentenyladenosine (iPA) is a modified adenosine characterized by an isopentenyl chain derived by dimethylallyl pyrophosphate (DMAPP), an intermediate of the metabolic pathway of mevalonate, that is known to be deregulated in cancer. iPA is an endogenous isoprenoid-derived product present in mammalian cells as a free nucleoside in the cytoplasm, or in a tRNA-bound form, displaying well established pleiotropic biological effects, including a direct anti-tumor activity against several cancers. However, the precise mechanism of action of iPA in inhibiting cancer cell proliferation remains to be clarified. In this work, we investigated whether iPA could directly interfere with the angiogenic process, fundamental to cancer growth and progression, and if the growth and proliferation of human melanoma cells, known for their highly angiogenic phenotype, could be affected by the treatment with iPA. Finally, we investigated if iPA could have an immunomodulatory role targeting directly human natural killer (NK) cells, components of innate immunity that participate in immunity against neoplastic cells, in order to provide a cooperative and multifactorial mode of action of iPA to arrest cancer growth. To evaluate the potential involvement of iPA in angiogenesis, we employed human umbilical vein endothelial cells (HUVECs) as a suitable in vitro model of angiogenesis, by evaluating the viability, proliferation, migration, invasion, tube formation, and molecular mechanisms involved. Data were corroborated in mice by using a gel plug assay. iPA dose- and time-dependently inhibited all the neoangiogenesis stages, with an IC50 of 0.98 μM. We demonstrated for the first time that iPA was monophosphorylated into iPA 5'-monophosphate (iPAMP) by adenosine kinase (ADK) inside the cells. iPAMP is the active form that inhibits angiogenesis through the direct activation of AMP-kinase (AMPK). Indeed, all effects were completely reversed by pre-treatment with 5-iodotubercidin (5-Itu), an ADK inhibitor. The isoprenoid intermediate isopentenyl pyrophosphate (IPP), which shares the isopentenyl moiety with iPA, was ineffective in the inhibition of angiogenesis, thus showing that the iPA structure is specific for the observed effects. Thus, iPA is a novel AMPK activator and could represent a useful tool for the treatment of diseases where excessive neoangiogenesis is the underlying pathology... [edited by author]