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The tyrosine kinase inhibitor imatinib (Gleevec, Novartis Pharmaceuticals Corporation; Basel, Switzerland) is a powerful drug for treatment of chronic myelogenous leukemia (CML) and other malignancies. It selectively targets various tyrosine kinases, thereby leading to growth arrest of respective cancer cells. Given its wide application, it is of high importance to know all related underlying molecular mechanisms. We had previously found that imatinib increases the cellular clearance of intracellular protein aggregates by targeting the abl pathway and thereby upregulating lysosomal activity. Here, we describe that imatinib dose dependently activates the cellular autophagy machinery in mammalian cells, independently of tissue type, species origin or immortalization status of cells. Autophagy is an archetypical cellular degradation mechanism implicated in many physiological and pathophysiological conditions. Our data link for the first time the process of autophagy with the mode of action of imatinib. Induction of autophagy might represent an additional mechanism of imatinib to induce growth arrest, promote apoptosis in cancer cells and eventually even promote tumour regression. Leukemia (2007) 21, 936-942.

Sp3 is a ubiquitous transcription factor closely related to Sp1. Previous analyses showed that, unlike Sp1, Sp3 fails to activate transcription in certain promoter settings. This is due to the presence of an inhibitory domain located between the second glutamine-rich activation domain and the DNA-binding domain. To further analyze the transcriptional properties of Sp3, we have expressed and purified recombinant Sp3 and Sp1 as epitope-tagged proteins from stable transfected insect cells. We found that Sp3 does act as a strong activator similar to Sp1 in an in vitro transcription assay using Sp1/Sp3-depleted HeLa nuclear extract. However, on the same promoter Sp3 is almost inactive when transfected into cells. Mutational studies demonstrate that a single lysine residue is responsible for the low transcriptional activity of Sp3 in vivo. We show that Sp3, but not a mutant of Sp3 that lacks this lysine residue, is highly acetylated in vivo. Our results strongly suggest that the transcriptional activity of Sp3 is regulated by acetylation. The consequences of acetylation for the activity of Sp3 are discussed.

Sc from >24 h to <9 h. Our data indicate that among the kinases known to be inhibited by STI571, c-Abl is likely responsible for the observed antiprion effect. Taken together, we demonstrate that treatment with STI571 strongly activates the lysosomal degradation of PrP Sc and that substances specifically interfering with cellular signaling pathways might represent a novel class of anti-prion compounds.

Prion diseases are fatal neurodegenerative disorders with no effective therapy. A hallmark of prion disease is the conversion of the normal cellular form of prion protein PrP(C) into a disease-associated isoform PrP(Sc). The authors recently have shown that a tyrosine kinase inhibitor, imatinib mesylate, induces clearance of PrP(Sc) via specific inhibition of c-Abl in prion-infected cell culture models. In this study, the authors assessed the in vivo effects of imatinib mesylate on prion disease using a scrapie-infected mouse model and further investigated prion infectivity of the drug-treated scrapie-infected neuroblastoma (ScN2a) cells. The authors found that imatinib mesylate abolished prion infectivity to almost undetectable level in ScN2a cells and the level of PrP(Sc) was significantly decreased by the drug in scrapie-infected mouse spleens as well as in ScN2a cells. Moreover, the drug treatment at an early phase of peripheral scrapie infection delayed the appearance of PrP(Sc) in the central nervous system (CNS) and onset of clinical disease in mice. However, neither intraperitoneal nor intracerebroventricular delivery of the drug exerted any PrP(Sc) clearance effect in the CNS.

Prion diseases are fatal transmissible neurodegenerative disorders linked to an aberrant conformation of the cellular prion protein (PrP c ). We have shown previously that the chemical compound suramin induced aggregation of fully matured PrP c in post-ER compartments, thereby, activating a post-ER quality control mechanism and preventing cell surface localization of PrP by intracellular re-routing of aggregated PrP from the Golgi/TGN directly to lysosomes. Of note, drug-induced PrP aggregates were not toxic and could easily be degraded by neuronal cells. Here, we focused on determining the PrP domains mediating these effects. Using PrP deletion mutants we show that intracellular re-routing but not aggregation depends on the N-terminal PrP (aa 23-90) and, more precisely, on the preoctarepeat domain (aa 23-50). Fusion of the PrP N-terminus to the GPI-anchored protein Thy-1 did not cause aggregation or re-routing of the chimeric protein, indicating that the N-terminus is only active in re-routing when prion protein aggregation occurs. Insertion of a region with a comparable primary structure contained in the PrP paralogue prnd/doppel (aa 27-50) into N-terminally deleted PrP re-established the re-routing phenotype. Our data reveal an important role for the conserved preoctarepeat region of PrP, namely controlling the intracellular trafficking of misfolded PrP.

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