title: Degradation of HIF-1alpha under hypoxia combined with induction of Hsp90 polyubiquitination in cancer cells by hypericin: a unique cancer therapy.
abstract: The perihydroxylated perylene quinone hypericin has been reported to possess potent anti-metastatic and antiangiogenic activities, generated by targeting diverse crossroads of cancer-promoting processes via unique mechanisms. Hypericin is the only known exogenous reagent that can induce forced poly-ubiquitination and accelerated degradation of heat shock protein 90 (Hsp90) in cancer cells. Hsp90 client proteins are thereby destabilized and rapidly degraded. Hsp70 client proteins may potentially be also affected via preventing formation of hsp90-hsp70 intermediate complexes. We show here that hypericin also induces enhanced degradation of hypoxia-inducible factor 1α (HIF-1α) in two human tumor cell lines, U87-MG glioblastoma and RCC-C2VHL-/- renal cell carcinoma and in the non-malignant ARPE19 retinal pigment epithelial cell line. The hypericin-accelerated turnover of HIF-1α, the regulatory precursor of the HIF-1 transcription factor which promotes hypoxic stress and angiogenic responses, overcomes the physiologic HIF-1α protein stabilization which occurs in hypoxic cells. The hypericin effect also eliminates the high HIF-1α levels expressed constitutively in the von-Hippel Lindau protein (pVHL)-deficient RCC-C2VHL-/- renal cell carcinoma cell line. Unlike the normal ubiquitin-proteasome pathway-dependent turnover of HIF-α proteins which occurs in normoxia, the hypericin-induced HIF-1α catabolism can occur independently of cellular oxygen levels or pVHL-promoted ubiquitin ligation of HIF-1α. It is mediated by lysosomal cathepsin-B enzymes with cathepsin-B activity being optimized in the cells through hypericin-mediated reduction in intracellular pH. Our findings suggest that hypericin may potentially be useful in preventing growth of tumors in which HIF-1α plays pivotal roles, and in pVHL ablated tumor cells such as renal cell carcinoma through elimination of elevated HIF-1α contents in these cells, scaling down the excessive angiogenesis which characterizes these tumors.
authors.affiliation: The Blood Center, Sheba Medical Center, Ramat-Gan, Israel.
authors.forename: Tilda
authors.initials: T
authors.lastname: Barliya
authors.forename: Mathilda
authors.initials: M
authors.lastname: Mandel
authors.forename: Tami
authors.initials: T
authors.lastname: Livnat
authors.forename: Dov
authors.initials: D
authors.lastname: Weinberger
authors.forename: Gad
authors.initials: G
authors.lastname: Lavie
mesh_terms: D000873:Anthracenes; D000970:Antineoplastic Agents / Q000494:pharmacology; D015153:Blotting, Western; D002401:Cathepsin B / Q000378:metabolism; D015687:Cell Hypoxia; D002460:Cell Line; D045744:Cell Line, Tumor; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects; D018841:HSP90 Heat-Shock Proteins / Q000378:metabolism*; D006801:Humans; D006863:Hydrogen-Ion Concentration / Q000187:drug effects; D051795:Hypoxia-Inducible Factor 1, alpha Subunit / Q000378:metabolism*; D008660:Metabolism / Q000187:drug effects; D009369:Neoplasms / Q000188:drug therapy / Q000378:metabolism / Q000473:pathology; D010569:Perylene / Q000031:analogs & derivatives* / Q000494:pharmacology; D011401:Promoter Regions, Genetic / Q000235:genetics; D011485:Protein Binding; D020218:Response Elements / Q000235:genetics; D020133:Reverse Transcriptase Polymerase Chain Reaction; D054875:Ubiquitination / Q000187:drug effects; D042461:Vascular Endothelial Growth Factor A / Q000235:genetics; D040301:Vascular Endothelial Growth Factor Receptor-2 / Q000235:genetics
chemical_list: D000873:Anthracenes; D000970:Antineoplastic Agents; D018841:HSP90 Heat-Shock Proteins; D051795:Hypoxia-Inducible Factor 1, alpha Subunit; D042461:Vascular Endothelial Growth Factor A; D010569:Perylene; C004965:hypericin; D040301:Vascular Endothelial Growth Factor Receptor-2; D002401:Cathepsin B