Degradation of HIF-1alpha under Hypoxia Combined with Induction of Hsp90 Polyubiquitination in Cancer Cells by Hypericin: a Unique Cancer Therapy

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 1 alpha (HIF-1 alpha) 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-1 alpha, the regulatory precursor of the HIF-1 transcription factor which promotes hypoxic stress and angiogenic responses, overcomes the physiologic HIF-1 alpha protein stabilization which occurs in hypoxic cells. The hypericin effect also eliminates the high HIF-1 alpha 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-alpha proteins which occurs in normoxia, the hypericin-induced HIF-1 alpha catabolism can occur independently of cellular oxygen levels or pVHL-promoted ubiquitin ligation of HIF-1 alpha. 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-1 alpha plays pivotal roles, and in pVHL ablated tumor cells such as renal cell carcinoma through elimination of elevated HIF-1 alpha contents in these cells, scaling down the excessive angiogenesis which characterizes these tumors.