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Abstract

The limited life span of non-transformed somatic cells is largely due to the induction of replicative (telomere dependent) or premature (telomere independent)senescence. Senescenceis astate of stable G1 arrest and thus represents a barrier to proliferation and transformation. Premature senescence is induced by several factors including stress, DNAdamage and oncogenes. Senescent cell phenotypic changes include altered morphology, increased cell volume and increased natural senescence-associated ß-galactosidase activity (SA-ß-gal). Based on the results of previous studies, I hypothesized that hyperforin,a compound from St. John's Wort that has anti-cancer activity in vitro and in vivo, may exert its effects in part through the induction of senescence.In my thesis work I therefore aimed to determine whether hyperforin induces senescence in tumor and endothelial cells in vitro and/or in vivo, and if so to study the molecular mechanism by which hyperforin induces senescence. As a subsidiary aim I set out to establish novel methodsfor analyzing and quantifying senescent cells. I found that at low concentrations, hyperforin and its derivative aristoforin induce senescence in cultured endothelial cells and tumor cells, and also in tumor tissues, whereas at higher concentrations it induces apoptosis. Hyperforin inhibits SIRT1, but my results suggest that this only partially explains how hyperforin induces senescence. The senescence-inducing concentration of hyperforin is also able to induce Noxa expression in MCF-7 cells after 24h, 4 and 6 days. Importantly, in hyperforin-treated MCF-7 cells in which Noxa was knocked down, a significant reduction of senescence compared to hyperforin-treated control cells was observed. Hyperforin-induced Noxa expression was found to be independent of p53 expression, but p53 is required for senescence induction by hyperforin. In addition to these mechanistic studies, I also established an improved method for the detection of senescent cells. Together these findings show that induction of senescence by hyperforin is a novel anti-cancer mechanism that affects both tumor cells and endothelial cells, and strengthen the notion that hyperforin is a promising anti-cancer agent.