Oxidative DNA damage is a known risk factor of cancer. Antioxidants, such as quercetin, are thought to play a important role in protecting cells from oxidative stress induced by reactive oxygen species. A study by the Yonsei University College of Medicine, Republic of Korea, tried to determine which phytochemicals were responsible for the anti-cancer effects of a Ginkgo biloba extract, as indicated by previous studies. Kang en coworkers found that quercetin and kaempferol induced induce caspase-3-dependent apoptosis in cultured oral cavity cancer cells and concluded that both phytochemicals could be considered as possible anti-oral cavity cancer agents . An in-vitro study conducted on several glioma cell lines by scientists of the Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil, demonstrated that a combined treatment of quercetin and resveratrol synergistically improved their antitumor activity, thereby reducing the therapeutic concentration needed for glioma treatment . Low doses of resveratrol or quercetin separately had no effect on apoptosis induction, but had a strong effect on caspase 3 and caspases 7 activation when administered together.
Scientists of the Henan University, China, concluded that quercetin could improve therapeutic index of doxorubicin, a drug used in cancer chemotherapy, by its opposing effects on hypoxia-inducible factor-1 alpha in tumor and normal cells . They considered quercetin as a promising candidate as anti-cancer agents. Doxorubicin is commonly used in the treatment of a wide range of cancers, including hematological malignancies, many types of carcinoma and soft tissue sarcomas. However, treatment with doxorubicin has many side effects, such as a decrease in white blood cells, hair loss, cardiotoxicity and immune suppression. The scientists tested the effect of quercetin on the therapeutic index of doxorubicin in breast tumor cells and spleen cells. Their in-vitro test showed that quercetin reversed cell resistance to doxorubicin under hypoxia and protected spleen cells against cytotoxicity. In-vivo tests with mouse with 4T1 breast cancer cells showed that quercetin suppressed tumor growth and prolonged survival. Quercetin enhanced therapeutic efficacy of doxorubicin and reduced toxic side effects. Another Chinese study by Shan and Wang of the Hebei Medical University, investigated the effects of quercetin on the growth of the colon carcinoma cells and the regulation effect of quercetin on the Wnt/beta-catenin signaling pathway . They found that quercetin reduced colon cancer cell viability in a dose- and time-dependent manner. Quercetin downregulated the transcription and protein expression of cyclin D1 and survivin. Cyclins are a family of proteins which control the progression of cells through the cell cycle by activating cyclin-dependent kinase enzymes. The decrease in survivin, which is a contributing factor to chemotherapy resistance and apoptosis induction therapies, would render the cancer cells more prone to such cancer treatments. The phytochemical also reduced the expressiony of beta-catenin/Tcf in the colon cancer cells transiently transfected with the TCF4 reporter gene.
Chinese researchers demonstrated that quercetin can inhibit the growth of HeLa cells and induce their apoptosis . HeLa cells are special cells used in cancer research. They proliferate abnormally rapidly and have an active version of the enzyme telomerase , which prevents the incremental shortening of telomeres that is implicated in aging and eventual cell death. The researchers studied the effects of quercetin on proliferation, apoptosis, adhesion and migration and invasion of HeLa cells. They found that quercetin inhibited the growth of HeLa cells and inducing apoptosis of the cells. The phytochemical also inhibited adhesion, migration and invasion of the cells. They concluded that quercetin may play an antimetastatic role. Aalinkeel and co-workers at State University of New York found that quercetin inhibited cell growth and induced cell death of prostate cancer cells, while exerting no effect on normal prostate epithelial cells . The scientists showed that quercetin promoted cancer cell apoptosis by down-regulating the levels of heat shock protein Hsp90. This protein appears to induce apoptosis through inhibition of the PI3K/AKT signaling pathway.
Quercetin may also have anti-mutagenic properties. A group of scientists lead by Gupta of the National Institute of Pharmaceutical Education and Research, Mohali,India, found that quercetin may be a potential candidate as chemoprotectant . They came to this conclusion after treating rats, which were exposed to the hepatocarcinogen diethylnitrosamine (found in tobacco smoke and processed meat) with quercetin. The hepatocarcinogen increased malondialdehyde and decreased glutathione levels in the liver, and increased plasma levels of aspartate transaminase and alanine transaminase. Treatment of the rats with quercetin restored these levels and also reduced diethylnitrosamine induced DNA damage and apoptosis.
 Kaempferol and quercetin, components of Ginkgo biloba extract (EGb 761), induce caspase-3-dependent apoptosis in oral cavity cancer cells. Phytother Res. 2009 Jul 7.
 Resveratrol and quercetin cooperate to induce senescence-like growth arrest in C6 rat glioma cells. Cancer Sci. 2009 May 12.
 Quercetin greatly improved therapeutic index of doxorubicin against 4T1 breast cancer by its opposing effects on HIF-1alpha in tumor and normal cells. Cancer Chemother Pharmacol. 2009 May 26.
 Quercetin inhibit human SW480 colon cancer growth in association with inhibition of cyclin D1 and survivin expression through Wnt/beta-catenin signaling pathway. Cancer Invest. 2009 Jul;27(6):604-12.
 Effects of quercetin on proliferation, apoptosis, adhesion and migration, and invasion of HeLa cells. Eur J Gynaecol Oncol. 2009;30(1):60-4.
 The dietary bioflavonoid, quercetin, selectively induces apoptosis of prostate cancer cells by down-regulating the expression of heat shock protein 90. Prostate. 2008 Dec 1;68(16):1773-89.
 Antioxidant and antimutagenic effect of quercetin against DEN induced hepatotoxicity in rat. Phytother Res. 2009 Jun 5.