Abstract. Propolis is a resinous substance collected by bees (Apis mellifera) from various tree buds which they then use to coat hive parts and to seal cracks and crevices in the hive. Propolis, a known ancient folk medicine, has been extensively used in diet to improve health and to prevent disease. In the present study, we have evaluated the effects of ethanolic extracts of Brazilian propolis group l2 and bud resins of botanical origin (B. dracunculifolia), and propolis group 3 on proliferation of metastasis (DU145 and PC-3) and primary malignant tumor (RC58T/h/SA#4)-derived human prostate cancer cells. The strongest inhibition was observed in propolis group 3 (sample #3) extracts whereas moderate growth inhibition was observed in human prostate epithelial cells. In the RC58T/h/SA#4 cells, resins of botanical origin of propolis group 12 (sample #1) and propolis group 12 (sample #2) induced growth inhibition that was associated with S phase arrest whereas propolis group 3 (sample #3) induced growth inhibition that was associated with G2 arrest. The mechanisms of cell cycle effects of propolis were investigated. The resins of botanical origin of propolis group 12 and propolis group 12 showed similar inhibition of cyclin D1, CDK4 and cyclin B1 expression. Propolis group 3 showed higher induction of p21 expression but no inhibition of cyclin D1, CDK4 and cyclin B1 expression. The results obtained here demonstrate that the Brazilian propolis extracts have significant inhibitory effect on proliferation of human prostate cancer cells. Inhibition was achieved through regulation of protein expression of cyclin D1, B1 and cyclin dependent kinase (CDK) as well as p21. Our results indicate that the Brazilian propolis extracts show promise as chemotherapeutic agents as well as preventive agents against prostate cancer. IntroductionProstate cancer is the most common male cancer in USA and other Western countries, and the second leading cause of male cancer death in USA (1). Despite its pervasive impact, the etiology of prostate cancer and the factors that promote its progression are not well understood. To date, there is no secure way to tell whether prostate cancer, once found, should receive treatment. Although prostatectomy, radiation therapy and hormone therapy have been used for curing prostate cancer patients, recurrence and metastasis of prostate cancer are major problems in prostate cancer therapy. Current available treatment often has troubling side-effects such as urinary incontinence and erectile dysfunction. There is no effective treatment modality once the cancer has evolved into the hormone refractory stage. Preventive strategies are currently emerging. These is an increase in the usage of nutritional supplements such as soybeans, garlic, green tea and Vitamin D3 etc., to augment the prescribed anticancer therapies.Propolis is a resinous substance collected by Apis mellifera from various tree buds which they then use to coat hive parts and to seal cracks and crevices in the hive (2). Propolis has been used as ...