Anti-cancer activity of compounds isolated from Bursera microphylla.

INTRODUCTION. Bursera microphylla A. Gray (Burseraceae), commonly known as elephant tree or torote blanco, is largely distributed in Sonoran Desert (Mexico). In folk medicine, B. microphylla is steeped in alcoholic beverages to make a tincture for gum sores, cold sores, and abscessed teeth. The dried stems and leaves are used in a tea to relieve painful urination and as a stimulating expectorant for bronchitis while the gum has been used to treat venereal diseases in Sonora.[1] Seri ethnic group in Sonoran Desert uses different parts of the plant (leaves, fruits, bark and sap) for treating several illnesses, such as sore throat, headache and for wound healing.[2] The aim of this study was to determine the effect of the B. microphylla hexane and dicholomethane fractions of resin methanol extract on cancer. RESULTS. At first, the BM1 fraction was tested against the acute myeloid cell line OCI-AML. We found that the concentrations of 2,5 and 0,25 but not 0,025 μ/mL of BM1 were able to significantly decrease OCI-AML cell number after 24 and 48 hours of treatment. The decrease of cell number was associated with an increased apoptosis, a decrease in S phase and a block of G2/M phase of the cell cycle. Thus, BM1 fraction of B. microphylla decreased OCI-acute myeloid leukemia cell number by an augmentation of apoptosis and inhibition of cell division. We have chosen the concentration of 0,25 μg/ml and the treatment of 24 hours for further testing on the acute myeloid cell line U937 and the results were similar to those obtained with the OCI-AML cell line. Next step was to test, under the same experimental conditions as for the BM1 fraction, the B. microphylla hexane (HEX-S) and dicholomethane (DCMS-S) fractions of resin methanol extract. We found that only HEX-S, but not DCMS-S or BM1, decreased OCI-AML cell number at the concentration of 0,025 μg/ml after 24 hours of treatment. In addition, HEX-S increased apoptosis and inhibited cell division of the same cells. Same results were obtained with different acute myeloid cell lines (U937, HL-60, K562), with a T-cell lymphoma (Jurkat) and with different solid tumors. Specifically, the anti-cancer effect of HEX-S was found in C643 (a thyroid tumor), MCF-7 (breast cancer) and HCT116 (colon cancer). The following step was to study the molecular mechanism of the effect seen on OCI-AML acute myeloid cells. To this end, apoptosis pathways were evaluated. Western blot analysis detected that, while caspase-8 was not activated, activation of caspase-3 was detectable at 4, 8, 14 or 24 hours of treatment of OCI-AML with HEX-S (0,025 μg/ml). this suggest that the intrinsic, but not the extrinsic, pathway of apoptosis was involved in the pro-apoptotic effect of HEX-S. Additional pro-apoptotic, such as Bim and Puma, or anti-apoptotic molecules, such as Bcl-2, seemed to be not involved in the pro-apoptotic action of HEX-S. An additional phase of our analysis was to dissect the anti-proliferative pathway triggered by HEX-S. We found that the inhibition of cell cycle progression was due to the up-regulation of p21 but not of p53 proteins, as evaluated by western blotting analysis of OCI-AML cells treated with 0.025 μg/ml of HEX-S fraction. CONCLUSIONS. The above mentioned results suggest that the HEX-S fraction of Bursera microphylla inhibits the proliferation of acute myeloid leukemia cells by triggering a p21-dependent, p53-independent pathway. This leads to the death of cells by stimulating the intrinsic pathway of apoptosis with final activation of caspase-3. Future experiments will focus on the isolation and identification of single molecules responsible for the antiproliferative effect exerted by the HEX-S fraction of Bursera microphylla in order to find a new anticancer drug with potential therapeutic utilization.