Gum resins harvested from Boswellia species have long been used in Ayurvedic and traditional Chinese medicine to treat a variety of health issues. In addition, frankincense has many pharmaceutical uses particularly in anti-inflammatory activity. In the present communication, we studied anti-tumor activity of frankincense essential oil hydrodistilled from Boswellia sacra gum resins against a panel of human pancreatic cancer cell lines. We demonstrated that frankincense essential oil is highly effective in suppressing proliferation and inducing cytoxocity of various pancreatic cancer cell lines; and these in vitro activities correlate with the repression of cell cycle regulators and activation of the caspase pathway. Consistent with the in vitro activities, frankincense essential oil was effective in reversing tumor growth, suppressing tumor cell growth, and inducing tumor cell death in a heterotopic xenograft human pancreatic cancer mouse model.
The abundance of high molecular weight compounds is positively correlated with distillation time and temperature, based on GC-MS and HPLC analyses. Longer duration and higher temperature distillations produce greater quantities of high molecular weight compounds (sesquiterpenes and boswellic acids). The presence of high molecular weight compounds might be positively correlated with frankincense essential oil-induced cytotoxity in human pancreatic cancer cells. First, Fraction IV is more potent than Fraction I, II, or III essential oil in suppressing tumor cell growth and inducing tumor cell death. Second, differential fractionations of Fraction IV essential oil enriched with high molecular weight compounds, when alpha-pinene contents were lowered to 5-10%, were significantly more potent than Fraction IV essential oil in inducing tumor cell death (data not shown). Results from cell viability and cell cytotoxicity assays suggest that high molecular weight compounds and/or ratios of these compounds present in Fractions III and IV essential oils play a significant role in anti-tumor activity. However, due the complexity of chemical constituents in essential oil, the active compound(s) responsible for anti-tumor activity has not been identified. Identification of active component(s) in frankincense essential oil and its activated molecular pathways in inducing tumor cell death will be a subject of future studies.
Boswellic acids have been suggested as a major component in frankincense for the observed anti-tumor activities. For example, ethanol extracts of Boswellia serrata gum resins contain a defined amount of boswellic acids, and have cytotoxic and pro-apoptotic activities against leukemia cell lines (HL-60, K 562, U937, MOLT-4, and THP-1) and brain tumor LN-18 and LN-229 cells
. Purified boswellic acids exhibit potent cytotoxic activities against cultured human neuroblastoma cell lines (IMR-32, NB-39, and SK-N-SH)
, and inhibit DNA, RNA, and protein synthesis in human leukemia HL-60 cells
. Furthermore, boswellic acids including acetyl-11-keto-β-boswellic acid (AKBA) have been shown to possess anti-tumor activity against a variety of human cancer cell lines including meningioma cells
, leukemia cells
[27, 29], hepatoma cells
, melanoma cells, fibrosarcoma cells
, colon cancer cells
[32, 33], prostate cancer cells
[34, 35], and pancreatic cancer cells
[36, 37] in both in vitro and in vivo conditions.
However, boswellic acids may not be the only compounds in frankincense essential oil for inducing pancreatic cancer cell death. Total boswellic acids contents were not proportionally related to essential oil-induced tumor cell cytotoxicity among different fractions by comparing Tables
3. Additionally, frankincense hydrosol, the aqueous distillate of hydrodistilled Boswellia sacra gum resins, contained 0.0 to 15.5% boswellic acids, but did not have detectible cytoxicity against tumor cells even when a 1:5 dilution was added to the cultures (data not shown). Finally, tirucallic acids purified from Boswellia carteri gum resins have been shown to induce human prostate cancer cells death
. We also observed that frankincense essential oil enriched with high molecular weight compounds but lower boswellic acids contents compared to Fraction IV essential oil was much more potent at inducing cytotoxicity in cultured pancreatic cancer cells (data not shown). With the complexity and mixture of chemical compounds in frankincense essential oil, our results are in agreement with other reports that crude extracts of frankincense are more potent than boswellic acids alone in inducing cytotoxicity in malignant cells
. The higher potency of total extracts, not just boswellic acids, may result from a combination of multiple active compounds.
Frankincense essential oil-regulated cell cycle regulators and signaling pathways were compared to boswellia acids-activated pathways in a variety of cancer cell lines. It has been reported that boswellic acids can regulate tumor cell viability by activating a variety of mechanisms. AKBA arrests cancer cells at the G1 phase of cell cycle, suppresses levels of cyclin D1 and E, cdk 2 and 4, and Rb phosphorylation, as well as increases expression of p21 through a p53-independent pathway
[33, 39]. AKBA activates death receptor-5 through elevated expression of CATT/enhancer binding protein homologus protein in human prostate cancer LNCaP and PC-3 cells
. Boswellic acids including AKBA strongly induce apoptosis through activation of caspase-3, -8, and −9 and cleavages of PARP in colon cancer HT29 cells and hepatoma HepG2 cells
[30, 32]. In addition, AKBA inhibits topoisomerases I and II without inhibiting DNA fragmentation in glioma and leukemia HL-60 cells
[40, 41]. Our results demonstrated that frankincense essential oil suppresses cyclin D1 and cdk4 proteins expression in pancreatic cancer cells. Cyclins function as regulators of CDK kinases; cyclin D1 forms a complex with and functions as a regulatory subunit of cdk4, this activity is required for G1/S transition in cell cycle
[42, 43]. Frankincense essential oil suppressed cyclin D1 and cdk4 expression may lead to suppressed Rb phosphorylation which results in suppressed cell cycle progression in pancreatic cancer cells
. Consistent with results from boswellic acids-treated HT29 and HepG2 cells, essential oil-induced apoptosis in pancreatic cancer cells is caspase-dependent based on the cleavages and activation of caspase-3, -8, -9, and PARP in these cells.
Boswellic acids have also been shown to activate multiple signaling pathways in human cancer cells. Boswellic acids and AKBA activate the PI3K/Akt pathway in colon cancer HT29, HCT-116, SW480, and LS174T cells
. Although AKBA has been reported to rapidly and potently inhibit the phosphorylation of Erk1/2 in primary cultures of meningioma cells
, other studies show that boswellic acids and AKBA activate Erk1/2 in human polymorphonuclear leukocytes and platelets
[46, 47]. Our results are in line with these reports that frankincense essential oil increases levels of Akt phosphorylation at Ser(473) and enhances Erk1/2 activation in most or all four pancreatic cancer cell lines. Activation of Akt and Erk1/2 signaling molecules in cancer cells by anti-cancer compounds with pro-apoptotic activity have been reported
[48, 49]. Biological activities and significances of the transient activation of PI3K/Akt and Erk1/2 pathways by frankincense essential oil in inducing pancreatic tumor cell death require further studies.
Boswellic acids can activate additional pathways in cancer cells. For example, boswellic acids can inhibit nuclear factor-κB and STATs activities in tumor cells
[36, 50]. In addition, Park et al. reported that AKBA down-regulates the expression of COX-2, MMP-9, CXCR4, and VEGF
. Pathways that are activated by a mixture of chemical components in frankincense essential oil are expected to be more complicate than the results presented in this communication. We reported that frankincense essential oil simultaneously modulates the activation of multiple signaling pathways and expression of multiple genes related to negative regulation of cell proliferation and cell cycle progression, as well as positive regulation of apoptosis in human bladder cancer J82 cells
Both heterotopic and orthotopic xenograft human cancer mouse models have been used to study anti-tumor activity of boswellic acids in vivo. Administration of boswellic acids have been shown to significantly suppress the progression of brain tumor
, prostate cancer
[34, 39], and pancreatic cancer
. In our heterotopic xenograft pancreatic cancer mouse model, wide variations of tumor growth were observed before the initiation of treatment in both groups receiving PBS (97–174 mm3) and frankincense essential oil (96–197 mm3). The wide variation of tumor growth provides an explanation why tumor weight is not statistically significant between these two groups. In addition, the duration of the present study is shorter than other reports using boswellic acids or AKBA in xenograft animal models
[34, 36, 39]; a more frequent and longer treatment regime might further enhance the differences between the control and experimental groups. In agreement with frankincense essential oil-induced DNA fragmentation in cultured pancreatic cancer cells, there are elevated numbers of TUNEL-positive tumor cells in the essential oil-treated group. Since apoptosis is observed within hours following Boswellia sacra Fraction IV essential oil treatment in cultured cancer cells, frankincense essential oil-activated anti-proliferative and pro-apoptotic activities may be more prominent if tumors were studied shortly after essential oil administration.
Frankincense has been used and investigated as a cancer therapeutic agent in clinical settings. For examples, frankincense gum resins are used as a main component in an anti-cancer drug in traditional Chinese medicine. In addition, frankincense is considered as an alternative medicine in Arab countries and has been used as an anti-cancer agent to treat neoplastic diseases. In a prospective, randomized, placebo-controlled, double-blind clinical trial conducted in patients irradiated for brain tumors, an extract prepared from Boswellia serrata gum resins (H15) significantly reduces cerebral edema with anti-tumor properties, without severe adverse effects
. Boswellia serrata extracts is included as an adjuvant agent in patients with high grade gliomas under an ongoing phase II clinical trial (
We expect that frankincense essential oil obtained from hydrodistillation of Boswellia sacra gum resins can be a novel and alternative therapeutic agent to suppress pancreatic cancer progression and metastasis. Nevertheless, a standardized procedures to process Boswellia species gum resins and to prepare essential oil are required for providing consistent anti-tumor activity. Although the identification of active compound(s) responsible for essential oil’s anti-cancer activity would be important, it may not be necessary if a standard assessment of chemical compositions and predicted biological functions can be established. Although no serious safety and toxicity issues have been raised in animal models and patients receiving oral administration of frankincense extracts
[51–55], the maximum safe dose of essential oil needs to be defined for cancer therapy, and pharmacokinetics and pharmacodynamic properties of essential oil need to be determined.