Chemopreventive properties of South African herbal teas, rooibos (Aspalathus linearis) and honeybush (Cyclopia spp) : mechanisms against skin carcinogenesis

[摘要] ENGLISH ABSTRACT: The present study employed a two-phased approach to investigate the possible mechanisms involved in the chemopreventive properties of rooibos (Aspalathus linearis) and different honeybush species (Cyclopia spp.) in vitro. In the first phase, the effect of unfermented methanol and aqueous herbal tea extracts against the growth parameters (cell viability, proliferation and apoptosis) of normal (CRL 7761); premalignant (HaCaT); and malignant (CRL 7762) skin cells was evaluated and compared to green tea extracts. The predictive potential of polyphenol content (total polyphenol and flavanol/proanthocyanidins) and antioxidant properties (ABTS; ORAC; FRAP and LPO) in the biological activity of extracts in cells was also assessed. Of the herbal teas, the methanol extract of rooibos was the most active and it inhibited the growth of skin cells presumably by inducing mitochondrial dysfunction via membrane depolarisation. At lower concentrations, this activity was associated with inhibition of cell proliferation that was selective for cancer cells whilst higher concentrations induced apoptosis that was more prominent in premalignant cells. The strong antioxidant properties of the extracts implicated the role of pro-oxidative polyphenol/iron interactions involving monomeric flavonoids and polymeric proanthocyanidins in the cytotoxic effects of rooibos. The strong relationship between total polyphenolic and flavanol/proanthocyanidins content, antioxidant properties and reduction of cell viability indicated that these parameters (polyphenols and antioxidant properties) can serve as predictive tools for the cytotoxic effects of rooibos in vitro. The aqueous extracts of honeybush species, although weaker, displayed similar effects to rooibos extracts in cells with C. genistoides being the most effective at selectively inhibiting the proliferation of cancer cells whilst the pro-apoptotic activity of C. subternata and C. intermedia was more prominent in premalignant cells. The underlying mechanisms are also likely to result from pro-oxidative mechanisms resulting from polyphenol/iron interactions that mainly involve polymeric flavanol-like proanthocyanidin compounds in honeybush. In contrast, the methanol extracts exhibited weaker cytotoxic effects and protected cancer cells from going into apoptosis. The cytoprotective effects of honeybush species are possibly mediated by the major monomeric compounds such as mangiferin and hesperidin through antioxidant mechanisms that result in reduction of oxidative stress. Due to the possible dual role of the monomeric and polymeric compounds in the honeybush extracts, the total polyphenolic content of these herbal teas may not be a goodindicator of biological activity in vitro. However, as aqueous extracts displayed highflavanol/proanthocyanidins content and exceptional activity in the ABTS assay, theseparameters may be considered as indicators of cytotoxicity. On the other hand,methanol extracts, particularly from the xanthone-rich species (C. genistoides and C.longifolia) which exhibited the weakest cytotoxic effects, were more active in theORAC thus this assay may be a useful predictor for cytoprotective activity. In thesecond phase, an in vitro UVB/HaCaT model which used IL-1α as a biomarker forearly inflammation was developed and validated with known anti-inflammatorycompounds, dexamethasone and ibuprofen. It was used to determine the specificmechanisms involved in the modulatory effects of the herbal tea extracts againstinflammation. Rooibos extracts and the aqueous extract of honeybush enhanced thecytotoxic effects of UVB in the model and exhibited indirect anti-inflammatory effectsas they removed icIL-1α containing cells via apoptosis. In contrast, methanol extractsof honeybush exacerbated icIL-1α by protecting UVB stimulated cells fromundergoing apoptosis. In conclusion, methanol extract of rooibos and aqueousextracts of honeybush species may be useful in protecting the skin after UVBexposure. These herbal tea extracts may block initiation and delay the promotionstage during skin carcinogenesis by removing premalignant cells via apoptosis andpreventing onset of inflammation. In contrast, due to their cytoprotective effects,methanol extracts of honeybush may be more effective at preventing oxidative stressin skin before UVB exposure. Future studies should focus on the effects of extractsand polyphenolic fractions on the oxidative status of the cells and development ofbiomarkers of chemoprevention that can be utilised in vivo and in human skin.

[发布机构] Stellenbosch University