Although widely used for millennia in China, until recently CHM has attracted minimal attention outside China as potential therapy for skin diseases. CHM are given systemically to patients with psoriasis, based upon the type and extent of the patient's condition. According to traditional Chinese medical theory, psoriasis is generally classified into four types, i.e. blood heat, blood dryness, blood stasis and blood toxicity. Tuhuai is a mixture of CHM, which has shown evidence of clinical effectiveness when used systemically in the treatment of psoriasis (42
). Most Chinese herbals used in the treatment of psoriasis must be heated for more than 1 h to extract the active ingredients prior to use, which is inconvenient for most patients, and could also inactivate some ingredients. Moreover, these herbals often leave an unpleasant taste after oral use. Furthermore, ready-to-eat herbal preparations are convenient, but usually large amounts of sugar are added to make the preparation more edible, which is not acceptable for many patients with diabetes. Thus, further development of topical CHM would be a highly desirable alternative for the treatment of psoriasis.
The pharmacological mechanisms by which CHM influence inflammatory dermatoses, including psoriasis, are not clear. It has been known that these medicinal herbs contain many ingredients, some of which are known to be active (see below). In addition, chemical reactions may occur to produce still-unknown, new chemical entities during preparation or in complex mixtures. Nevertheless, Tse et al. studied the effects of 60 CHM, previously shown to display variable effectiveness in the treatment of psoriasis, on cell proliferation in HaCaT keratinocyte cell culture (44
). Significant inhibition of cell proliferation has been observed with some of ingredients in Tuhuai mixture, such as glycyrrhizin (45
). In agreement with these in vitro
finding, we demonstrated here that topical Tuhuai also inhibits epidermal proliferation in an in vivo
model of hyperproliferative epidermis. Moreover, scutellarin, flavonoids and paeonol, which are chemical compounds found in the Tuihuai mixture, inhibit cell proliferation in cancer cell lines (47
). Yet the molecular mechanism by which Tuhuai inhibits keratinocyte proliferation is not known. Furthermore, the decreased epidermal thickness induced by topical Tuhuai extract is likely due to the inhibition of keratinocyte proliferation, as keratinocyte apoptosis did not change in either normal or hyperproliferative epidermis following topical Tuhuai treatment even though some anti-psoriatic agents induce keratinocyte apoptosis (51
). As these active chemicals are ethanol extractable, it can be assumed that they were in the Tuhuai extract used in the present study.
Cutaneous inflammation is another prominent pathological feature of certain dermatoses, including psoriasis. Accordingly, immunomodulators and steroids often are used in the treatment of psoriasis. Previous clinical observations demonstrated that systematic administration of CHM decreased cutaneous inflammation in psoriasis and eczema (53
). Topical applications of either glycyrrhiza containing gel or Atopiclair®, a US FDA approved glycyrrhetinic acid-containing cream for atopic dermatitis treatment, improve atopic dermatitis (55
). Moreover, CHM inhibit the production of cytokines from stimulated human monocytes (58
). Smilax glabra roxb and astilbin (a chemical from smilax glabra roxb) inhibit inflammation by decreasing TNF alpha expression, lymphocyte migration, as well as matrix metalloproteinase (MMP-2, MMP-9) activities (59
). Both blood IgE and IL-4 levels decreased in atopic dermatitis patients treated with glycyrrhizin (65
). Here, we demonstrated that topical applications of Tuhuai, containing all of the above ingredients, inhibits cutaneous inflammation, as indicated by a reduction of inflammation in both irritant and acute allergic contact dermatitis models. Yet, again the molecular mechanism(s) whereby Tuhuai inhibits inflammation remain unknown. Nevertheless, while topical Tuhuai does not completely eliminate skin inflammation, it could be useful for human inflammatory dermatoses, including psoriasis.
The epidermal hyperproliferative model is easy to establish and provides a large skin area for experimental study. Previously, we have utlilized this model to successfully evaluate the anti-proliferative effects of topical PPAR and LXR activators (66
). Although this model is not an exact analogue of psoriasis, the epidermal changes, i.e. hyperplasia and abnormal barrier function mimic similar changes in psoriasis, making it a useful tool for assessing potential therapies for psoriasis. Likewise, the inflammatory models employed here are also easy to establish, and highly reproducible, and therefore useful models to assess anti-inflammatory therapies (38
). Coupling the epidermal hyperproliferative model with the inflammatory models provides a useful matrix for the testing and development of topical anti-inflammatory and proliferative medications.
In conclusion, a combination of the epidermal hyperproliferative model and the ear inflammation models provide a reliable and useful approach to assess the efficacy of anti-proliferative and inflammatory agents. In addition to its anti-inflammatory effect in both TPA and oxazolone models, topical Tuhuai extract also inhibits epidermal proliferation in the epidermal hyperproliferative model, but not in normal skin. These results explain, at least partially, the mechanism by which psoriasis reportedly is effectively treated with Tuhuai, and suggest that topical Tuhuai may be clinically useful in the treatment other inflammatory dermatoses as well. In addition to delineating the molecular mechanisms accounting for Tuhuai's activity, further clinical studies will also be required to evaluate the efficacy of topical Tuhuai on psoriasis and other inflammatory skin disorders.