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A brief listing of these cell types, relationships and the quantitative features of dose responses is presented in Table 1.
Skin is a complex biological model that is nonetheless highly approachable. Several methods exist to study it, including animal skin and human skin, both in vitro and in vivo models; regional variation models; and stem cell and hair follicle biological models. Through such models, pharmaceutical preparations in dermatology have been found to affect cell regulation. However, the US Food and Drug Administration (FDA) usually exempts dose justifications for dermatologic preparations. With oral and parenteral dosing, dose justification is generally done during phase II. With topicals, a relatively arbitrary percentage is often dosed—without the benefit of dose justifications. As a result, the presence of any hormetic effects could be missed.
Dermatology literature indicates that several cell types in the skin provide evidence of hormetic-like biphasic dose/concentration-response relationships. A brief listing of these cell types, relationships and the quantitative features of dose responses is presented in Table 1. This review examines hormetic effects of various agents on skin biology. Recognition of this emerging biological phenomenon in dermatology could lead to markedly improved integrative assessments of animal/human skin responses to toxic substances and pharmacological agents, as well as endogenous agonists.
Mechanisms of Hormesis
In toxicology, hormesis is a dose-response phenomenon characterized by a low-dose stimulation effect, followed by a high-dose inhibition effect. One cosmetic example of this is demonstrated by the effect of the surfactant sodium lauryl sulfate on keratinocytes in vitro: dose response studies demonstrate that low levels increase or stimulate cell replication, while higher levels decrease or inhibit it. 1 This biphasic dose response has been noted in a wide range of biological model systems ranging from immunology to cancer biology.6, 7
Calabrese6, 7 has been the primary advocate in bringing this interesting and not uncommon phenomenon to the attention of the scientific community. As noted by Calabrese, the quantitative features of the hermetic-like biphasic dose response are remarkably similar with respect to the amplitude of the stimulatory response, the width of the stimulation, and the relationship of the maximum stimulatory response to the zero equivalent point (ZEP), i.e., the threshold. Typically, the low-dose hormetic biphasic dose response stimulation is modest, with a maximum stimulation from 30% to 60% greater than controls, and produces a similar appearance in different cell types with various chemicals.7
Most stimulatory ranges were less than 100-fold, having averages of 10- to 20-fold, measuring back from the ZEP. These low-dose stimulatory responses often occur following an initial disruption in homeostatis and appear to represent a modest overcompensation response.