Managing epidermal melanin unit signals

Indeed, homogeneous tone is an aesthetic criterion shared across the globe, and cosmetic products that lighten the complexion or reduce inhomogeneities in pigmentation are very strongly in demand. All pigmentation disorders, irrespective of their nature and origin, are related to abnormal accumulation of melanin pigments in the epidermis or dermis. Here, we focus our research on ephelides, lentigines1 and melasma2 which are epidermal pigment disorders accentuated by UV radiation and age, and inflammatory hyperpigmentation related to mechanical stress (scratching, shaving, depilation) or irritant stress (acne). The intensity and shade of the complexion are directly dependent on the basal melanin activity. Conventionally, a “melanic unit” is considered to consist of one melanocyte surrounded by about 40 keratinocytes.3 When UVAB radiation penetrates the epidermis, it first encounters keratinocytes on its trajectory, where the radiation activates, via oxidative stress, a cascade of events. The keratinocytes react and signal their change of state to neighbouring cells by secreting cytokines: endothelin-1 and IL-1??(UVB), IL-6 and GM-CSF.4,5 The cytokine activators enable the tissue to adjust its response and, in particular, to activate melanin synthesis (tanning by a paracrine reaction), to promote cell proliferation and differentiation (epidermal thickening by autocrine action, IL-1?, neo-melanocytes) and also to modulate vasoconstriction. In the skin, each melanocyte receives the paracrine signals from 40 keratinocytes. Thus, the cytokine signal is “amplified” and interpreted as a signal to stimulate production of melanin via activation of melanogenesis and stimulation of proliferation. Thus, the “melanic unit” concept is highly significant with regard to protection against the sun. Among the various cytokines, endothelin-1 gives the instruction to stimulate mitosis and melanogenesis in response to UVB radiation stress by a PKC-dependent pathway.4 Furthermore, cytokine GM-CSF (granulocyte macrophage-colony stimulating factor) is reported to give the instructions following both UVA irradiation and UVB irradiation. According to literature, GM-CSF is one of the rare cytokines to be secreted after both types of irradiation.4,5,6 GM-CSF binds to a specific receptor, GMCSFR,7 then activates transduction signals which contribute to melanocyte stimulation and, in particular, activate the tyrosinase proteins TRP1 and TRP2. We selected cytokine GM-CSF as the cosmetic target since it is induced just as well by both the UVA and UVB radiation and also because it is produced in response to cutaneous irritation (mechanical or acne8) and is frequently present at higher levels in the blood in women.9 We also made the reasonable hypothesis that localised variations in the secretion of GM-CSF by hyperactive keratinocytes locally forces melanocytes into over-activity, generating lentigines. It would thus appear opportune to inhibit GM-CSF production by keratinocytes in order to prevent stimulation of melanocytes. A specific extract of hop (Humulus lupulus), trademarked Wonderlight, was prepared and investigated for its ability to regulate melanogenesis by a tissue approach: control of the “instructing” keratinocytes (via GM-CSF) limits the activity of melanocytes.

Inhibition of GM-CSF secretion by keratinocytes

In vitro studies were conducted on normal human keratinocytes cultured in an appropriate medium (KSFM medium) until confluence. The cells were pretreated for 24 hours in the presence or absence of 0.5% Wonderlight (now referred to as ‘the hop extract’), then subjected to UVB radiation stress (100 mJ/cm2) or oxidising chemical stress with PMA (=0.2 ng/mL phorbol myristate acetate) for 24 hours, followed by 48 hours of incubation with the product. GM-CSF was assayed by a sandwich immunoenzymatic method. Dexamethasone was used as the positive control of inhibition of GM-CSF secretion. Standard curves showing the effect of PMA and UVB radiation on GM-CSF release were used to quantify the percentage of inhibition of GM-CSF secretion in the presence of the hop extract: an approximately threefold increase in GM-CSF for stress induced by 0.2 ng/mL PMA and a fourfold increase for stress induced by UVB radiation (100 mJ/cm2). As expected, the dexamethasone positive control reduced GM-CSF release by 57% post-PMA stress and by 45% post-UVB radiation stress. The hop extract was shown to be a good inhibitor of stress-induced GM-CSF released by normal human keratinocytes.