Sulforaphane mobilizes cellular defenses that protect skin against damage by UV radiation

Abstract

UV radiation (UVR) is a complete carcinogen that elicits a constellation of pathological events, including direct DNA damage, generation of reactive oxidants that peroxidize lipids and damage other cellular components, initiation of inflammation, and suppression of the immune response. Recent dramatic increases in the incidence of nonmelanoma skin cancers are largely attributable to higher exposure of an aging population to UVR. Therefore, the development of cellular strategies for intrinsic protection of the skin against the deleterious effects of UVR is imperative. Here we show that erythema resulting from UVR is a comprehensive and noninvasive biomarker for assessing UVR damage and can be precisely and easily quantified in human skin. Topical application of sulforaphane-rich extracts of 3-day-old broccoli sprouts up-regulated phase 2 enzymes in the mouse and human skin, protected against UVR-induced inflammation and edema in mice, and reduced susceptibility to erythema arising from narrow-band 311-nm UVR in humans. In six human subjects (three males and three females, 28-53 years of age), the mean reduction in erythema across six doses of UVR (300-800 mJ/cm(2) in 100 mJ/cm(2) increments) was 37.7% (range 8.37-78.1%; P = 0.025). This protection against a carcinogen in humans is catalytic and long lasting.

Conflict of interest statement

Conflict of interest statement: P.T. and J.W.F. are unpaid consultants to Brassica Protection Products, LLC (BPP), which licenses the technology to produce broccoli sprouts from The Johns Hopkins University. P.T., J.W.F., and The Johns Hopkins University are equity owners in BPP (whose chief executive officer is Antony Talalay, son of P.T.), and their stock is subject to certain restrictions under university policy. The terms of this arrangement are being managed by The Johns Hopkins University in accordance with its conflict of interest policies.

Figures

Fig. 1.

SF and SF-rich BSEs protect mouse skin against edema and inflammatory effects of 311-nm UVR. The backs of SKH-1 hairless mice were treated topically with three doses at 24-h intervals of: (i) BSE containing 0.5 μmol of SF in 50 μl of 80% acetone/20% water (vol/vol) applied to the caudal area, and (ii) solvent applied to the rostral area. The animals received 700 mJ/cm2 of 311-nm UVR 24 h after the last dose and were euthanized 24 h later, and their dorsal skin was harvested. (A) Fresh frozen 9-μm-thick sections of skin were fixed with paraformaldehyde and stained with H&E. (Scale bar: 100 μm.) (B and C) Mice were irradiated with a range of doses of UVR and euthanized 24 h later. MPO-specific activity (B) was measured in supernatant fractions of total homogenates prepared from liquid nitrogen-frozen and pulverized dorsal skin, and its protein levels (C) were detected by Western blots with anti-MPO antibody (Hycult Biotechnology, Uden, The Netherlands). Uniformity of protein levels was confirmed by Coomassie blue staining of a parallel gel (data not shown). (D and E) MPO-specific (D) and NQO1-specific (E) activities were measured in supernatant fractions of total skin homogenates from mice treated with solvent (black bars), SF (gray bars), and BSE (white bars) and are expressed as ratios of each treatment to the nonirradiated control. Average values ± SD are shown. Eight animals were used in the control group, and four in each of the treatment groups. Treatment with either SF or BSE led to equivalent protection against the UVR-induced MPO elevation (SF, P = 0.005; BSE, P = 0.001), and restoration of the NQO1 levels depressed by UVR (SF, P = 0.003; BSE, P = 0.00001).

Fig. 2.

Intensity of erythema depends linearly on the dose of UV radiation. (A) Adhesive vinyl templates placed on the back of the chest in the paraspinal regions. The apertures are 2.0-cm diameter and can be individually occluded to allow delivery of a range of UVR doses. The positions of the small holes at the four corners of each template are marked with a skin marker to locate the templates precisely in the same positions on successive days. (B) Intensity of erythema as a function of UVR dose. The erythema values (a*) were measured on 2.0-cm-diameter circles on the back of a male subject immediately before and 24 h after exposure to 100–800 mJ/cm2 of 311-nm UVR. Two pairs of adjacent spots were assigned to each UV dose. The mean changes in a* values after radiation are shown (filled circles), together with bars indicating the range of the duplicate values. The mean a* value for all 16 spots before radiation was 6.22 ± 1.91 (CV = 30.7%). The linear correlation coefficient (r2) of the increment of a* values with respect to UV dose is 0.986.

Fig. 3.

SF-rich BSEs protect human skin against erythema caused by 311-nm UVR. (A) Inhibition of skin erythema development by topical treatment of a male volunteer with a range of SF doses. The circular 2.0-cm-diameter spots received 100, 200, 400, or 600 nmol SF as BSE in 25 μl of 80% acetone/20% water on 3 days at 24-h intervals. Control spots received 25 μl of solvent only. Chromometer measurements of a* were obtained 4 days before radiation with 500 mJ/cm2 of UVR and 24 h after radiation. The 4-day mean a* value for the solvent-treated areas before radiation was 6.70 ± 1.16. Inhibition of erythema formation (%) was calculated from [a* (untreated) − a* (treated)/a* (untreated)] × 100. The untreated values (zero dose) were calculated from the increment of two areas that received 25 μl of BSE in 80% acetone/20% water containing 400 nmol of unhydrolyzed glucoraphanin (the inactive glucosinolate precursor of SF). (B) Photograph of four pairs of spots of individuals (described in A) who received 100, 200, 400, or 600 nmol doses of SF (as BSE) or solvent only. (C) Effect of topical treatment with SF-containing BSE on erythema response to a range of doses of UVR. With the use of 16-window template, horizontally adjacent pairs of spots were treated with either 200 nmol of SF in 25 μl of 80% acetone/20% water or solvent alone on 3 successive days at 24-h intervals and 24 h later were radiated with 100–800 mJ/cm2 of UVR. The increments in a* values for each spot after UVR with respect to their 4-day means before UVR are plotted as a function of UV dose. The visually determined minimum erythema dose was 600 mJ/cm2. (D) Photographs of pairs of BSE- and solvent-treated spots that received 500, 600, or 700 mJ/cm2 of UVR. The complete set of percent reduction values for this subject are shown in Table 1 (subject 2).