Edward C. De Fabo

Neonatal sunburn and melanoma in mice.

Retrospective epidemiological data have indicated that cutaneous malignant melanoma may arise as a consequence of intense, intermittent exposure of the skin to ultraviolet radiation, particularly in children, rather than from the cumulative lifetime exposure that is associated with other forms of skin cancer. Here we use a genetically engineered mouse model to show that a single dose of burning ultraviolet radiation to neonates, but not adults, is necessary and sufficient to induce tumours with high penetrance which are reminiscent of human melanoma. Our results provide experimental support for epidemiological evidence that childhood sunburn poses a significant risk of developing this potentially fatal disease.

Immunosuppression by ultraviolet B radiation: initiation by urocanic acid

Irradiation with UV-B, a component of natural sunlight, initiates systemic immunosuppression of delayed-type hypersensitivity responses. This may be a fundamental regulatory mechanism, controlling the interaction between mammals and potentially deleterious environmental UV radiation. Here, Frances Noonan and Edward De Fabo assess the evidence that suppression is initiated by the photoisomerization of trans-urocanic acid (UCA) in the stratum corneum, discuss the significance of this mechanism for skin cancer outgrowth and propose applications for UCA in transplantation.

Ultraviolet B but not Ultraviolet A Radiation Initiates Melanoma

Cutaneous malignant melanoma is one of the fastest increasing cancers with an incidence that has more than doubled in the last 25 years. Sunlight exposure is strongly implicated in the etiology of cutaneous malignant melanoma and the UV portion of the sunlight spectrum is considered responsible. Data are, however, conflicting on the roles of ultraviolet B [UVB; 280–320 nanometers (nm)] and ultraviolet A (UVA; 320–400 nm), which differ in their ability to initiate DNA damage, cell signaling pathways and immune alterations. To address this issue, we have used specialized optical sources, emitting isolated or combined UVB or UVA wavebands or solar simulating radiation, together with our hepatocyte growth factor/scatter factor-transgenic mouse model of UV-induced melanoma that uniquely recapitulates human disease. Only UVB-containing sources initiated melanoma. These were the isolated UVB waveband (>96% 280–320 nm), the unfiltered F40 sunlamp (250–800 nm) and the solar simulator (290–800 nm). Kaplan-Meier survival analysis indicated that the isolated UVB waveband was more effective in initiating melanoma than either the F40 sunlamp or the solar simulator (modified log rank P < 0.02). The latter two sources showed similar melanoma effectiveness (P = 0.38). In contrast, transgenic mice irradiated with either the isolated UVA waveband (>99.9% 320–400 nm, 150 kJ/m2), or an F40 sunlamp filtered to remove > 96% of the UVB, responded like unirradiated control animals. We conclude that, within the constraints of this animal model, UVB is responsible for the induction of mammalian cutaneous malignant melanoma whereas UVA is ineffective even at doses considered physiologically relevant. This finding may have major implications with respect both to risk assessment from exposure to solar and artificial UVB, and to development of effective protection strategies against melanoma induction by UVB. Moreover, these differences in wavelength effectiveness can now be exploited to identify UV pathways relevant to melanomagenesis.

Melanoma induction by ultraviolet A but not ultraviolet B radiation requires melanin pigment.

Malignant melanoma of the skin (CMM) is associated with ultraviolet radiation exposure, but the mechanisms and even the wavelengths responsible are unclear. Here we use a mammalian model to investigate melanoma formed in response to precise spectrally defined ultraviolet wavelengths and biologically relevant doses. We show that melanoma induction by ultraviolet A (320–400 nm) requires the presence of melanin pigment and is associated with oxidative DNA damage within melanocytes. In contrast, ultraviolet B radiation (280–320 nm) initiates melanoma in a pigment-independent manner associated with direct ultraviolet B DNA damage. Thus, we identified two ultraviolet wavelength-dependent pathways for the induction of CMM and describe an unexpected and significant role for melanin within the melanocyte in melanomagenesis.

BIOLOGICALLY EFFECTIVE DOSES OF SUNLIGHT FOR IMMUNE SUPPRESSION AT VARIOUS LATITUDES AND THEIR RELATIONSHIP TO CHANGES IN STRATOSPHERIC OZONE

Abstract— Using information on solar irradiance at different latitudes derived from a radiative transfer model and a detailed in vivo action spectrum for immune suppression in a murine system, we report here calculations of the “biologically effective” irradiance of sunlight for immune suppression. From 40°N to 40°S in summer, under normal stratospheric ozone concentrations this value ranged from 0.27 W/m2 (40°N or S) to a peak of 0.33 W/m2 (20°N or S) predicting that 50% immune suppression in the Balb/c mouse would occur after 21–26 min of sunlight exposure within this latitude range. We also found that the most effective wavelengths for immune suppression shift from a peak of 270 nm in the laboratory to near 315 nm in sunlight. Furthermore, using ozone depletion scenarios of 5 to 20%, at latitudes 20°S and 40°N, a 0.6% increase in biologically effective irradiance levels of solar UVB for immune suppression was predicted for each 1% decrease of ozone. This value rose to a nearly 1% increase for each 1% decrease in ozone at 60°N latitude in wintertime. These data indicate that activation of immune suppression, in a murine model, requires relatively low levels of sunlight and that these levels are easily obtainable over most of the populated regions of the world. Since a UVB‐activated photoreceptor, urocanic acid, regulates immune suppression in mice and since this same compound exists on other mammalian skin, including human skin, suppression of the mammalian immune system is predicted to increase if substantial stratospheric ozone depletion takes place.

Suppression of contact hypersensitivity by ultraviolet radiation: An experimental model

Studies on the immunologic factors involved in skin cancer induction in mice by ultraviolet (UV) radiation have suggested that several immunologic processes can be altered by UV exposure. Following the observation that many UV-induced murine skin cancers are rejected immunologically upon transplantation into normal syngeneic animals [12], studies were initiated to investigate how these highly antigenic squamous carcinomas and fibrosarcomas escaped immunologic destruction in the primary host [15]. These studies led to the discovery that UV irradiation of the shaved dorsum of mice produced systemic changes in the animals that interfered with the normal immunologic rejection of these skin cancers [13,15]. One aspect of this systemic change has been elucidated in recent experiments, which produced the rather startling finding that a subcarcinogenic exposure of mice to UV radiation resulted in the induction of suppressor T lymphocytes. These cells, present in spleens and lymph nodes of UV-irradiated mice, prevent the development of an immune response against UV-induced tumors [2, 5, 6, 24]. The specificity of this immunologic alteration in UV-irradiated mice was investigated by testing their immune responses to a variety of antigens [15–17,22,25]. Although many immune responses proceeded normally in UV-irradiated animals, a depressed response to the contact sensitizer, dinitrochlorobenzene (DNCB), was observed [16]. Studies on the mechanism of this systemic suppression of immunity to DNCB demonstrated that there was a second immunologic alteration in UV-irradiated mice and suggested that this alteration occurred at the level of the cells involved in antigen uptake, processing, and presentation, i.e., macrophages or Langerhans cells [11].

Cis‐UROCANIC ACID DOWN‐REGULATES THE INDUCTION OF ADENOSINE 3', 5'‐CYCLIC MONOPHOSPHATE BY EITHER trans‐UROCANIC ACID OR HISTAMINE IN HUMAN DERMAL FIBROBLASTS in vitro

Abstract— It has been demonstrated that UVB radiation (290–320 nm) suppresses mammalian cell‐mediated immunity by effecting the trans to cis isomerization of urocanic acid (UCA) in the stratum corneum, the uppermost layer of the skin. Trans‐urocanic acid has been shown to be the photoreceptor for UVB‐induced immune suppression and the cis‐isomer has been demonstrated to be immunosuppressive. Little is known, however, about how the isomerization of UCA may affect the proximal or distal cells of the skin or the immune system. We report here that trans‐UCA is biologically active in vitro in human dermal fibroblasts, inducing adenyl cyclase as measured by cAMP (adenosine 3, 5‐cyclic monophosphate) formation in a dose‐dependent manner similar to the action of histamine. Trans‐UCA and histamine stimulate 50% of maximum activity at concentrations of 3.3 μM end 13.8 μM respectively. Cis‐UCA does not increase cAMP in these human fibroblasts but actively down regulates the increase of cAMP induced by either histamine or trans‐UCA. Cis‐UCA down regulated the histamine response by 75% and the trans‐UCA response by 60% at a concentration range of 1 mM to 1 nM. The trans‐UCA induction of cAMP can also be downregulated with an H2 histamine receptor antagonist cimetidine. These results support the hypothesis that a cellular target for cis‐UCA is the dermal fibroblast and the effects reported here may represent the initial biochemical and cellular event for UVB‐induced immune suppression i.e. the immediate step following the isomerization of trans to cis‐UCA is the down regulation of cAMP by cis‐UCA. Regulation of such an important second messenger such as cAMP could then allow cascading signals to occur, leading to immune suppression.

Deficient inflammatory response to UV radiation in neonatal mice

Mechanisms of juvenile susceptibility to cancer are not well understood. The immune response in neonates favors nonresponsiveness or TH2‐dominant responses, raising the question of a role for neonatal immunity in this susceptibility. We have investigated the postulate that the inflammatory response differs in neonatal and adult skin. We found no inflammatory infiltrate into neonatal mouse skin in response to UV irradiation as a function of time, dose, or wavelength, although UV‐induced DNA damage was readily detected. In contrast, UV irradiation of adult mice initiated a dose‐ and time‐dependent influx of inflammatory cells, chiefly CD11b+Ly6G+ neutrophils, into the skin, detected by immunohistochemistry and quantitated by FACS analysis. This inflammatory response was initiated by UVB (290–320 nm) but not by UVA (320–400 nm). Further, in neonates, in contrast to adults, neither topical trinitrochlorobenzene (TNCB) nor i.p. thioglycollate initiated an inflammatory infiltrate. Conversely, topical TNCB applied to neonates was tolerogenic, resulting in a subsequent antigen‐specific decrease of the contact‐hypersensitivity response in adults. Neonatal blood contained abundant neutrophils, which exhibited impaired chemotaxis to the chemokine growth‐related oncogene‐α but efficient chemotaxis to the bacterial product fMLP, concomitant with decreased expression of CXCR2 but normal levels of CD11b. We propose this neonatal deficiency in the inflammatory response is a significant, previously unrecognized factor in neonatal immune tolerance and may contribute to neonatal susceptibility to cancer, including melanoma and other UV‐induced cancers.

Arctic stratospheric ozone depletion and increased UVB radiation: potential impacts to human health

Abstract Contrary to popular belief, stratospheric ozone depletion, and the resultant increase in solar UV-B (280–320 nm), are unlikely to fully recover soon. Notwithstanding the success of the Montreal Protocol in reducing the amount of ozone destroying chemicals into the stratosphere, the life-times of these compounds are such that even with full compliance with the Protocol by all countries, it will be decades before stratospheric ozone could return to pre-1980 levels. This raises the question, therefore, of what will happen to biological processes essential to the maintenance of life on earth which are sensitive to damage by increased UV-B radiation, particularly those involved with human health? The polar regions, because of the vagaries of climate and weather, are the bellwether for stratospheric ozone depletion and will, therefore, be the first to experience impacts due to increases in solar UV-B radiation. The impacts of these are incompletely understood and cannot be predicted with certainty. While some UV-B impacts on human health are recognized, much is unknown, unclear and uncertain. Thus, this paper attempts, as a first approximation, to point out potential impacts to the health and welfare of human inhabitants of the Arctic due to increased solar UV-B radiation associated with stratospheric ozone depletion. As will be seen, much more data is critically needed before adequate risk assessment can occur. (Int J Circumpolar Health 2005; 64(5):509-522)

Shedding Light on Melanocyte Pathobiology In Vivo

Cutaneous malignant melanoma is rapidly increasing in the developed world and continues to be a challenge in the clinic. Although extensive epidemiologic evidence points to solar UV as the major risk factor for melanoma, there is a significant gap in our knowledge about how this most ubiquitous environmental carcinogen interacts with the largest organ of the mammalian body (skin) at the microenvironmental and molecular level. We review some recent advances that have started to close this gap.