Terminal differentiation is a key element in the maintenance of tissue homeostasis in the epidermis. We show here that methotrexate (MTX) induces differentiation of human epidermal keratinocytes in vitro. MTX inhibits proliferation of keratinocytes and also induces several markers of differentiation: a change in cell morphology, a marked increase in cell size, an increase in the proportion of cells that express involucrin, and an increase in the amount of cornifled envelope protein. These effects of MTX are dose-and exposure-timedependent and become irreversible after 24 hr, approximately one population doubling time. These effects of MTX cannot be attributed to cytotoxicity since keratinocytes not only remain viable but also actively synthesize proteins. MTX causes reproducible changes in the SDS/PAGE profiles of newly synthesized proteins and, in particular, increases the amount of involucrin synthesis. Thymidine completely prevents these effects of MTX, suggesting that they are caused by a depletion of thymine deoxyribonucleotides. The effect of MTX on keratinocytes may provide a model for studying the relationship between deoxyribonucleotide metabolism and differentiation in normal cells. In addition, the ability of MTX to induce differentiation in keratinocytes suggests a mechanism to explain its therapeutic action in psoriasis.

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A DTM culture is a relatively rapid, easy, and inexpensive method to confirm dermatophyte infections in patients with signs of onychomycosis in the primary care setting. Because the available drugs for treating onychomycosis are effective against all dermatophyte species, the confirmation of dermatophyte infection, without further identification of genus and species, is sufficient evidence to begin treatment.

To examine the effects of chronic ultraviolet light on human epidermal cells, we grafted white human skin onto recombinase activating gene-1 knockout mice. We found previously that the maximal concentration of ultraviolet B radiation (290-320 nm) tolerated by human skin xenografts was 500 J per m2 when given three times weekly. One hundred and fifty-eight grafted mice were randomized and observed for a median of 10 mo in four groups: (i) no treatment; (ii) one treatment with the chemical carcinogen dimethyl-(a)benzanthracene; (iii) ultraviolet B three times weekly; and (iv) a combination of dimethyl-(a)benzanthracene and ultraviolet B. Approximately half of the skin specimens treated with ultraviolet B developed superficial milia and epidermal cysts. Grafts contained up to seven milia lesions between 4 and 8 mo after initiation of treatment, whereas the number of larger epidermal cysts was rarely more than two. Milia and cysts developed in the skin regardless of pigmentation or tanning. Actinic keratoses arose in 9% of grafts treated with ultraviolet B alone and in 19% of grafts treated with the combination of dimethyl-(a)benzanthracene and ultraviolet B. Invasive squamous cell carcinomas developed in 10% of grafts after combined dimethyl-(a)benzanthracene and ultraviolet B treatment and lesions were restricted to skin grafts that did not tan. These findings demonstrate that (i) development of ultraviolet-induced lesions can be experimentally accelerated in human skin, (ii) xenografted recombinase activating gene-1 deficient mice are superior to severe combined immunodeficiency disease mice for chronic ultraviolet B studies, and (iii) benign cystic tumors and squamous cell carcinomas are caused by ultraviolet B.