PCR-based subtractive hybridization identifies repressed genes in growth-arrested human dermal fibroblasts following combined treatment with 8-methoxypsoralen and UVA irradiation (PUVA)

The gene expression profile of psoralen plus UVA-induced premature senescence in skin fibroblasts resembles a combined DNA-damage and stress-induced cellular senescence response phenotype

After a finite number of population doublings, normal human cells undergo replicative senescence accompanied by growth arrest. We previously described a model of stress-induced premature senescence by treatment of dermal fibroblasts with psoralen plus UVA, a common photodermatological therapy. Psoralen photoactivation has long been used as a therapy for hyperproliferative skin disorders. The repetitive therapeutical treatment is accompanied by premature aging of the skin. Treatment of fibroblasts in vitro with 8-methoxypsoralen (8-MOP) and subsequent ultraviolet A (UVA) irradiation results in growth arrest with morphological and functional changes reminiscent of replicative senescence. For gene expression profiling in two strains of human skin fibroblasts after PUVA treatment, we used a low-density DNA array representing 240 genes involved in senescence and stress response. Twenty-nine genes were differentially expressed after PUVA treatment in the two strains of human skin fibroblasts. These genes are involved in growth arrest, stress response, modification of the extracellular matrix and senescence. This study contributes further to the elucidation of the PUVA model and its validation as a useful stress-induced premature senescence model aiming to characterize the premature senescence of fibroblasts and to identify biomarkers that could be applied in vivo.

Long-term growth arrest of PUVA-treated fibroblasts in G2/M in the absence of p16INK4a, p21CIP1or p53

Premature aging of the skin is a prominent side-effect of psoralen photoactivation, a therapy used for different skin disorders. Recently, we demonstrated that treatment of fibroblasts with 8-methoxypsoralen and ultraviolet A irradiation resulted in growth arrest with morphological and functional changes reminiscent of replicative senescence. To further elucidate the underlying molecular mechanisms, we analysed the cell-cycle phases of the growth-arrested fibroblasts. After PUVA treatment, fibroblasts arrested in G2/M, in contrast to spontaneously senesced fibroblasts arresting in a cell-cycle phase with many features similar to G1. To address the role of the cell-cycle controlling genes p16(INK4a), p21(CIP1) and p53, we analysed the expression of these genes. p16(INK4a), p21(CIP1) and p53 protein levels increased substantially with different time kinetics in growth-arrested fibroblasts. Because p16(INK4a), p21(CIP1) and p53 are involved in replicative senescence, we applied the PUVA regimen to fibroblasts deficient in either of these genes. p16(INK4a), p21(CIP1) and p53 null mutant fibroblast strains underwent growth arrest with a senescent phenotype similar to wild-type human fibroblasts. Based on these results, we propose that redundant or alternate pathways are involved in the response of dermal fibroblasts to PUVA treatment resulting in a phenocopy of replicative senescence in vitro.

Psoralen plus UVA (PUVA) induced premature senescence as a model for stress-induced premature senescence

Following psoralen photoactivation (PUVA treatment) human dermal fibroblasts undergo long-term growth arrest as well as morphological and functional changes reminiscent of replicative senescence. Although the molecular description of cellular senescence is still incomplete, replicative senescence of cultured human cells has been suggested to reflect cellular aging in vitro. Recently, the term stress-induced premature senescence (SIPS) was introduced to define in vitro models with longterm growth arrest upon exposure to sublethal stressors (i.e. hyperoxia, hydrogen peroxide, ethanol), which are characterized by morphological and functional changes common for replicative senescence. This mini review focuses on the morphological and functional changes in the fibroblast phenotype following exposure to psoralen plus UVA (PUVA) leading to SIPS and the role of reactive oxygen species in the switch from the proliferative to the post mitotic cell. Additionally, we will discuss the possible in vivo relevance of PUVA-SIPS fibroblasts in PUVA-treated patients.