Role of INK4a/Arf locus-encoded senescent checkpoints activated in normal and psoriatic keratinocytes

During malignant transformation in skin, epidermal keratinocytes (KCs) frequently acquire the capacity to by-pass cellular senescence, a response that normally limits their unrestricted proliferation. Despite growing interest in the role for senescence during aging of skin and cutaneous carcinogenesis, little is known regarding regulation of three proteins encoded by the INK4a/ARF locus (p12, p14ARF, p16) in KCs. In this study, several molecular pathways are explored using cultured KCs and KCs freshly isolated from psoriatic plaques. p16 and p14ARF are predominantly expressed spontaneously when foreskin-derived early-passage KCs undergo confluency-induced premature senescence. Induction of p14ARF on confluency occurred with low E2F-1 levels. Suspension of KCs in methylcellulose induced p12 expression. Addition of various cytokines (interferon-γ, tumor necrosis factor-α) or a phorbol ester [12-O-tetradecanoylphorbol-13-acetate (TPA)] only induced p16, but not p14ARF. Confluent KCs up-regulated Ras activity and the downstream signaling involving ERK. Addition of MAPK inhibitor blocked cytokine and TPA-induced p16 expression. Confluency and interferon-γ induced premature senescence and p16 expression was linked to induction of the transcription factor Egr-1. KCs derived from chronic psoriatic plaques were characterized by enhanced p16, p14ARF, and p12 expression accompanied by elevated Egr-1 levels. These results demonstrate that multiple and highly divergent stimuli can trigger the senescent checkpoint in human KCs with differential regulation of p16, p14ARF, and p12. Although abnormal mitogenic signaling by oncogenic Ras is generally cited as being responsible for induction of premature senescence, our findings indicate that a broader perspective is warranted, to include confluency and cytokine-/TPA-induced pathways for KCs.