Activation of ribosomal protein S6 kinase in psoriatic lesions and cultured human keratinocytes by epidermal growth factor receptor ligands

p70 Ribosomal protein 56 kinase is a critical downstream effector of a mitogen-stimulated signaling pathway that is selectively inhibited by the immunosuppressant rapamycin. The purpose of this study was to quantify S6 kinase expression in psoriatic involved, uninvolved, and normal epidermis and to characterize regulation of S6 kinase activity in cultured normal human keratinocytes. S6 kinase activity was increased 4-fold in psoriatic lesions (1.63 ± 0.25 pmol per min per mg, n = 6), compared to nonlesional (0.44 ± 0.12 pmol per min per mg, n = 6, p < 0.01), and normal (0.35 ± 0.14 pmol per min per mg, n = 7, p < 0.01) epidermis. In contrast, S6 kinase mRNA and protein levels were not significantly different among psoriatic lesional, nonlesional, and normal epidermis. In keratinocytes, S6 kinase activity was stimulated 3-fold by mitogenic epidermal growth factor (EGF) receptor ligands, EGF and transforming growth factor-α (TGF-α), but not by cytokines interleukin-1α, tumor necrosis factor-α, interferon-γ, or transforming growth factor-β1. TGF-α stimulation of S6 kinase activity was inhibited in a concentration-dependent manner by rapamycin (IC₅₀ < 0.2 nM) and the specific EGF receptor antagonist PD153035 (IC₅₀ = 20 nM). Rapamycin also inhibited EGF-stimulated proliferation of keratinocytes (IC₅₀ = 0.2 ng per ml) with a potency similar to that reported for inhibition of T-cell proliferation. We conclude: (i) the mitogenic signaling pathway(s) regulating S6 kinase is activated in psoriatic lesions, thus accounting for increased S6 kinase activity in the absence of increased S6 kinase gene or protein expression; (ii) S6 kinase activation in lesional keratinocytes likely occurs in response to EGF receptor stimulation by TGF-α and/or amphiregulin, which are known to be elevated in psoriatic lesions; and (iii) keratinocyte as well as T-cell mitogenic signaling pathways are susceptible to inhibition by rapamycin, suggesting that rapamycin may be of therapeutic benefit in the treatment of psoriasis.