Interleukin-6 in psoriasis: expression and mitogenicity studies

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Abstract
InterIeukin-6 (IL-6) is a multifunctional cytokine which has been suggested to function as an autocrine mitogen in psoriatic epidermis. We report here the results of several experiments designed to further examine this hypothesis. Blot hybridization was unable to detect 1.3 kb IL-6 transcripts in RNA extracted from normal or psoriatic epidermal (keratome) biopsies, suggesting that IL-6 expression is very low in normal and psoriatic epidermis. Therefore, qualitative and semiquantitative PCR/Southern blot analyses were performed on keratome-derived RNA, and revealed variable but significantly increased IL-6 mRNA levels in lesional psoriatic relative to normal tissue. To further examine the ability of normal human keratinocytes (NHK) to express IL-6, RNA was extracted from rapidly proliferating secondary NHK cultures. IL-6 transcripts were nearly undetectable by blotting in keratinocytes grown in low-calcium serum-free medium, but low levels could be induced by treatment with 1.8 mM CaCl2. IL-6 transcripts were strongly superinduced after cycloheximide treatment, suggesting that a labile protein regulates IL-6 mRNA levels in these cells. Finally, the mitogenic activity of IL-6 was examined in NHK under varying conditions of cell density and added growth factors. IL-6 did not stimulate high density keratinocyte growth in the presence or absence of other growth factors, but did stimulate clonal growth in epidermal growth factor (EGF)-deficient media at high concentrations (≧10 ng/ml). The proliferative effects of IL-6, but not of basic fibroblast growth factor, were abrogated by monoclonal antibodies directed against the EGF receptor. Taken together, these results suggest that the proliferative effects of IL-6 are mediated indirectly via the EGF/TGF-α receptor, and that autocrine overexpression of IL-6 may be limited in psoriatic keratinocytes.