Fig. 2. (A) Schematic outline of the double paracrine regulation of keratinocyte growth and differentiation by fibroblast interactions (figure modified from Szabowski et al., 2000). (B) Reduced IL-1 expression in HaCaT cultures. Expression levels of IL-1 and IL-1ß in monocultures of postmitotic (irradiated) fibroblasts (HDFi), keratinocytes (NEK) and HaCaT cells as well as in 2D co-cultures of NEK and HaCaT cells, both with HDFi, and in epithelia of organotypic cultures of NEK and HaCaT cells (OTC). Steady-state mRNA levels were determined by semiquantitative RT-PCR using GAPDH expression as internal standard. The RNA expression profile is representative of duplicate determinations of three independent experiments. (C) Lack of IL-1 production in HaCaT cells. Concentrations of IL-1 in supernatants of 3-, 5- and 7-day-old co-cultures with irradiated fibroblasts of NEK and HaCaT cells were determined in aliquots of 2-day-conditioned media by ELISA and calculated as pg/10⁵ cells. Bars represent means ± s.d. of duplicate measurements performed in three independent experiments.