Fig. 4. Influence of calcium-induced differentiation and UV irradiation on SPRR expression in vitro. Normal human keratinocytes (NHK) were cultured in complete medium to confluency and subsequently incubated with calcium-free medium to remove differentiated cells. Cells were either irradiated with 500 J/m2 UV-A, 300 J/m2 UV-B or 30 J/m2 UV-C, or mock irradiated, and maintained for 24 or 48 hours in medium containing 1.8 mM Ca2+. Total RNA was isolated from the different cultures and hybridized with either SPRR class specific, involucrin, or 28S ribosomal RNA probes.