IL-1Ra mRNA isoforms. Total RNA from HepG2 cells and human primary hepatocytes stimulated with IL-1β and IL-4, from human keratinocyte cell lines A431 and KB, and from the U937 human monocytic cell line cultured with PMA and LPS were examined by RNase protection assay using a riboprobe that recognized the different IL-1Ra mRNA isoforms. Ten micrograms of total RNA were hybridized simultaneously with ³²P-labeled riboprobes complementary to IL-1Ra and GAPDH mRNA, then digested with RNase A and RNase T1 (see Materials and Methods). The protected fragments were analyzed in a 6% denaturing polyacrylamide gel. Lane 1, undigested probes; lane 2, U937 cells; lane 3, A431 cells; lane 4, KB cells; lane 5, HepG2 cells; lane 6, human primary hepatocytes.