We examined the signal transduction mechanism responsible for the IFN-gamma-induced HLA class II molecule expressions on glioblastoma cell lines, T98G and A172. A series of experiments demonstrated that the activation of protein kinase C (PKC) is involved in the DR and DP molecule expressions on T98G cells. In addition to the activation of PKC, calcium influx appeared to be involved in the DR and DP molecule expressions on T98G. Northern blot analyses with actinomycin D or cycloheximide revealed that these second messengers induce the transcription of DRA and B and DPA and B genes without de novo protein synthesis. Furthermore, we examined the region of the DPB gene that is responsible for IFN-gamma-induced gene transcription by gene transfer of a series of 5' and 3' deletion mutants in which the upstream region of the DPB was linked to a reporter gene, chloramphenicol acetyltransferase. By using these deletion mutants, it appeared that the region between -152 and -126 bp contains a critical IFN-gamma-responsive element. Taken together, these results suggest that IFN-gamma activates PKC and stimulates calcium influx, resulting in the induction of transcription of DRA and B and DPA and B genes without de novo protein synthesis. In DPB gene, we speculate that preexiting protein(s) phosphorylated by PKC in the presence of Ca2+ might directly bind or indirectly interact with the region between -152 and -126 bp of the upstream sequence, leading to the induction of the transcription (possibly in concert with other nuclear protein(s) bound to the promoter sequences).