Euxanthone, a potent neuritogenic compound isolated from the roots of the medicinal herb Polygala caudata, has recently been shown to induce the differentiation of murine neuroblastoma Neuro 2A (BU-1) cells. In this study, the role of protein kinase C (PKC) and the expression of various PKC isoforms in euxanthone-treated BU-1 cells were examined. mRNA phenotyping using the reverse-transcription polymerase chain reaction (RT-PCR) showed that BU-1 cells express six different PKC isoforms, namely PKC-alpha, -beta, -delta, -epsilon, -lambda, and -zeta. Differential regulation and expression of PKC isoforms was observed in BU-1 cells treated with 100 microM euxanthone. PKC-apha, -beta, -delta, -lambda and -zeta were all up-regulated, with 1.7- to 9.5-fold increase, at around 30 to 60 minutes after euxanthone treatment. The expression level of PKC-epsilon remained relatively constant during the treatment. PKC-gamma, -eta, and -theta were not detected in both untreated and euxanthone-treated BU-1 cells. Staurosporine, a broad spectrum PKC inhibitor, was found to inhibit both spontaneous and euxanthone-induced neuritogenesis in BU-1 cells. A significant reduction of the euxanthone-induced neuritogenic effect was also observed when the PKC isoform-specific inhibitor Go6976 was included in the culture. These results suggest that the euxanthone-induced differentiation of the neuroblastoma BU-1 cells may be mediated through the differential expression of PKC-alpha, -beta, -delta, -lambda and -zeta isoforms.