Increased expression of rat ribosomal protein L4 mRNA in 5-azacytidine-treated PC12 cells prior to apoptosis.

5-Azacytidine (5AzC), a cytidine analogue, is thought to induce apoptosis in fetal neuronal cells and PC12 cells through DNA hypomethylation. However, apoptosis can be inhibited by adding protein synthesis inhibitors, indicating de novo protein synthesis may be partially responsible for apoptosis. Therefore, genes expressed just before apoptosis from 5AzC-treated PC12 cells were cloned. cDNA libraries were prepared from both 5AzC-treated and untreated PC12 cells and these libraries were subtracted. One clone overexpressed in 5AzC-treated PC12 cells was obtained, and was identified as the nearly full length (9 nt at 5' end and 1 nt at 3' end missing) rat ribosomal protein L4 (rpL4) gene. Time course study of Northern blot analysis in 5AzC-treated PC12 cells revealed that the peak of rat rpL4 gene expression preceded DNA fragmentation. COS-7 cells transfected with different amounts of cDNA from the subtracted clone expressed rat rpL4 dose-dependently. DNA fragmentation in the transfected COS-7 cells occurred proportional to the amount of the cDNA used for transfection. The present study indicates that rat rpL4 gene expression selectively increases in PC12 cells prior to 5AzC-induced apoptosis and that COS-7 cells transfected with and expressing the rat rpL4 gene also undergo apoptosis.