Differentiation-inducing effect of thymidine on human neuroblastoma cells.

Human neuroblastoma cells (KP-N-RT(BMI) treated with thymidine underwent morphological differentiation, as revealed by the extension of neurites. The morphological differentiation was caused by deoxyadenosine as well, but not by thymine or deoxyribose. The neurite-extending effect of thymidine was counteracted by deoxycytidine, indicating that inhibition of ribonucleotide reductase was involved. Similar morphological change was indeed brought about by hydroxyurea, a specific inhibitor of the enzyme. Azidothymidine and dideoxythymidine were also effective in induction of neurite extension, suggesting that inhibition of DNA replication, rather than the reductase per se, is responsible for the induction of neurite extension. Supporting this notion, various inhibitors of DNA synthesis induced the morphological differentiation of the cells. Although a-amanitin and cycloheximide were suppressive, actinomycin D promoted the thymidine-induced neurite extension. Morphological changes caused by thymidine were similar to those induced by cyclic AMP, rather than retinoic acid. Intracellular cAMP content was however not increased by the thymidine treatment.