Poly(ADP-ribosyl)ation, synthesized mainly by poly(ADP-ribose) polymerase-1 (PARP-1) using nicotinamide adenine dinucleotide as a substrate, has been identified as a post-translational modification of nuclear proteins in many eukaryotic cells. At present, studies on poly(ADP-ribosyl)ation have focused primarily on its roles in DNA repair, replication, transcription, and cell death. It has been suggested that poly(ADP-ribosyl)ation activity occurs in organelles, including mitochondria, but its role is not fully understood. On the other hand, it has been reported that PARP-1 and poly(ADP-ribosyl)ation play an important role in the differentiation of many cell types. Recently, we have demonstrated that niacin and its related compounds induced granulocytic differentiation in human promyelocytic leukemia HL-60 cells. In this study, we investigated changes in poly(ADP-ribosyl)ation levels during granulocytic differentiation by niacin, its related compounds and well-known granulocytic inducers. Results indicated no significant changes in expression levels and cellular localization of PARP-1 protein was observed in HL-60 cells during the differentiation. However, poly(ADP-ribosyl)ation was detected in cells treated with nicotinamide (NA) compared to the controls. Intriguingly, this phenomenon was detected in the mitochondrial fractions, but not in the nucleus. In addition, nicotinic acid, which is equivalent to NA induce neither differentiation nor the change of poly(ADP-ribosyl)ation. Because NA promotes the differentiation activities in several types of cells, these results provide important clues towards elucidating the mechanisms of NA. Additionally, these findings provide new information about mitochondrial poly(ADP-ribosyl)ation. (C) 2009 Elsevier Ltd. All rights reserved.