Differential nuclear proteomes in response to N-methyl-N'-nitro-N-nitrosoguanidine exposure

As a model carcinogen in studying the mutagenicity and carcinogenicity of N-nitroso alkylating agent, N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) can induce DNA damages and many cellular defensive events, such as DNA repair, G2/M-phase arrest and apoptosis. Noticeably, diverse cellular responses were observed to occur following MNNG treatment in our previous studies, including nontargeted mutations (NTM) at undamaged DNA bases, endoplasmic reticulum stress (ER stress) induction and the activation of several signal transduction pathways. In addition, whole cell proteome analysis also revealed that comprehensive and various changes were triggered by this mutagen. However, low-abundance proteins with key functions, such as nuclear proteins, are always underrepresented in proteomic studies. To reduce the complexity of protein samples and monitor subcellular alterations in response to MNNG exposure, nuclear extracts were fractionated from MNNG-treated cells and compared using two-dimensional fluorescence difference gel electrophoresis (2-D DIGE). Twenty-three differentially expressed protein spots were observed after 0.25 and 1 muM MNNG exposure, and 8 of them were shared between these two treatment groups. When MALDI-TOF MS was used, 17 nuclear proteins affected by MNNG were identified. These proteins were involved in regulation of RNA processing, signal transduction, metabolism, DNA repair, protein biosynthesis and microtubule or cytoskeleton organization. Among them, two nuclear proteins with nucleocytoplasmic shuttling activity, 14-3-3 zeta and hnRNP K, were further demonstrated to undergo different dynamic changes in response to MNNG exposure. These results will aid our understanding of the complicated mechanisms of MNNG-induced cellular responses.