No abstract

DNA topoisomerase II (TOP2) cleavable complexes represent an unusual type of DNA damage characterized by reversible TOP2-DNA cross-links and DNA double strand breaks. Many antitumor drugs and physiological stresses are known to induce TOP2 cleavable complexes leading to apoptotic cell death and genomic instability. However, the molecular mechanism(s) for repair of TOP2 cleavable complexes remains unclear. In the current studies, we show that TOP2 cleavable complexes induced by the prototypic TOP2 poison VM-26 are proteolytically degraded by the ubiquitin/26 S proteasome pathway. Surprisingly the TOP2␤ isozyme is preferentially degraded over TOP2␣ isozyme. In addition, transcription inhibitors such as 5,6-dichlorobenzimidazole riboside and camptothecin can substantially block VM-26-induced TOP2␤ degradation. These results are consistent with a model in which the repair of TOP2␤ cleavable complexes may involve transcription-dependent proteolysis of TOP2␤ to reveal the protein-concealed double strand breaks.DNA topoisomerases are double-edged swords. They are essential for many important processes of DNA such as DNA replication, RNA transcription, chromosome condensation/decondensation, and chromosome segregation (1). However, due to their delicate act on DNA, they are also highly vulnerable to xenobiotics and physiological stresses to produce topoisomerase-mediated DNA damage, mostly in the form of topoisomerase cleavable complexes (2-5). So far, five human DNA topoisomerases, topoisomerase I (TOP1), 1 TOP2␣, TOP2␤, TOP3␣, and TOP3␤, have been identified and characterized, and the first three have been demonstrated to be important molecular targets for antitumor drugs (1, 6 -10).Both hTOP2 isozymes have been demonstrated to be the cellular targets for many clinically useful anticancer drugs such as VP-16 (etoposide) and doxorubicin (11-13). In the presence of these TOP2-directed drugs (TOP2 poisons), TOP2 isozymes are trapped as their covalent reaction intermediates, the reversible TOP2 cleavable complexes in which each TOP2 subunit is covalently linked to the 5Ј-phosphoryl ends of the four-base staggered double strand breaks (14,15). While the double strand breaks within the TOP2 cleavable complexes are normally concealed by TOP2, many of the cellular effects of TOP2 cleavable complexes are clearly indicative of DNA damage. For example, TOP2 cleavable complexes induced by TOP2 poisons are known to induce DNA damage responses (e.g. G 2 arrest, elevation of sister-chromatid exchanges, NFB activation, and p53 stabilization) (16 -19). DNA repair mutant cells (e.g. ataxia telangiectasia, progeroid Werner's syndrome, and Rad52) are also known to be hypersensitive to TOP2 poisons (20 -22). However, how TOP2-concealed DNA strand breaks are converted to DNA damage signals is still unknown. Inhibitor studies have suggested that both DNA replication and RNA transcription may be important for processing TOP2 cleavable complexes into DNA damage signals (23-25).Repair of topoisomerase cleavable complexes is conceptually challe...

Long-term cytokine-mediated inflammation is a risk factor for obesity and type 2 diabetes mellitus (T2DM). Our previous studies reveal significant associations between promoter single nucleotide polymorphisms (SNPs) of interleukin (IL)-4 and T2DM, as well as between SNPs in genes encoding IL-4/IL-4 receptor and high density lipoproteins. Our animal study reveals that IL-4 regulates glucose/lipid metabolism by promoting glucose tolerance and inhibiting lipid deposits. The above results strongly suggest the involvement of IL-4 in energy homeostasis. In the present study, we focus on examining the regulatory mechanism of IL-4 to lipid metabolism. Our results show that IL-4 inhibits adipogenesis by downregulating the expression of peroxisome proliferator-activated receptor-γ and CCAAT/enhancer-binding protein-α. Additionally, IL-4 promotes lipolysis by enhancing the activity and translocation of hormone sensitive lipase (HSL) in mature adipocytes, which suggests that IL-4 plays a pro-lipolytic role in lipid metabolism by boosting HSL activity. Our results demonstrate that IL-4 harbors pro-lipolysis capacity by inhibiting adipocyte differentiation and lipid accumulation as well as by promoting lipolysis in mature adipocytes to decrease lipid deposits. The above findings uncover the novel roles of IL-4 in lipid metabolism and provide new insights into the interactions among cytokine/immune responses, insulin sensitivity, and metabolism.