Biochemical characterization of the meiosis-essential yet evolutionarily divergent topoisomerase VIB-like protein MTOPVIB from Arabidopsis thaliana.

Formation of programmed DNA double-strand breaks is essential for initiating meiotic recombination. Genetic studies on Arabidopsis thaliana and Mus musculus have revealed that assembly of a type IIB topoisomerase VI (Topo VI)-like complex, composed of SPO11 and MTOPVIB, is a prerequisite for generating DNA breaks. However, it remains enigmatic if MTOPVIB resembles its Topo VI subunit B (VIB) ortholog in possessing robust ATPase activity, ability to undergo ATP-dependent dimerization, and activation of SPO11-mediated DNA cleavage. Here, we successfully prepared highly pure A. thaliana MTOPVIB and MTOPVIB-SPO11 complex. Contrary to expectations, our findings highlight that MTOPVIB differs from orthologous Topo VIB by lacking ATP-binding activity and independently forming dimers without ATP. Most significantly, our study reveals that while MTOPVIB lacks the capability to stimulate SPO11-mediated DNA cleavage, it functions as a bona fide DNA-binding protein and plays a substantial role in facilitating the dsDNA binding capacity of the MOTOVIB-SPO11 complex. Thus, we illustrate mechanistic divergence between the MTOPVIB-SPO11 complex and classical type IIB topoisomerases.

Compound cellular stress maximizes apoptosis independently of p53 in glioblastoma.

We examined the apoptotic response of two glioblastoma cells, p53 wild type U87 and p53 mutated T98G, to doxorubicin, bortezomib, and vorinostat, which respectively target DNA, 26S proteasome and histone deacetylase, to clarify p53's function in apoptosis. We demonstrated that doxorubicin induced apoptosis in U87 cells but not in T98G cells. The level of p53 was definitively correlated to the extent of DNA damage and apoptosis initiation. Dominant-negative p53 reduced p21 expression, but did not affect doxorubicin-induced apoptosis, so the transcriptional activity of p53 seemed not to participate in doxorubicin-induced apoptosis. However, p53 concentrated into the nucleus during heavy apoptosis. Bortezomib could induce apoptosis in U87 with high sensitivity and T98G cells with low sensitivity. In contrast, vorinostat promoted apoptosis in both U87 and T98G cells and reduced the basal level of p53 in U87 cells, indicating that p53 played no role in the vorinostat-induced apoptosis. To clearly define the role of p53 in bortezomib- and doxorubicin-induced apoptosis, we combined doxorubicin with bortezomib to treat U87 cells to assess this combination's effect on apoptosis and p53 status. Interestingly, the combination of doxorubicin with bortezomib engendered compound stress, resulting in a synergistic outcome for apoptosis in U87 cells. However, the amounts of p53 in the total count and in the nucleus were much lower with the combination than with doxorubicin alone, suggesting that p53 played no role in either the compound stress, doxorubicin-only or bortezomib-induced apoptosis.

P53 enhances apoptosis induced by doxorubicin only under conditions of severe DNA damage.

We previously demonstrated that Bim is the main BH3-only protein replacing Bak/Bax from Bcl-xl to activate apoptosis in a p53-independent manner in response to doxorubicin in prostate cancer. However, the comparison of doxorubicin treatment between LNCaP cells carrying p53-wild type and PC3 cells carrying p53-null suggested that p53 might be essential for maximizing apoptosis. Inhibition of ATM did not affect doxorubicin-induced apoptosis. Overexpression of p53 did not affect ABT-263-induced apoptosis and nevertheless, the combination of doxorubicin with ABT-263 induced higher apoptotic responses than did doxorubicin or ABT-263 alone. These results advocated that doxorubicin-induced DNA damage controls p53 function for intensifying apoptosis. Indeed, overexpression of p53 only enhanced apoptosis under conditions of severe DNA damage induced by high concentrations of doxorubicin in LNCaP cells. Immunofluorescence staining showed vague γH2AX foci and enlarged nuclei in LNCaP cells in response to high concentrations of doxorubicin, en route to apoptosis. In addition, our results revealed that the apoptosis in response to DNA replication stress induced by CFS-1686, a catalytic inhibitor of topoisomerase, is p53-independent. Interestingly, the combination of doxorubicin with CFS-1686 generated DNA damage and replication stress simultaneously, resulting in a synergistic apoptotic effect in prostate cancer cells. Thus, we concluded that p53 is a sensor for enhanced apoptosis in response to DNA damage stress, not DNA replication stress, at least in prostate cancer.