Microbial and acute phase stimuli disrupt promyelocytic leukemia tumor suppressive nodes.

The promyelocytic leukaemia (PML) nuclear domain (PML-ND) is a nuclear sentinel for stress. Each PML-ND cradles a delicate scaffold of nucleoproteins, many of which can trigger the apoptotic death cascade if disrupted. Given their place in integrating stress and death, PML-NDs are obvious targets for excision from injury pathways by viruses and cancers. Viruses express proteins dedicated to silencing the PML-ND network and their failure can presage the suppression of viral replication. To understand how PML-NDs protect the cell from stress we must discover those damage pathways with which they connect. Such data will reveal the panoply of signal pathways lost in PML null cells and the extent to which infection and cancer can desensitise the cell to therapeutic intervention. A convenient and sensitive method with which to detect PML-ND stress induction is its biophysical reorganisation, as well defined dose responsive modifications of PML protein accompany damage recognition. The experiments that we present in this manuscript arose from an observation that lipid mediated transfection of plasmid DNA triggered a dramatic modification of PML-NDs that was identical to that seen following their recognition of DNA damage. In later experiments, we identified lipoprotein and IL-6 as potential mediators of this response. Collectively these data are the first to link an endotoxin component and IL-6 to the PML-ND compartment and, given the role of PML in cell fate, suggest increasing complexity at the interface of immunity and carcinogenesis.

Evidence for the receipt of DNA damage stimuli by PML nuclear domains.

Promyelocytic leukaemia nuclear domains (PML-NDs) comprise a shell of PML protein and many labile cargo proteins. The nature of their cargo, their juxtaposition to foci of damaged DNA following ionizing radiation (IR), and the altered DNA damage responses in PML null cells all implicate PML-NDs in the DNA damage response. In this work, the propensity of PML-NDs to increase in number and decrease in size following IR has been studied. Serial quantitative studies of endogenous PML-NDs prove that the PML-ND response to IR is not the result of the asymmetry in cell cycle distribution that can follow IR, but reflects more directly the process of DNA damage. The response is swift, sensitive (evident after 1 Gy), and potentially reversible in untransformed fibroblasts. In these cells and in HCT116 colon cancer cells, failure to restore PML-ND number within 24 h correlates with later loss of growth potential--in fibroblasts, through prolonged cell cycle arrest and in HCT116 cells, through apoptosis. Failure to express an intact ATM/CHK2 DNA damage signalling pathway in either cell type leads to a delay in the PML-ND response to IR. Conversely, cell cycle progression following IR in cells that detect damaged DNA accelerates PML-ND reorganization. Collectively, these data show that the increase in PML-ND number seen after irradiation is, in part, triggered by the receipt of the DNA damage stimulus. The senescent cell state is also associated with chronic DNA damage and Hayflick-limited fibroblasts were found to express nuclei with elevated numbers of PML-NDs before IR that remained unresponsive to IR. Though the underlying reasons for damage-induced PML alteration remain obscure, it is noteworthy that significant numbers of PML-NDs juxtapose with ionizing radiation-induced foci after IR. The co-regulation of these structures may necessitate the stereotyped increases in PML-ND number following damage.