Heat shock and Cd²⁺ exposure regulate PML and Daxx release from ND10 by independent mechanisms that modify the induction of heat-shock proteins 70 and 25 differently

Nuclear domains called ND10 or PML bodies might function as nuclear depots by recruiting or releasing certain proteins. Although recruitment of proteins through interferon-induced upregulation and SUMO-1 modification level of PML had been defined, it is not known whether release of proteins is regulated and has physiological consequences. Exposure to sublethal environmental stress revealed a sequential release of ND10-associated proteins. Upon heat shock Daxx and Sp100 were released but PML remained, whereas exposure to subtoxic concentrations of CdCl₂ induced the release of ND10-associated proteins, including PML, with Sp100 remaining in a few sites. In both cases, recovery times were similar and were followed by a burst of mitotic activity. Cadmium-induced release of proteins from ND10 could be blocked by inhibiting activation of p38 MAPK or ERK1/2. By contrast, heat-shock-induced desumolation of PML and release of proteins from ND10 are unaffected by these inhibitors but can be recapitulated by overexpression of the SUMO isopeptidase SENP-1. Therefore, activation of SENP-1-like SUMO isopeptidase(s) during heat shock is not affected by these kinases. Thus, the release of ND10-associated proteins is not due to a general dispersal of nuclear domains but seems to be regulated by rapid desumolation during thermal stress and through the phosphorylation cascade of stress and mitogenic signaling pathways in the case of CdCl₂. Whether the release of certain proteins had consequences was tested for heat-shock-protein transcription and synthesis. Release of Daxx correlated with Hsp25 suppression, suggesting that Daxx normally inhibits immediate Hsp25 production. Release of PML correlated with lower production of Hsp70. These results suggest that segregation or release of PML or Daxx have differential physiological relevance during the stress response. The fact that enzymatic activation of protein release or segregation after stress modifies the heat-shock response strengthens the concept of ND10 as a regulated depot of effector proteins.