Osmotic regulation of MG-132-induced MAP-kinase phosphatase MKP-1 expression in H4IIE rat hepatoma cells

Background/Aims: Proteasome inhibitors such as MG132 are considered as potential therapeutical tools in different clinical settings. The dual specificity MAP-kinase phosphatase MKP-1 plays a role in balancing signals mediating cell death or survival. Here the effect of cell hydration on MG-132-induced MKP-1 expression was investigated in H4IIE rat hepatoma cells. Results: Hyperosmolarity ( 405mosmol/l) increased MKP-1 expression by MG- 132, which was accompanied by an induction of c-Fos, c-Jun, cJun Ser(73) phosphorylation, and AP-1 DNA binding. MKP-1 induction by MG-132 plus hyperosmolarity was sensitive to inhibition of p38(MAPK) and c-Jun-N-terminal kinases (JNKs) but not extracellular signal-regulated kinases Erk-1/Erk-2, and was accompanied by a decline of MAP-kinase activities. Although hyperosmolarity increased overall protein ubiquitination in presence of MG- 132, ubiquitination of MKP-1 was found under normo-, but not hyperosmotic conditions. Hyperosmolarity also enabled MG-132 to induce poly-ADP-ribose polymerase ( PARP) cleavage which was sensitive to inhibition of p38(MAPK) and JNKs but not Erk-1/Erk-2. PARP cleavage and caspase-3 activation in H4IIE cells treated with hyperosmolarity plus MG-132 was further increased by vanadate, consistent with a contribution of MKP-1 to counterbalance proapoptotic MAP-kinase signals. Conclusion: The findings suggest that among other factors cell hydration critically determines the cellular response to proteasome inhibitors. Copyright (c) 2005 S. Karger AG, Basel.