Stress-induced accumulation of heme oxygenase-1 in Xenopus laevis A6 kidney epithelial cells
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Abstract Previous studies have examined stress-induced heme oxygenase-1 (HO-1) expression primarily in mammalian systems. The present study examines, for the first time in amphibians, the effect of heat shock, sodium arsenite, cadmium chloride, and the proteasomal inhibitor MG132 on HO-1 accumulation in Xenopus laevis A6 kidney epithelial cells. Western blot analysis revealed that exposure of A6 cells to a range of heat shock temperatures (30-35 °C), which induced HSP30 accumulation, did not induce HO-1 accumulation. In contrast, cells treated with sodium arsenite (5-50 μM), cadmium chloride (50-200 μM) or MG132 (5-30 μM) exhibited a dose- and time-dependent accumulation of HO-1. Additionally, immunocytochemical analysis revealed that HO-1 and HSP30 accumulation occurred in a granular pattern primarily in the cytoplasm in cells treated with sodium arsenite, cadmium chloride, or MG132. In cells recovering from sodium arsenite or cadmium chloride treatment, HO-1 and HSP30 accumulation initially increased to a maximum at 12 h and 24 h recovery, respectively, followed by a 50% reduction at 48 h. This initial increase in the relative levels of stress proteins was likely the result of new synthesis as it was inhibited by cycloheximide. In comparison, cells recovering from MG132 treatment displayed reduced but prolonged accumulation of HO-1 and HSP30. Interestingly, cells treated with low concentrations (10 μM) of sodium arsenite or MG132 but not cadmium chloride in combination with a mild 30 °C heat shock had enhanced accumulation of HO-1 and HSP30 accumulation compared to either of the stressors individually. This study has shown for the first time in amphibians that HO-1 accumulation is induced in response to metals and proteasomal inhibitors, suggesting that it may play a role in mediating the cellular stress response in X. laevis.