Calcium-Dependent Oxidation of Thioredoxin during Cellular Growth Initiation

The fraction of cell thiol proteins in the oxidized disulfide form were quantified during mitogen-induced HaCaT keratinocyte growth initiation. Oxidized thioredoxin increased from 11 ± 1.2% in resting cells to 80 and 61% 2 min after addition of bradykinin or EGF. Thioredoxin oxidation was transient returning toward normal values by 20 min. The disulfide forms of other cellular proteins rose in parallel with thioredoxin oxidation. The oxidation of thioredoxin depended on a rise in cytosolic calcium. It was prevented by preloading cells with BAPTA, a Ca2+ chelator and induced by addition of Ca2+-ionophore A23187 or of thapsigargin. In cell extracts, thioredoxin reductase was inhibited by micromolar calcium. The rise in cytosolic Ca2+ led to a concomitant burst of H2O2 formation. The oxidizing intracellular milieu suggests that redox regulation actively participates in the growth initiation cascade. The role of peroxiredoxins and ASK 1 cascade activation are discussed in this context.