Redox regulation of neutral sphingomyelinase-1 activity in HEK293 cells through a GSH-dependent mechanism

Phospholipases are essential enzymes in cellular signalling processes such as cellular differentiation, proliferation and apoptosis. Based on its high degree of homology with sequences of prokaryote SMases, a type of Mg²⁺-dependent PLC (nSMase-1) was recently discovered which displayed strong redox dependence for activity in vitro [F. Rodrigues-Lima, A.C. Fensome, M. Josephs, J. Evans, R.J. Veldman, M. Katan (2000), J. Biol. Chem. 275 (36) 28316-28325]. The aim of this work was to test the hypothesis that glutathione could be a natural regulator of nSMase-1 activity ex vivo. We studied how altering glutathione levels and redox ratio modulate nSMase-1 activity in a HEK293 cell line that ectopically overexpressed the nSMase-1 gene. Diminishing total glutathione with BSO without altering significantly the GSH/GSSG ratio did not affect nSMase-1 activity. Treatment of cells with diamide produced a transient decrease of total glutathione and a sharp, but also transient, decrease of the GSH/GSSG ratio. Under these conditions, nSMase-1 activity was temporarily activated and then returned to normal levels. Simultaneous treatment with BSO and diamide that resulted in permanent decreases of total glutathione and GSH/GSSG redox ratio produced a sustained activation of nSMase-1 activity. Taken together, these data indicate that altering the GSH/GSSG ratio by increasing GSSG or decreasing GSH levels, but not the total concentration of glutathione, modulates nSMase-1 activity. Our findings are the first evidence supporting the ex vivo regulation of nSMase-1 through a redox glutathione-dependent mechanism.