SPHINGOMYELINASE AND PHOSPHOLIPASE A(2) REGULATE TYPE-I DEIODINASE EXPRESSION IN FRTL-5 CELLS

We have previously reported (Mol. Cell. Endocrinol. (1994) 101, R31-R3 5) that the proinflammatory cytokines, tumor necrosis factor-alpha (TN F), interleukin-1 beta (IL-1 beta), and interferon-gamma (IFN-gamma), have a marked inhibitory effect on the expression and activity of type I iodothyronine deiodinase (D1) in FRTL-5 rat thyroid cells, while th e antiinflammatory cytokine, transforming growth factor-beta(1) (TGF-b eta(1)) had no effect. These three proinflammatory cytokines utilize a number of intracellular second messenger systems including the pathwa ys beginning with activation of sphingomyelinase and phospholipase A(2 ). We have studied the time-dependent and dose-dependent effects of sp hingomyelinase, ceramide, phospholipase A(2) (PLA(2)), and arachidonic acid on the expression and activity of D1 in FRTL-5 cells. Sphingomye linase (0.3 U/mL) inhibited D1 activity 55% and reduced D1 mRNA levels 70% to 90% by 8 hours. Similar treatment with 10 U/mL PLA(2) inhibite d D1 activity 54%. Treatment with 15 mu M 5,8,11-eicosatriynoic acid ( ETI), a nonmetabolizable analog of arachidonic acid, or 15 mu M cerami de for 3 hours reduced D1 activity with a half-time of disappearance ( t(1/2)) of 4.2 hours and 3.7 hours, respectively, but ETI and ceramide did not alter the D1 immunoreactivity or mRNA levels. Treatment for 8 hours with cycloheximide (5 or 10 mu g/mL) had no effect on the D1 mR NA level, but blocked the TNF-induced reduction of this mRNA. We concl ude that proinflammatory cytokines inhibit D1 expression and activity in FRTL-5 cells, in part, by activation of sphingomyelinase and PLA(2) that results in (1) competitive inhibition of D1 activity by the enzy matic products ceramide and arachidonic acid and (2) reduction of D1 m RNA stability by protein synthesis-dependent mechanisms.