Sphingomyelinase and Phospholipase A 2 Regulate Type I Deiodinase Expression in FRTL-5 Cells

We have previously reported (Mol. Cell. Endocrinol. (1994) 101, R31–R35) that the proinflammatory cytokines, tumor necrosis factor-α (TNF), interleukin-1β (IL-1β), and interferon-γ (IFN-γ), have a marked inhibitory effect on the expression and activity of type I iodothyronine deiodinase (D1) in FRTL-5 rat thyroid cells, while the anti-inflammatory cytokine, transforming growth factor-β1 (TGF-β1) had no effect. These three proinflammatory cytokines utilize a number of intracellular second messenger systems including the pathways beginning with activation of sphingomyelinase and phospholipase A₂. We have studied the time-dependent and dose-dependent effects of sphingomyelinase, ceramide, phospholipase A₂ (PLA₂), and arachidonic acid on the expression and activity of D1 in FRTL-5 cells. Sphingomyelinase (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 PLA2 inhibited D1 activity 54%. Treatment with 15 μM 5,8,11-eicosatriynoic acid (ETI), a nonmetabolizable analog of arachidonic acid, or 15 μM ceramide 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 βg/mL) had no effect on the D1 mRNA level, but blocked the TNF-induced reduction of this mRNA. We conclude that proinflammatory cytokines inhibit D1 expression and activity in FRTL-5 cells, in part, by activation of sphingomyelinase and PLA₂ that results in (1) competitive inhibition of D1 activity by the enzymatic products ceramide and arachidonic acid and (2) reduction of D1 mRNA stability by protein synthesis-dependent mechanisms.