Effect of okadaic acid and calyculin-A, two protein phosphatase inhibitors, on thyrotropin-stimulated triiodothyronine secretion in cultured sheep thyroid cells

We have studied the effect of two protein phosphatase inhibitors on thyrotropin (TSH)-stimulated triiodothyronine (T₃) production by sheep thyroid cells grown in primary culture. Incubation of sheep thyrocytes with okadaic acid (OA) and calyculin-A (CL-A), two potent inhibitors of type 1 (PP1) and type 2A (PP2A) protein phosphatases, resulted in an increase of TSH-stimulated T₃ production. This effect was detected using concentrations as low as 0.1 pM with OA and 1 fM with CL-A. An inhibitory effect on T₃ production, due to cellular death, was observed with 6 nM OA and 1 nM CL-A. In the absence of TSH, OA or CL-A had no effect on T₃ production by thyrocytes. Forskoline (10 μM), an activator of adenylate cyclase, increased the basal and TSH-stimulated T₃ release by sheep thyroid cells; this effect was increased by OA in cells grown in the basal state but not in the presence of TSH. These results suggest that the marine toxins OA and CL-A, two potent inhibitors of PP-1 and PP-2A, have significant stimulatory effects on T₃ secretion promoted by TSH and FK. These observations indicate that these proteins could be important mediators of thyroid hormone production.