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 thyrotr opin (TSH)-stimulated triiodothyronine (T-3) production by sheep thyroid ce lls grown in primary culture. Incubation of sheep thyrocytes with okadaic a cid (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-stimula ted T-3 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 T-3 production, due to cellular death, was observed with 6 nM OA and 1 nM CL-A, In the abs ence of TSH, OA or CL-A had no effect on T-3 production by thyrocytes, Fors koline (10 mu M), an activator of adenylate cyclase, increased the basal an d TSH-stimulated T-3 release by sheep thyroid cells; this effect was increa sed 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 inhib itors of PP-1 and PP-2A, have significant stimulatory effects on T-3 secret ion promoted by TSH and FK. These observations indicate that these proteins could be important mediators of thyroid hormone production.