INOSITOL 1,4,5-TRISPHOSPHATE 3-KINASE ACTIVITY IN FRTL-5 CELLS - REGULATION OF THE ENZYME-ACTIVITY BY TSH

Inositol 1,4,5-trisphosphate (InsP(3)) 3-kinase phosphorylates the Ca2 +-mobilizing second messenger InsP(3) to inositol 1,3,4,5-tetrakisphos phate (InsP(4)). InsP(3) 5-phosphatase dephosphorylates InsP(3) to ino sitol 1,4-bisphosphate (InsP(2)). We compared the effects of TSH added to a culture of FRTL-5 thyroid cells on the activity of InsP(3) 5-pho sphatase and InsP(3) 3-kinase. InsP(3) 3-kinase activity was decreased at a physiological concentration of TSH. Inhibition of this activity started after 3 h of incubation with TSH and was maximal after 24 h. I n contrast, InsP(3) 5-phosphatase activity was not affected by TSH und er the same conditions. The inhibitory effect of TSH on InsP(3) 3-kina se was characterized as follows: a) inhibition of activity was mimicke d by both dibutyryl cyclic AMP and forskolin; b) activity obtained by mixing lysates of TSH-stimulated and non-stimulated cells was the sum of each activity measured separately; c) inhibition persisted after a crude lysate of TSH-stimulated cells had been subjected to SDS/polyacr ylamide gel electrophoresis and the extraction of InsP(3) 3-kinase act ivity. The data suggest that TSH reduced the activity of InsP(3) 3-kin ase in FRTL-5 cells either by a phosphorylation/dephosphorylation mech anism, or by affecting expression of the enzyme.