Thyrotropin-induced elevation of 1,2-diacylglycerol and stimulation of growth of FRTL-5 cells are not dependent on inositol lipid hydrolysis.

We reported that TSH, which stimulates cAMP accumulation and proliferation of FRTL-5 thyroid cells, chronically increases the 1,2-diacylglycerol (1,2-DG) content of FRTL-5 cells. Because activation of inositol lipid hydrolysis by a phospholipase-C enzyme would generate 1,2-DG, we compared the effects of TSH on inositol lipid metabolism to TSH-induced increases in 1,2-DG content and stimulation of cAMP accumulation and cell growth. Acute stimulation of inositol lipid hydrolysis did not occur with doses of 1000 microU/ml or lower, but did occur with TSH doses of 3000 microU/ml and higher, with rates between 1-4%/h. More importantly, in cells chronically exposed to TSH, the rate of inositol lipid hydrolysis was increased only at TSH doses of 10,000 microU/ml or greater, and the maximum rate was 4-5%/h. When cells were growth arrested by TSH deprivation, there was no change in the content of inositol phosphates or polyphosphoinositides. In contrast to the high doses of TSH required to stimulate inositol lipid hydrolysis, TSH-induced elevation of 1,2-DG content and stimulation of cAMP accumulation and growth occurred at physiological TSH concentrations, with minimal effective doses in the range of 1-10 microU TSH/ml, and half-maximally effective doses between 50-200 microU TSH/ml. These data suggest that inositol lipid hydrolysis does not mediate the proliferative response to TSH in FRTL-5 cells and is not the mechanism by which increases in 1,2-DG content occur at physiological TSH concentrations.