THYROID-STIMULATING HORMONE-REGULATED GROWTH AND CELL-CYCLE DISTRIBUTION OF THYROID-CELLS INVOLVE TYPE-I ISOZYME OF CYCLIC AMP-DEPENDENT PROTEIN-KINASE

Optimal growth and differentiation of normal rat thyroid FRTL5 cells d epend strictly on the presence of thyroid-stimulating hormone (TSH). F RTL5 cells deprived of TSH cease dividing and become quiescent. Additi on of TSH to quiescent cells, which activates the cyclic AMP-mediated pathway, is sufficient to stimulate cell entry into S phase of the cel l cycle. We have previously shown that the differential expression of the two isozymes, type I and type II, of the cyclic AMP-dependent prot ein kinase (PKA) correlates with cell growth and differentiation of se veral rodent and human cell lines. We have studied the role of PKA in the TSH-regulated growth and cell cycle distribution of FRTL5 cells. U pon addition of TSH to FRTL5 cells deprived of hormone, a rapid induct ion of RIalpha mRNA species occurred within 30 min after treatment, re aching the levels of proliferating FRTL5 cells at 12 h. RIIalpha mRNA levels slightly increased after TSH addition, whereas Calpha mRNA leve ls did not show major changes. Photoaffinity labeling of PKA receptor proteins showed that addition of TSH to quiescent FRTL5 cells induced a progressive increase in RIalpha levels starting at 6 h after stimula tion, whereas RIIalpha receptor levels increased only slightly. When F RTL5 cells were treated with an antisense oligodeoxynucleotide targete d against the RIalpha regulatory subunit, their growth was arrested, w hereas an antisense against the RIIalpha regulatory subunit produced o nly a mild growth inhibition. Moreover, exposure to the antisense RIal pha oligomer resulted in accumulation of cells in the G0-G1 compartmen t, as during TSH deprivation. Unlike FRTL5 cells deprived of TSH, cell s arrested by the specific RIalpha antisense oligomer failed to enter S phase upon stimulation with TSH. These results demonstrate that type I isozyme of PKA is directly involved in TSH-regulated cell prolifera tion and may be involved in cell cycle distribution of thyroid FRTL5 c ells.