DEMONSTRATION OF CELL-CYCLE KINETICS IN THYROID PRIMARY CULTURE BY IMMUNOSTAINING OF PROLIFERATING CELL NUCLEAR ANTIGEN - DIFFERENCES IN CYCLIC AMP-DEPENDENT AND AMP-INDEPENDENT MITOGENIC STIMULATIONS

In this study, experimental conditions are described that allowed us t o follow the fate of the DNA polymerase delta-associated proliferating cell nuclear antigen (PCNA), by immunolabeling during the overall cel l cycle. Differences in subcellular localization or the presence of PC NA allowed us to identify each phase of the cell cycle. Using these ce ll cycle markers in dog thyroid epithelial cells in primary culture, w e found unexpected differences in cell cycle kinetics, in response to stimulations through cAMP-dependent and cAMP-independent pathways. The se provide a new dimension to the view that the two pathways are large ly separate, but co-operate on DNA synthesis initiation. More precisel y, thyrotropin (TSH), acting via cAMP, exerts a potent triggering effe ct on DNA synthesis, associated with a precocious induction of PCNA ap pearance. This constitutes the major influence of TSH (cAMP) in determ ining cell cycle progression, which is only partly moderated by TSH-de pendent lengthening of S- and G2-phases.