The regulation of cell cycle progression and apoptosis in thyroid follicular epithelial cells

The data presented in this thesis provide new insight into the mechanisms governing normal and Ras oncogene-stimulated proliferation in rat thyroid epithelial cells. Although strictly required for the proliferation of thyroid cells, TSH is devoid of mitogenic activity but collaborates with serum to stimulate proliferation. My work revealed that TSH and serum contribute to cell cycle progression through differential effects on p27 localization and expression. Alone, TSH enhanced nuclear accumulation of p27, while serum modestly induced its export. In contrast, only combined treatment with TSH and serum led to rapamycin-sensitive p27 turnover, which was directly correlated with an increase in DNA synthesis. These findings suggest that TSH contributes to cell cycle progression at least in part by increasing the number of cycling cells, effects that may be elicited through p70 ribosomal S6 protein kinase. Ras mutations are frequent in human thyroid tumors, where Ras has been implicated in tumor initiation and progression. Paradoxically, acute expression of activated Ras in rat thyroid cells induced deregulated cell cycle progression followed by apoptosis. My studies revealed that cell cycle progression was an obligate intermediate in Ras-stimulated apoptosis. Overexpression of p27, a potent inhibitor of cell cycle progression, abrogated Ras-induced apoptosis, demonstrating for the first time that Ras engages apoptosis through effects on the cell cycle. Furthermore, deregulation of Rb/E2F through expression of HPV-E7 was sufficient to induce both cell cycle progression and apoptosis. My studies further revealed that Ras activates a DNA damage response checkpoint, with characteristics similar to those reported in precancerous lesions in humans. Additional studies revealed that a subpopulation of Ras-expressing cells bypass checkpoint activation, evade apoptosis and emerge with features of cellular transformation and chromosomal instability. These data suggest that the ability of Ras to transform thyroid cells relies on its ability to bypass cell cycle checkpoints and evade apoptosis, rather than to simply increase proliferative potential. These data highlight the intricate linkage between cell cycle progression, DNA damage, and apoptosis, and confirm that deregulation of the regulatory proteins controlling these pathways is an important contribution to cell transformation