Regulation of Thyroid Cell Proliferation by TSH and Other Factors: A Critical Evaluation of in Vitro Models

Abstract

TSH via cAMP, and various growth factors, in cooperation with insulin or IGF-I stimulate cell cycle progression and proliferation in various thyrocyte culture systems, including rat thyroid cell lines (FRTL-5, WRT, PC Cl3) and primary cultures of rat, dog, sheep and human thyroid. The available data on cell signaling cascades, cell cycle kinetics, and cell cycle-regulatory proteins are thoroughly and critically reviewed in these experimental systems. In most FRTL-5 cells, TSH (cAMP) merely acts as a priming/competence factor amplifying PI3K and MAPK pathway activation and DNA synthesis elicited by insulin/IGF-I. In WRT cells, TSH and insulin/IGF-I can independently activate Ras and PI3K pathways and DNA synthesis. In dog thyroid primary cultures, TSH (cAMP) does not activate Ras and PI3K, and cAMP must be continuously elevated by TSH to directly control the progression through G 1 phase. This effect is exerted, at least in part, via the cAMP-dependent activation of the required cyclin D3, itself synthesized in response to insulin/IGF-I. This and other discrepancies show that the mechanistic logics of cell cycle stimulation by cAMP profoundly diverge in these different in vitro models of the same cell. Therefore, although these different thyrocyte systems constitute interesting models of the wide diversity of possible mechanisms of cAMP-dependent proliferation in various cell types, extrapolation of in vitro mechanistic data to TSH-dependent goitrogenesis in man can only be accepted in the cases where independent validation is provided. (Endocrine Reviews 22: 631-656, 2001) I. Introduction II. The in Vitro Models III. Methods of Measuring Cell Proliferation IV. Proliferation Characteristics of the Various Systems A. FRTL-5 rat thyroid cell line B. WRT rat thyroid cell line C. PC Cl3 rat thyroid cell line D. Rat thyroid follicles in primary culture E. Dog thyrocytes in primary culture F. Human thyrocytes in primary culture G. Comparison of cell systems V. Kinetics of TSH-Insulin/IGF-I Synergy and Cell Cycle Progression VI. Hypertrophy vs. Mitogenesis VII. Signaling Cascades A. Expression of membrane receptors B. Coupling of TSH receptor C. Involvement of cAMP and PKA D. Involvement of Ras and its effector pathways E. Summary VIII. Immediate/Early Genes IX. Cell Cycle-Regulatory Proteins X. In Vivo Models XI. Discussion