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.

Autocrine stimulation by osteopontin plays a pivotal role in the expression of the mitogenic and invasive phenotype of RET/PTC-transformed thyroid cells

“…This not withstanding, very little is known about the mechanisms by which RET/PTC oncogenes transform thyroid follicular cells. Model systems of thyroid follicular cells have been widely used to study growth regulation and neoplastic transformation (Kimura et al, 2001). By using PC Cl 3 cells, here we identify OPN as a major transcriptional target of RET/PTC in thyroid cells.…”

Critical role of cyclin D3 in TSH-dependent growth of thyrocytes and in hyperproliferative diseases of the thyroid gland

“…In fact, rat thyroid PC Cl 3 cells are routinely maintained in 5% calf serum with a mixture of six hormones, of which only TSH and insulin are required for proliferation (see Materials and methods) (Fusco et al, 1987;Medina and Santisteban, 2000;Kimura et al, 2001). Although TSH alone does not induce DNA synthesis in this cell line (Florio et al, 2001), TSH associated with insulin or a low serum concentration is mitogenic for PC Cl 3 cells (Kimura et al, 1999).…”

“…According to the current model, progression through G1 is determined by the sequential phosphorylation of the retinoblastoma susceptibility gene product, p105 Rb , whose phosphorylation by the cyclin D-Cdk4/6 and E-Cdk2 complexes, activates the E2F family of transcription factors (Weinberg, 1995). Studies of the TSH-dependent regulation of cell cycle progression suggested that cyclin D3 is a critical regulator of the thyroid follicular cell cycle (Kimura et al, 2001;Medina and Santisteban, 2000). Cyclin D3 is the predominant D-type cyclin in cultured thyrocytes of dogs and humans, and in the mouse thyroid in vivo (Coppee et al, 1998;Baldassarre et al, 1999;Kimura et al, 2001).…”

“…Studies of the TSH-dependent regulation of cell cycle progression suggested that cyclin D3 is a critical regulator of the thyroid follicular cell cycle (Kimura et al, 2001;Medina and Santisteban, 2000). Cyclin D3 is the predominant D-type cyclin in cultured thyrocytes of dogs and humans, and in the mouse thyroid in vivo (Coppee et al, 1998;Baldassarre et al, 1999;Kimura et al, 2001). Furthermore, inhibition of cyclin D3 function induced by neutralizing antibodies suppressed TSH-dependent proliferation of dog thyrocytes Wollman et al, 1978).…”

G13-dependent Activation of MAPK by Thyrotropin

“…Transient Transfection with shRNA or Dominant Negative G␣ 12 and G␣ 13 Expression Vectors-To investigate the role of G␣ 12 and G␣ 13 in TSHR-mediated MAPK activation, the RNA interference technique was used (31). For this purpose, shRNA was transiently expressed via a pSuper NeoGFP expression vector (32).…”