TGF-β1 actions on FRTL-5 cells provide a model for the physiological regulation of thyroid growth

Abstract

Little is known about the TGF-b1 mechanism that promotes thyroid cell growth arrest. We assessed TGFb1 e ects on Fisher rat thyroid cell line . This allowed us to study TGF-b1 action on thyroid cells in various physiological situations such as actively proliferating cells, resting cells stimulated to proliferate by the action of various mitogens, and resting cells. TGF-b1 arrested proliferating FRTL-5 cells, increasing c-myc mRNA levels and reducing p27-free cyclin D1 protein levels, without a ecting either the cellular content of p27 or the cyclin D1-p27 complexes. Moreover, TGF-b1 treatment reduced the activity of cyclin E-CDK2 complexes and, consequently, pRB was found to be hypophosphorylated. TGF-b1 prevented resting cells to enter in the cell cycle when stimulated with growing medium (newborn calf serum plus a mixture of ®ve hormones) but not when TSH (thyroid stimulating hormone) plus IGF-1 (Insulin-like growth factor I) were used as mitogens. Both stimuli increased the levels of cyclins D1, D3 and E but TGF-b1 had a greater e ect in decreasing these cyclin levels in growing-medium stimulated cells than in TSH+IGF-1. This suggests that for FRTL-5 cells, the content of these cyclins must exceed a threshold to progress through the cell cycle. TGF-b1 induced apoptosis in quiescent cells, accompanied by a reduction in p27 protein levels and an increase in c-myc expression. Interestingly, TGF-b1-induced variations in prothymosin alpha and c-myc mRNA levels were not correlated. TGF-b1 always promoted an increase of p15 mRNA levels. In summary, our results point to the fact that TGF-b1 could play a physiological role in the control of thyroid growth through the modi®cation of cell cycle regulatory proteins.

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

“…In PC Cl 3 cells, the excess of cyclin D3 appears to titrate p27 kip1 from cyclin E-or cyclin A-containing complexes thereby leading to Cdk2 activation. Sequestration of p27 kip1 by cyclin D3/Cdk complexes in PC Cl 3 cells could be necessary because, unlike FRTL-5 and WRT cells, p27 kip1 is not downregulated in PC Cl 3 cells in response to TSH or insulin (Carneiro et al, 1998;Medina et al, 1999).…”

“…Furthermore, these results are reminiscent of those obtained with single-cell microinjection, showing that cyclin D3 was necessary for G1 progression of dog thyrocytes . In the FRTL-5 rat thyroid cell line model, TSH and/or IGF-1, accelerate G1 phase by inducing the expression of cyclins D1, D3 and E and by decreasing p27 kip1 expression (Yamamoto et al, 1996;Carneiro et al, 1998;Medina et al, 1999).…”

“…Furthermore, inhibition of cyclin D3 function induced by neutralizing antibodies suppressed TSH-dependent proliferation of dog thyrocytes Wollman et al, 1978). Conversely, in another rat cell line (the FRTL-5 cells), TSH-dependent proliferation occurs both through upregulation of D-type cyclins and downregulation of the cyclin-dependent kinase inhibitor p27 kip1 (Yamamoto et al, 1996;Carneiro et al, 1998;Medina et al, 1999).…”

Somatostatin Interferes with Thyrotropin-induced G1-S Transition Mediated by cAMP-dependent Protein Kinase and Phosphatidylinositol 3-Kinase

“…FRTL-5 cells provide an excellent system to study the mechanisms that govern progression from G 1 to S phase, because in this cell type the transition from quiescent to proliferative cells requires the action of hormones and growth factors such as TSH, insulin, and insulin-like growth factor-I (9 -12). Studies in FRTL-5 thyroid cells, although performed in different hormonal backgrounds, show that TSH increases the expression of G 1 cyclins such as cyclin D1, D3, and E (13,14) as well as its partners Cdk2 and Cdk4 (13,15). Moreover, these effects correlate with down-regulation of p27 kip1 protein levels * This work was supported by DGICYT Grant PM97-0065, CAM Grant 08.1/0025/97-99, and a grant from Fundación Salud 2000 (Spain).…”

“…(8,13) and with an increase in the phosphorylation state of Rb (13), leading to the activation of cyclin⅐Cdk complexes and the progression of the cells through the cell cycle. TSH cell cycle induction is counteracted by cytostatic signals such as TGF-␤1 (13) and somatostatin (8,16,17). TGF-␤1 interference with TSH action has been studied in FRTL-5 cells (13); however, the mechanism of interference between somatostatin and TSH is unknown.…”