Activation by thyroid stimulating hormone of nerve growth factor-induced gene-B expression in thyrocytes in culture: relation with proliferation and specific gene expression

Nerve growth factor-induced gene-B (NGFI-B) is an immediate early gene first found as a part of the PC12 cell response to NGF (Milbrandt, J., Science 238 (1987) 797-799). We have previously reported that NGFI-B mRNA is strongly upregulated by thyroid-stimulating hormone (TSH) in dog thyrocytes in culture (Pichon et al., Endocrinology 137 (1996) 4691-4698). In this study, we have analyzed the regulation of NGFI-B mRNA expression by a variety of agents acting on thyrocytes proliferation and/or differentiation. We show that: (1) the induction of NGFI-B mRNA is stronger after stimulation of the cAMP cascade, but it is not restricted to this signaling pathway; (2) the powerful mitogens for thyroid cells EGF and HGF have little or no effect on NGFI-B mRNA induction; (3) NGFI-B mRNA is induced by anisomycin at a subinhibitory concentration for protein synthesis, and is superinduced by the combination of TSH and anisomycin; this treatment decreases the TSH-induced proliferation levels, but does not inhibit the induction of some differentiation markers; and (4) both in dog and in pig thyrocytes, NGFI-B mRNA induction is observed after a variety of treatments stimulating differentiation, but without proliferative effects. Our results therefore suggest that NGFI-B mRNA induction might not be related to TSH-induced thyrocyte proliferation, but could participate in the differentiation program triggered by TSH.     

Fas (APO-1, CD95)-mediated apoptosis in thyroid cells is regulated by a labile protein inhibitor

To determine whether thyroid cell apoptosis observed in autoimmune thyroid disease could be related to activation of the Fas pathway, we examined the expression and function of Fas on thyroid follicular cells in vitro. Fas messenger RNA was found to be present using two different techniques and was expressed at equal levels in thyrocytes cultured either in the presence or absence of TSH. Fas antigen protein expression was demonstrated by Western blot of thyroid cell lysates and by immunohistochemical staining of thyrocytes, and the amount of Fas protein present did not appear to vary regardless of culture conditions. Despite expressing substantial amounts of Fas protein, thyrocytes treated with anti-Fas monoclonal antibody failed to undergo apoptosis. The addition of either interferon-gamma or interleukin-1beta to the anti-Fas-treated cell cultures also did not promote apoptotic signaling through this pathway. In contrast, the concomitant administration of cycloheximide allowed the induction of apoptosis through the activation of Fas in thyrocytes. These results suggest that Fas is constitutively expressed in thyrocytes, but that the induction of apoptosis through the Fas pathway is blocked by a labile protein inhibitor.