The effects of the tri-iodothyronine (T(3)) secreted by thyroid cells on the growth of the thyrocyte are poorly known. In this study we analyzed the effects of T(3) on the proliferation of bovine thyroid follicles in primary culture previously depleted of endogenous T(3). Cellular deoxiribonucleic acid (DNA) synthesis, determined by [(3)H]thymidine incorporation, was stimulated by T(3) (0.1-5.0 nM) for 24 h in a concentration-dependent fashion with a maximal effect at 1.0 nM T(3) (P<0.01). This T(3) action was time-dependent when assayed from 12 to 72 h. The induction of mitogenic activity was corroborated by the increase in proliferating cell nuclear antigen (PCNA) measured by Western blot analysis. PCNA increased after treatment with T(3) (0.1-5.0 nM) in a concentration-dependent manner. Since T(3) modifies the activity of growth factors whose actions are mainly mediated by tyrosine kinase (TK) activation in diverse cellular types, we assayed the effects of genistein, a general TK inhibitor, and tyrphostin A25, a specific epidermal growth factor (EGF)-receptor (EGFR)-dependent TK activity inhibitor, on the proliferative effects of T(3). The T(3)-induced [(3)H]thymidine incorporation was inhibited by both agents in a concentration-dependent manner. A significant increase in the total TK activity measured in cellular protein extracts was induced by 0.5 and 1.0 nM T(3) (P<0.001). Tyrosine phosphorylation of the EGFR was also stimulated by T(3) (P<0.001) with no change in the EGFR expression as determined by Western blot analysis. Both, the T(3)-stimulated [(3)H]thymidine incorporation and the TK activity were inhibited by a anti-mouse EGF antibody. These results lead us to propose that T(3) could operate as a proliferative agent in bovine thyroid cells through a mechanism involving an autocrine/paracrine EGF/EGFR-dependent regulation.
M Di Fulvio, AH Coleoni, CG Pellizas and AM Masini-Repiso
CG Pellizas, AH Coleoni, ME Costamagna, M Di Fulvio and AM Masini-Repiso
Tri-iodothyronine (T3) is known to be involved in the regulation of the growth hormone (GH)-insulin-like growth factor I (IGF-I) axis. In previous studies we demonstrated that IGF-I and GH reduced the metabolic response to T3 measured as the activity of two T3-dependent enzymes, mitochondrial alpha-glycerophosphate dehydrogenase (alpha-GPD) and cytosolic malic enzyme (ME) in cultured rat liver cells. In this study we analysed in vivo the effect of IGF-I administered to rats on the activity of alpha-GPD and ME. IGF-I (240 micrograms/100 g body weight (BW) every 12 h for 48 h) significantly diminished alpha-GPD (P < 0.01) and ME (P < 0.05) activities. Serum basal glucose concentration was not significantly modified 12 h after the administration of recombinant human IGF-I (240 and 480 micrograms/100 g BW every 12 h for 48 h). Under similar conditions, no significant change in serum total thyroxine (TT4) concentration was observed, although free thyroxine (FT4) was diminished (P < 0.02) and total T3 (TT3) was increased (P < 0.03). To explore the participation of the nuclear thyroid hormone receptor (THR) in the mechanism of IGF-I action we measured the maximal binding capacity and the affinity constant (Ka) of THR by Scatchard analysis, and concentrations of messenger RNAs (mRNAs) that code for the isoforms of THR present in the liver (beta 1, alpha 1 and alpha 2) by Northern blot. IGF-I (240 micrograms/100 g BW every 12 h for 48 h) significantly reduced maximal binding capacity to 37% of the control value (P < 0.01) without changes in the Ka. beta 1, alpha 1 and alpha 2 THR mRNAs were significantly reduced (P < 0.01) by 120-480 micrograms/100 g BW IGF-I administration every 12 h for 48 h. Time-course studies indicated that this effect was obtained 12 h after the administration of 240 micrograms/100 g BW IGF-I (P < 0.05). These results indicate that IGF-I administration to rats diminishes the metabolic thyroid hormone action in the liver by a mechanism that involves, at least in part, a reduction in the number of THRs and in their level of expression.