Characterization of epidermal growth factor receptor in a rat thyroid cell line, FRTL-5

Impaired response to hepatocyte growth factor in FRTL-5 rat thyroid cells expressing a functional hepatocyte growth factor receptor

The FRTL-5 rat thyroid cell line is widely used for studies of thyrocyte function and growth. The objective of the present study was to investigate the effects of hepatocyte growth factor (HGF) on FRTL-5 cells. HGF has previously been known to be a potent regulator of thyrocyte growth and differentiation. Met, the receptor for HGF was expressed in FRTL-5 cells, as well as in primary cultures of porcine thyrocytes included in the study as control. On HGF stimulation Met was tyrosine phosphorylated in both porcine and FRTL-5 cells, indicating an activation of the receptor. Addition of HGF induced changes of cell shape, scattering and proliferation of the porcine thyrocytes, but not in the FRTL-5 cells; yet, a functional coupling of Met to the p85 subunit of the phosphatidylinositol-3'-kinase (PI3'-K) in coprecipitation experiments, formation of focal adhesions detected in immunofluorescence staining with an antivinculin antibody, and induction of c-fos mRNA in Northern blot analysis was observed in FRTL-5 cells, showing a transduction of the HGF/Met signal. In summary, despite the expression of apparently functional Met, the FRTL-5 cells are unable to properly respond to HGF.

A novel function of thyrotropin as a potentiator of electrolyte secretion from the tracheal gland

Accumulating evidence suggests that thyrotropin (thyroid-stimulating hormone [TSH]) plays some roles in immunoregulation by an extrathyroidal action. Because airway submucosal glands are responsible for nonspecific and specific airway defense, we tested the effect of TSH on feline tracheal submucosal gland using a whole-cell patch-clamp technique, immunohistochemistry, and reverse transcription/polymerase chain reaction (RT-PCR). TSH potentiated neurotransmitter-induced ionic currents significantly in a dose-dependent manner. Acetylcholine (10(-)(8) M)- and norepinephrine (10(-)(7) M)-induced inward current (I(i)), which we previously showed to be a Cl(-) current, were increased to about 3-fold the pre-TSH control responses, respectively, by 2.0 ng/ml TSH; and to 6- and 23-fold the control values by 20.0 ng/ml TSH, respectively. TSH alone was without effect up to 20.0 ng/ml. Follicular stimulating hormone only slightly affected the I(i) (1. 5-fold the control). Analyses with immunohistochemistry and RT-PCR failed to identify TSH receptors on the glandular tissue. Maneuvers to raise the cellular adenosine 3',5'-cyclic monophosphate also failed to mimic the TSH-mediated potentiation. The TSH effect appeared to be mediated by a signaling pathway involving tyrosine kinase because its inhibitors (genistein and herbimycin A) abolished the augmentation completely, and interferon-gamma, a tyrosine kinase activator, imitated the TSH action on submucosal gland. Thus, TSH may be an important regulator of airway fluid secretion.

TGF-beta 1 inhibits the cell proliferation stimulated by IGF-I by blocking the tyrosine phosphorylation of 175kDa substrate

TGF-beta strongly inhibits the mitogenic effect of growth factors such as insulin-like growth-I (IGF-I) on thyroid follicular cells. To study the mechanism of this inhibiting action, the effect of TGF-beta 1 on tyrosine phosphorylation of 175kDa substrate induced by IGF-I was investigated in FRTL-5 thyroid epithelial cells. After treatment with TGF-beta 1, tyrosine phosphorylation of 175kDa protein induced by IGF-I was inhibited in parallel with the degree of the inhibition of cell proliferation. However, TGF-beta 1 had no effect on tyrosine phosphorylation of 120kDa substrate. These results suggest that TGF-beta 1 blocks the mitogenic signal of IGF-I by inhibiting the tyrosine phosphorylation of 175kDa substrate in FRTL-5 cells.