Smad proteins are targets of transforming growth factor beta1 in immortalised gonadotrophin-releasing hormone releasing neurones

Transforming growth factor beta (TGFbeta) is one of the growth factors involved in the neuroendocrine control of the gonadotrophin-releasing hormone (GnRH) neurones. It is produced and released by the astrocytes surrounding GnRH neurones and directly controls their secretory activity. TGFbeta signalling is based on a complex of two receptors that transduces the signal through peculiar intracellular substrates, the Smad proteins, which, upon activation, move into the nucleus, and modify the transcription of TGFbeta responsive genes. The present study aimed to verify whether TGFbeta1 is able to regulate the Smad pathway in GT1-1 cells (i.e. an immortalised neuronal cell line releasing GnRH). We show that: (i) GT1-1 cells express Smad 2, 3, 4, and 7; (ii) TGFbeta1 enhances the phosphorylation of Smad 2 and 3 at short times of exposure (15-30 min); (iii) TGFbeta1 induces the synthesis of the inhibitory Smad 7 at longer times (60-120-240 min); (iv) the conditioned medium of type 1 astrocytes enhances the phosphorylation of Smad 2 and 3 in GT1-1 cells and a TGFbeta1 neutralising antibody counteracts this effect. The results indicate that Smads are targets of TGFbeta1 and that astrocytes are able to modulate Smads proteins in GT1-1 cells through the release of TGFbeta1. Taken together, the data provide new evidence that glial cells are important regulators of the GnRH neuronal activity.