TMEFF2: a new endogenous modulator of the CRH signaling in corticotroph cells

Abstract

We have previously demonstrated that retinoic acid (RA) inhibits CRH signaling in corticotroph cells. In order to elucidate the mechanisms controlling RA action, cDNA microarray analysis was perfomed. From 87 genes regulated by RA in the corticotroph cell line AtT-20, we selected TMEFF2 for further studies. TMEFF2 is a transmembrane protein containing an altered epidermal growth factor (EGF)-like motif, two follistatin-like domains and a cytosolic tail which a putative G protein-activating motif. TMEFF2 is predominantly expressed in brain and prostate and has been implicated in cell signaling, neuronal cell survival and tumor suppression. Although TMEFF2 function is unknown, its structural domains suggest that it may have a role in the regulation of growth factor signalling. Since there is no previous evidence for an involvement of TMEFF2 in Cushing's disease or CRH signalling, we have studied the effects of TMEFF2 on the signal transduction pathway of CRH. Our results show that CRH-signaling is under the inhibitory effects of endogenous TMEFF2. TMEFF2 expression in corticotroph cells inhibits the effects of CRH on the production of intracellular cAMP, CREB and POMC transcriptional activity. Regulation of CRH activity by TMEFF2 does not require the cytoplasmic tail, while deletion of the follistatin and EGF modules abolishes the inhibitory function of TMEFF2. Moreover, a soluble secreted protein containing these three extracellular domains is sufficient for CRH signaling inhibition. TMEFF2-induced inhibition depends on serum components. Furthermore, TMEFF2 seems to regulate non-cannonical Activin/BMP-4 signaling, leading to the inhibition of the PI3K pathway (Akt) and Erk1/2 activation. Whether TMEFF2 acts upstream to Akt to regulate cAMP production and PKA activity remains to be elucidated. We found that TMEFF2 expression in human Cushing's adenoma is reduced as compared to the normal human pituitary, which may indicate that TMEFF2 acts as a tumor suppressor in these adenomas. Consistent with this hypothesis, the overexpression of TMEFF2 decreased cell proliferation in corticotroph cells. Our results also indicate a potential therapeutic use of TMEFF2 or factors that stimulate TMEFF2 activity for the treatment of corticotroph tumors in order to reduce their ACTH secretion and proliferation.