Regulation and actions of Smad7 in the modulation of activin, inhibin, andtransforming growth factor-beta signaling in anterior pituitary cells

Activins and transforming growth factor-beta (TGF beta) are crucial autocri ne, paracrine, and endocrine modulators of anterior pituitary function. Act ivins regulate most pituitary cells and lactotropes are targets of TGF beta . Smad2 and Smad3 are two cellular mediators of activin/TGF beta signaling, whereas Smad7 is as an inducible, negative modulator of the pathway. This study was undertaken to evaluate Smad7 regulation in the pituitary. Activin A rapidly and transiently increased Smad7 messenger RNA (mRNA) levels of r at anterior pituitary (RAP), clonal gonadotrope (alpha T3-1 and L beta T2), and corticotrope (AtT20) cells with an EC50 of 0.1-0.2 nM. In RAP cells, a ctivin A or TGF beta1 had equivalent effects that were additive. Follistati n, known to bind and inactivate activins, prevented Smad7 induction by acti vin. Inhibin A partially antagonized activin A, perhaps reflecting gonadotr ope-selective actions. This antagonism was also evident with alpha T3-1 and L beta T2 gonadotropes. Forskolin had no measurable effect in RAP cells, b ut increased Smad7 mRNA levels in alpha T3-1 cells and decreased them in L beta T2 cells. Transient transfection of Smad7 along with 3TPLux, an activi n/TGF beta -responsive reporter, blocked activin-mediated promoter activati on in alpha T3-1 and AtT20 cells. In alpha T3-1 cells, which express endoge nous follistatin mRNA, a follistatin-luciferase reporter, rFS(rin3)-Luc, wa s transcriptionally activated by activin A, or when cotransfected with a co nstitutively active ActRIB [Alk4(T>D)], Smad2, or Smad3. Smad7 blocked rFS( rin3)-Luc activation by activin A or Alk4(T>D). Together, these results poi nt to a role of Smad7 in modulating activity/TGF beta signaling in the pitu itary.