179. CHARACTERISATION OF THE IN VITRO FUNCTION OF ACTIVIN AC

Abstract

Activins are members of the TGF-β superfamily that signal via type II and type I receptor subunits and intracellular Smads¹. Activin A stimulates FSH release from the pituitary and is also a potent growth and differentiation factor in many physiological systems². Over-expression of the activin-β_C subunit in vitro leads to a reduction in activin A and an increase in activin AC³. Transgenic mice over-expressing activin-β_C show decreased circulating activin A, implying that activin AC may also be formed in vivo⁴. Recently recombinant activin AC has become available, therefore this study examines the in vitro function and mechanism of action of activin AC. Activin AC stimulates FSH release in LβT2 cells and is a negative growth regulator in LNCaP cells, however the potency of activin AC is 8-10 fold less than activin A. Incubation of LNCaP cells with activin receptor antibodies (ALK4, ActRIIA, ActRIIB) abolishes the growth inhibitory effects of activin AC. Activin AC binds to ActRIIB, however a 20-30 fold decrease in both the potency and affinity of activin AC is evident compared to activin A. In addition, activin AC increases Smad-2 phosphorylation. These results indicate activin AC utilises the same receptors and intracellular signalling molecules as activin A. The activin A antagonists, follistatin and activin C⁴, also antagonise the growth inhibitory effects of activin AC and reduce Smad-2 phosphorylation and Smad-4 expression. This study shows for the first time that the in vitro function of activin AC is similar to activin A, albeit at a lower potency and provides the impetus to determine the physiological role of activin AC in vivo.