Effect of Cyclic Adenosine 3′,5′-Monophosphate/Protein Kinase A Pathway on Markers of Cell Proliferation in Nonfunctioning Pituitary Adenomas

Abstract

Alterations in cAMP signaling have been identified as a cause of endocrine neoplasia. In particular, activating mutations of the G(s)alpha gene and protein kinase A (PKA) overactivity due to low expression of PKA regulatory subunit 1A (R1A) have been implicated in somatotroph proliferation. The objective of this study was to evaluate the effects of cAMP-PKA cascade activation in nonfunctioning pituitary adenomas (NFPA). By immunohistochemistry, R1A, R2A, and R2B expression was evaluated in cells obtained from eight surgically removed NFPA positive for gonadotropins. Cyclin D1 expression and ERK1/2 activity were analyzed under basal conditions and after cAMP-PKA cascade activation. Immunohistochemistry studies demonstrated a low R1/R2 ratio in all NFPA. Additional unbalance of R1/R2 ratio by 8-chloroadenosine cAMP (8-Cl-cAMP) and direct adenylyl cyclase stimulation by forskolin did not increase cyclin D1 expression or ERK1/2 activity in five NFPA (group 1), but even caused 74 +/- 15% and 85 +/- 13% inhibitions of cyclin D1 and ERK1/2 activity, respectively, in the remaining NFPA (group 2). Moreover, in group 2, PKA blockade by the specific inhibitor PKI increased cyclin D1 expression (96 +/- 25% over basal) and ERK1/2 activity (116 +/- 28% over basal). These data show that in contrast with what was previously observed in transformed somatotrophs, activation of the cAMP-PKA pathway did not generate proliferative signals in tumoral cells of the gonadotroph lineage, and in a subset of tumors even exerted a tonic inhibitory effect, thus confirming a different role for the cAMP-mediated pathway in promoting proliferation in the pituitary.