Effect of cyclic adenosine 3',5'-monophosphate/protein kinase a pathway on markers of cell proliferation in nonfunctioning pituitary adenomas.

CONTEXT: 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.

OBJECTIVE: The objective of this study was to evaluate the effects of cAMP-PKA cascade activation in nonfunctioning pituitary adenomas (NFPA).

DESIGN AND METHODS: 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.

RESULTS: 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).

CONCLUSIONS: 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.