Activation of calcium and cAMP-signalling in GH-secreting pituitary adenomas

Introduction: Pituitary adenomas are relatively common and understanding of their molecular basis is of great interest. The most frequent molecular cause of growth hormone (GH) secreting tumours is an activation of the cAMP signalling pathway by gsp (1) or PRKAR1A (2) mutations. The cAMP – PKA pathway may however be activated also in tumours without these mutations (3) and, together with the second messenger calcium, is pivotal for pituitary somatotroph growth and function (4,5). We therefore studied the activity of the calcium and cAMP pathways and the effects of somatostatin receptor agonists in a series of 11 GH-secreting pituitary adenomas. Methods: GH-secreting adenoma cells were isolated from surgical specimens of 11 pituitary adenomas and cultured for 3–5 days. Intracellular free calcium [Ca²⁺]_i and the response to somatostatin agonists were measured by fura-2 dual wavelength excitation microfluorometry. After treatment for 8 hours with somatostatin agonists, total RNA was isolated and analysed by quantitative RT-PCR for the cAMP target gene ICER and sequencing for gsp mutations. Results: Baseline [Ca²⁺]_i was variable: While some cells showed constant [Ca²⁺]_i of about 50 nM, many cells displayed spontaneous oscillations with peaks up to 100 nM [Ca²⁺]_i. 9 of 11 adenomas, including all gsp-positive tumours, showed a marked drop in elevated [Ca²⁺]_i in response to SRIF-14 or octreotide, but no change in [Ca²⁺]_i peaks triggered by depolarization with 45 mM KCl. All adenoma cells insensitive to octreotide were also resistant to SRIF-14. Most adenomas including gsp-negative cells appeared to have activated cAMP signalling and responded to somatostatin agonists. Inhibition of PKA by H89 not only blocked cAMP signalling but also suppressed [Ca²⁺]_i level and oscillations in some adenomas. Discussion: Calcium and cAMP-signalling appear to be frequently activated in GH-secreting pituitary adenoma cells and are often suppressible by somatostatin receptor agonists in vitro. Correlation of these in vitro data with in vivo effects must await clinical follow-up data. Activated cAMP-signalling found in gsp-negative adenomas suggest that the cAMP pathway may also be involved in the pathogenesis of gsp-negative adenomas. References: [1] Vallar, L, ea Nature 1987: 566–8. [2] Kirschner, LS, ea Nat Genet 2000: 89–92. [3] Bertherat, J, ea Mol Endo 1995: 777–83. [4] Billestrup, N, ea PNAS 1986: 6854–7. [5] Struthers, RS, ea Nature 1991: 622–4.