1. ¹Endocrine Unit, Department of Medical Sciences, University of Milan, Fondazione Ospedale Maggiore IRCCS, Milan, Italy
2. ²Unit of Melanoma Genetics, Istituto Nazionale per lo Studio e la Cura dei Tumori IRCCS, Milan, Italy
3. ³Istituto Clinico Humanitas IRCCS, Department of Immunology and Inflammation, Rozzano, Milan, Italy
4. ⁴Pathology Unit, Department of Medicine, Surgery and Dentistry, AO San Paolo and Fondazione Ospedale Maggiore IRCCS, Milan, Italy

Correspondence: Dr A Spada, Endocrine Unit, Department of Medical Science, University of Milan, Fondazione, Policlinico, Ospedale Maggiore IRCCS, Via F Sforza, 35, Milan 20122, Italy. E-mail: anna.spada@unimi.it

Received 16 March 2007; Revised 29 August 2007; Accepted 3 September 2007; Published online 1 October 2007.

Abstract

The cAMP–protein kinase A (PKA) pathway is the major signal transduction pathway involved in melanocyte-stimulating hormone receptor-mediated signaling and melanin production, whereas its role in the control of melanocyte proliferation is still controversial. In this study, we evaluated the effects of selective activation of the different PKA regulatory subunits type 1A (R1A) and type 2B (R2B) on melanocyte proliferation. Immunohistochemistry demonstrated that normal melanocytes lacked R1A protein whereas this subunit was highly expressed in all human melanomas studied (N=20) and in six human melanoma cell lines. Pharmacological activation of the R2 subunits by the cAMP analogue 8-Cl-cAMP inhibited proliferation and increased caspase-3 activity by 68.7710.5 and 729% respectively, in all cell lines with the exception of the only p53-mutated one. Similar effects were obtained by activating R2 subunits with other analogues and by silencing R1A expression. The antiproliferative and proapoptotic effects of 8-Cl-cAMP were comparable to those observed with commonly used antitumoral drugs. Moreover, 8-Cl-cAMP potentiated the effects of these drugs on both cell proliferation and caspase-3 activity. In conclusion, this study first reports that human melanomas are characterized by a high R1/R2 ratio and that pharmacological and genetic manipulations able to revert this unbalanced expression cause significant antiproliferative and proapoptotic effects in melanoma cells.