Inactivation of the Carney Complex Gene 1 (Protein Kinase A Regulatory Subunit 1A) Inhibits SMAD3 Expression and TGFβ-Stimulated Apoptosis in Adrenocortical Cells

The cyclic AMP signaling pathway can be altered at multiple levels in endocrine tumors. Its central component is the protein kinase A (PKA). Carney complex (CNC) is a hereditary multiple neoplasia syndrome resulting from inactivating mutations of the gene encoding the PKA type I α regulatory subunit (PRKAR1A). Primary pigmented nodular adrenocortical disease is the most frequent endocrine tumor of CNC. Transforming growth factor β (TGFβ) regulates adrenal cortex physiology and signals through SMAD2/3. We used an interference approach to test the effects of PRKAR1A inactivation on PKA and TGFβ pathways and on apoptosis in adrenocortical cells. PRKAR1A silencing stimulates PKA activity and increases transcriptional activity of a PKA reporter construct and expression of the endogenous PKA target, NR4A2, under basal conditions or after forskolin stimulation. PRKAR1A inactivation also decreased SMAD3 mRNA and protein levels via PKA, altering the cellular response to TGFβ. SMAD3 expression was also inhibited by adrenocorticorticotropic hormone in the mouse adrenal gland and by forskolin in H295R cells. TGFβ stimulates apoptosis in H295R cells, and this effect was counteracted by PRKAR1A inactivation. PRKAR1A silencing decreased the percentage of apoptotic cells and the cleavage of apoptosis mediators [caspase-3, poly(ADP-ribose) polymerase, and lamin A/C]. Inactivating mutations of PRKAR1A observed in adrenocortical tumors alter SMAD3, leading to resistance to TGFβ-induced apoptosis. This cross-talk between the PKA and the TGFβ signaling pathways reveals a new mechanism of endocrine tumorigenesis. [Cancer Res 2009;69(18):7278–84]