Cyclic AMP, at the hub of the cystic cycle

Half a century after its discovery, new roles for adenosine 3′,5′-cyclic monophosphate (cAMP) continue to be discovered. The article by Belibi et al1.Belibi F.A. Reif G. Wallace D.P. et al.Cyclic AMP promotes growth and secretion in human polycystic kidney epithelial cells.Kidney Int. 2004; 66: 964-973Abstract Full Text Full Text PDF PubMed Scopus (202) Google Scholar in this issue of Kidney International, implicates cAMP in the pathogenesis of autosomal-dominant and recessive polycystic kidney diseases (ADPKD and ARPKD) and provides a target for the pharmacologic treatment of these disorders. Cyclic AMP is a ubiquitous and versatile second messenger. Extracellular ligands bind to G protein–coupled receptors (GPCR) that positively (Gs) or negatively (Gi) regulate closely associated adenylyl cyclases (ACs). Cyclic AMP levels are controlled within specific cellular compartments by the activity of cAMP phosphodiesterases (PDEs), themselves subject to complex regulatory mechanisms. Maximal rates of degradation by PDEs exceed those of synthesis by ACs, pointing to the importance of PDEs in the regulation of cAMP signaling. Cyclic AMP may exit the cells by an energy-dependent transport mechanism, but in most cells efflux does not have a major effect on cAMP levels. Biological effects of cAMP are mediated by protein kinase A (PKA), cyclic nucleotide gated cation channels, and guanine nucleotide exchange factors. PKA is targeted to specific cellular compartments by A kinase–anchoring proteins (AKAPs). The specificity of cAMP signaling is achieved by the extraordinary diversity of ACs, PDEs, PKAs, and AKAPs. It also depends on cell-type specific cross-talk with other signaling systems, such as mitogen-activated protein kinase/extracellular-regulated kinase (MAPK/ERK) cascades that couple growth factor receptors and Ras to cell proliferation. They consist of three protein kinases acting in series, a MAP kinase kinase kinase (MAPKKK), which activates a MAP kinase kinase (MAPKK or MEK), which phosphorylates a MAP kinase (MAPK or ERK). In the ERK1/2 cascade, the Raf family of serine/threonine protein kinases (A-Raf, B-Raf, and Raf-1) function as MAPKKK. Activated PKA catalytic subunits and ERKs can translocate to the nucleus where they phosphorylate transcription factors. Calcium in intracellular compartments ([Ca2+]i) that may be directly affected by polycystic kidney disease (PKD)-causing genetic alterations can regulate positively and negatively cAMP and MAPK signaling. In the last 15 years, Grantham et al have gathered a wealth of evidence pointing to a major cAMP role in cystogenesis by promoting fluid secretion and cell proliferation (reviewed in2.Grantham J.J. Lillian Jean Kaplan International Prize for advancement in the understanding of polycystic kidney disease. Understanding polycystic kidney disease: A systems biology approach.Kidney Int. 2003; 64: 1157-1162Abstract Full Text Full Text PDF PubMed Scopus (68) Google Scholar). Initially, they reported that cAMP enhances the growth of Madin-Darby canine kidney (MDCK) cell subconfluent monolayers, the transepithelial movement of fluid across polarized MDCK cell monolayers on permeable supports, and the enlargement of MDCK and ADPKD microcysts in hydrated collagen gels. Further studies established that fluid secretion by ADPKD epithelia, as well as by normal collecting ducts, is driven by chloride entering the cell via basolateral Na+/Cl− or Na+/K+/2Cl− cotransporters and exiting via apical cAMP-dependent Cl− channels. Then they noted, along with Hanaoka et al3.Hanaoka K. Guggino W. cAMP regulates cell proliferation and cyst formation in autosomal polycystic kidney disease cells.J Am Soc Nephrol. 2000; 11: 1179-1187PubMed Google Scholar, that cAMP activates the ERK cascade and increases the proliferation of ADPKD cells, while exerting inhibitory effects on normal human kidney cortex cells. More recently, their research showed that the treatment of collecting duct (M-1) cells with calcium channel blockers replicates the abnormal proliferative response of the ADPKD cells to cAMP, thus linking the cystic phenotype to the reduction in [Ca2+]i that likely results from disrupting the polycystin pathway. While the precise underlying mechanism remains to be elucidated, their preliminary observations suggest an inhibition of Ca2+ dependent PI3 kinase activity with downstream inhibition of Akt and activation of B-Raf kinase (Yamaguchi et al, ISN 2003 abstract). Because cAMP modulates cellular functions in a cell-specific manner, the cellular composition of the cysts may be relevant to their pathogenesis and the capacity of pharmacologic interventions to influence the course of ADPKD and ARPKD. While there is general agreement that the cysts in ARPKD derive from collecting ducts, the cysts in ADPKD have been thought to arise from all segments of the nephron and collecting duct. A detailed histochemical study of 10 ADPKD kidneys, in which cysts were defined as tubular dilatations at least 1 mm in diameter, challenged this view4.Verani R.R. Silva F.G. Histogenesis of the renal cysts in adult (autosomal dominant) polycystic kidney disease: A histochemical study.Mod Pathol. 1988; 1: 457-463PubMed Google Scholar. In this study, 70% of the cysts stained positively for collecting duct markers, while the remaining 30% were negative for all markers, possibly due to cellular dedifferentiation. Similar results have been obtained in the Pkd2−/WS25 mouse model of ADPKD5.Wu G. D'agati V. Cai Y. et al.Somatic inactivation of PKD2 results in polycystic kidney disease.Cell. 1998; 93: 177-188Abstract Full Text Full Text PDF PubMed Scopus (432) Google Scholar,6.Torres V.E. Wang X. Qian Q. et al.Effective treatment of an orthologous model of autosomal dominant polycystic kidney disease.Nature Med. 2004; 10: 363-364Crossref PubMed Scopus (358) Google Scholar. Other studies using aquaporin (AQP) antibodies showed that 30% of the cysts, but only 11% of large cysts, were positive for AQP1 (presumably from proximal tubular origin)7.Devuyst O. Burrow C.R. Smith B.L. et al.Expression of aquaporins-1 and -2 during nephrogenesis and in autosomal dominant polycystic kidney disease.Am J Physiol. 1996; 271: F169-183PubMed Google Scholar. Because these observations were made mostly in kidneys with advanced or end-stage renal disease, it is possible that obstruction and acquired renal cystic disease could account for the observed proximal tubular dilatations and small cysts. Therefore, it seems likely that in both cases, ARPKD and ADPKD, cysts are predominantly of collecting duct origin, at least at early stages. The markedly higher cAMP production in response to vasopressin as compared to parathyroid hormone, in the article by Belibi et al, is consistent with this interpretation1.Belibi F.A. Reif G. Wallace D.P. et al.Cyclic AMP promotes growth and secretion in human polycystic kidney epithelial cells.Kidney Int. 2004; 66: 964-973Abstract Full Text Full Text PDF PubMed Scopus (202) Google Scholar. Recent publications suggest that an increased renal accmulation of cAMP is a feature common to several rodent models of PKD, including the pcy mouse, the PCK rat, and the Pkd2−/WS25 mouse6.Torres V.E. Wang X. Qian Q. et al.Effective treatment of an orthologous model of autosomal dominant polycystic kidney disease.Nature Med. 2004; 10: 363-364Crossref PubMed Scopus (358) Google Scholar, 8.Yamaguchi T. Nagao S. Kasahara M. et al.Renal accmulation and excretion of cyclic adenosine monophosphate in a murine model of slowly progressive polycystic kidney disease.Am J Kidney Dis. 1997; 30: 703-709Abstract Full Text PDF PubMed Scopus (103) Google Scholar, 9.Gattone V.H. Wang X. Harris P.C. et al.Inhibition of renal cystic disease development and progression by a vasopressin V2 receptor antagonist.Nat Med. 2003; 9: 1323-1326Crossref PubMed Scopus (483) Google Scholar. This is surprising because previous studies have shown reduced cAMP accmulation in cells cultured from cpk kidneys10.Marfella-Scivittaro C. Quinones A. Orellana S.A. cAMP-dependent protein kinase and proliferation differ in normal and polycystic kidney epithelia.Am J Physiol Cell Physiol. 2002; 282: C693-707Crossref PubMed Scopus (19) Google Scholar and adenylyl cyclase hyporesponsiveness to agonist stimulation in cultured cells and cysts freshly isolated from human ADPKD kidneys11.Wilson P.D. Schrier R.W. Breckon R.D. et al.A new method for studying human polycystic kidney disease epithelia in culture.Kidney Int. 1986; 30: 371-378Abstract Full Text PDF PubMed Scopus (86) Google Scholar. In this issue of Kidney International, Belibi et al1.Belibi F.A. Reif G. Wallace D.P. et al.Cyclic AMP promotes growth and secretion in human polycystic kidney epithelial cells.Kidney Int. 2004; 66: 964-973Abstract Full Text Full Text PDF PubMed Scopus (202) Google Scholar have rigorously evaluated the responses of human ADPKD and ARPKD cyst epithelial cells to physiologically important adenylyl cyclase agonists. The rank order of responses was identical for both types of cells: epinephrine> desmopressin> adenosine> prostaglandin E2> parathyroid hormone, with effects ranging from 5.7% (ARPKD cells, PTH) to 95.8% (ADPKD cells, epinephrine) of maximal forskolin-induced stimulation. While these results demonstrate that these agonists are capable to stimulate cAMP production in target cells from human renal cysts, the relative strengths of stimulation, as the authors recognize, may not hold in situ. A previous study has shown that adenylyl cyclase stimulation by epinephrine acting on β-adrenergic receptors was markedly enhanced, while the adenylyl cyclase inhibition by epinephrine acting on α2-adrenergic receptors was markedly blunted in cultured as compared to freshly isolated inner medullary collecting ducts12.Yasuda G. Jeffries W.B. Regulation of cAMP production in initial and terminal inner medullary collecting ducts.Kidney Int. 1998; 54: 80-86Abstract Full Text Full Text PDF PubMed Scopus (30) Google Scholar. In freshly isolated collecting ducts the changes in cAMP production induced by β-adrenergic and other adenylyl cyclase agonists are very small compared to those induced by physiologic concentrations of vasopressin. Furthermore, agonists in vivo do not work in isolation; agents, that in isolation stimulate adenylyl cyclase in vitro, may have an inhibitory effect in vivo by interacting with other more potent agonists13.Maeda Y. Terada Y. Nonoguchi H. et al.Hormone and autacoid regulation of cAMP production in rat IMCD subsegments.Am J Physiol. 1992; 263: F319-F327PubMed Google Scholar. In addition, the maximally effective agonist concentrations used by Belibi et al may be different from those that are physiologically relevant. Among the cAMP agonists tested in their study, the novel cyst-activating factor (CAF), present in the renal cyst fluids, appears to be particularly potent. Its precise chemical structure and its role in early and advanced polycystic kidney disease remain to be elucidated. The mechanism underlying the increased levels of cAMP in polycystic kidneys has not been established, but a number of hypotheses can be considered. The PC1 C-terminal tail binds heterotrimeric G proteins and may act as a GPCR, which, when activated by ligand binding or extracellular matrix proteins, initiates Gi-mediated signaling14.Parnell S.C. Magenheimer B.S. Maser R.L. et al.The polycystic kidney disease-1 protein, polycystin-1, binds and activates heterotrimeric G-proteins in vitro.Biochem Biophys Res Commun. 1998; 251: 625-631Crossref PubMed Scopus (185) Google Scholar. By this mechanism PC1 might regulate intracellular cAMP, and a disruption of its function could result in the elevated levels. A reduction in intracellular calcium due to disruption of the polycystin pathway could result in the stimulation of calcium inhibitable AC615.Helies-Toussaint C. Aarab L. Gasc J.-M. et al.Cellular localization of type 5 and type 6 ACs in collecting duct and regularion of cAMP systhesis.Am J Physiol Renal Physiol. 2000; 279: F185-194PubMed Google Scholar and the inhibition of calcium/calmodulin dependent PDE116.Kusano E. Murayama N. Werness J.L. et al.Effects of calcium on the vasopressin-sensitive cAMP metabolism in medullary tubules.Am J Physiol. 1985; 249: F956-966PubMed Google Scholar. AC6 is the main AC isoform, and PDE1, along with PDE4, is the main PDE isoform in the collecting duct principal cells. Some PDE4 isozymes are phosphorylated and inhibited by ERK217.Conti M. Richter W. Mehats C. et al.Cyclic AMP-specific PDE4 phosphodiesterases as critical components of cyclic AMP signaling.J Biol Chem. 2003; 278: 5493-5496Crossref PubMed Scopus (382) Google Scholar, which is known to be activated in PKD. Finally, abnormal AQP2 traffic due to altered [Ca2+]i homeostasis or disrupted medullary architecture due to cysts could cause the concentration defect common to all forms of PKD and explain the increased cAMP levels as a maladaptive response attempting to compensate for this defect. Consistent with this hypothesis are the increased circulating vasopressin levels in human ADPKD18.Michalski A. Grzeszczak W. The effect of hypervolemia on electrolyte level and and level of volume regulating hormones in patients with autosomal dominant polycystic kidney disease.Pol Arch Med Wewn. 1996; 96: 329-343PubMed Google Scholar and the renal up-regulation of vasopressin V2 receptor and AQP2 mRNA in several animal models of polycystic kidney disease, including the cpk, pcy, and Pkd2−/WS25 mice, and the PCK rat6.Torres V.E. Wang X. Qian Q. et al.Effective treatment of an orthologous model of autosomal dominant polycystic kidney disease.Nature Med. 2004; 10: 363-364Crossref PubMed Scopus (358) Google Scholar, 9.Gattone V.H. Wang X. Harris P.C. et al.Inhibition of renal cystic disease development and progression by a vasopressin V2 receptor antagonist.Nat Med. 2003; 9: 1323-1326Crossref PubMed Scopus (483) Google Scholar, 19.Gattone V.H. Maser R.L. Tian C. et al.Developmental expression of urine concentration-associated genes and their altered expression in murine infantile-type polycystic kidney disease.Develop Gen. 1999; 24: 309-318Crossref PubMed Scopus (110) Google Scholar. Observations that the collecting duct is the predominant site of cystogenesis in ARPKD and ADPKD, that vasopressin (via V2 receptors) is the major adenylyl cyclase agonist in principal cells, and that cAMP promotes cystogenesis, have provided a strong rationale for treating animal models of PKD with vasopressin V2 receptor (VPV2R) antagonists6.Torres V.E. Wang X. Qian Q. et al.Effective treatment of an orthologous model of autosomal dominant polycystic kidney disease.Nature Med. 2004; 10: 363-364Crossref PubMed Scopus (358) Google Scholar,9.Gattone V.H. Wang X. Harris P.C. et al.Inhibition of renal cystic disease development and progression by a vasopressin V2 receptor antagonist.Nat Med. 2003; 9: 1323-1326Crossref PubMed Scopus (483) Google Scholar. The administration of the VPV2R antagonist OPC31260 has recently been shown to lower renal cAMP and markedly inhibit disease development in models of ARPKD (PCK rat), ADPKD (Pkd2WS25/− mouse), and adolescent nephronophthisis (pcy mouse). Administration of OPC31260 when the disease is already established either halts progression (PCK) or causes regression (pcy) of disease. OPC31260 has also been effective in the cpk mouse. The renal selectivity (VPV2R expression restricted to collecting duct principal cells and endothelial cells) and apparent safety of these drugs in preclinical and clinical studies (for disorders of water retention such as congestive heart failure and cirrhosis) makes them particularly attractive. Clinical trials for polycystic kidney disease are currently under consideration.