Endocrine DOI 10.1007/s12020-015-0621-y
REVIEW
Phosphodiesterase 8B and cyclic AMP signaling in the adrenal cortex Leticia Ferro Leal1 • Eva Szarek1 • Fabio Faucz1 • Constantine A. Stratakis1
Received: 2 March 2015 / Accepted: 27 April 2015 Ó Springer Science+Business Media New York (outside the USA) 2015
Abstract Bilateral adrenocortical hyperplasia (BAH) in humans and mice has been recently linked to phosphodiesterase (PDE) 8B (PDE8B) and 11 (PDE11A) defects. These findings have followed the discovery that defects of primary genes of the cyclic monophosphatase (cAMP) signaling pathway, such as guanine nucleotide binding alpha subunit and PRKAR1A, are involved in the pathogenesis of BAH in humans; complete absence of Prkar1a in the adrenal cortex of mice also led to pathology that mimicked the human disease. Here, we review the most recent findings in human and mouse studies on PDE8B, a cAMP-specific PDE that appears to be highly expressed in the adrenal cortex and whose deficiency may underlie predisposition to BAH and possibly other human diseases. Keywords PDE8B PRKAR1A PRKACA cAMP signaling Adrenal
Introduction Adrenal lesions and/or related adrenal hyperplasias have been linked to abnormalities in the adenosine 30 ,50 -cyclic monophosphatase (cAMP)/Protein Kinase A (PKA) signaling pathway, of which several of the phosphodiesterases (PDE) family members play an important regulatory role & Eva Szarek [email protected] 1
Section on Endocrinology and Genetics, Program on Developmental Endocrinology & Genetics (PDEGEN) Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), National Institutes of Health, 10 Center Drive, Building 10, NIH-Clinical Research Center, Room 1-3216, Bethesda, MD 20892, USA
[1]. PDEs exert their functions not only via the cAMPdependent protein kinase (PKA), but also via the cGMPdependent protein kinase (PKG), as well as cyclic nucleotide-gated ion channels (CNGCs). PDEs can be cAMPspecific only, cGMP-specific only, or have dual specificity towards cAMP and cGMP. Any abnormalities in this pathway and/or PDE will result in an imbalance of cAMP/ cGMP. Among the PDEs that are important for adrenocortical function, the relatively recently identified PDE8 family, consisting of two molecules (PDE8A and PDEB), is among the most prominent. PDE8B is highly expressed in the adrenal cortex and a single mutation of this gene was first identified in a patient with a form of bilateral adrenocortical hyperplasia (BAH) [2]: the p.H305P missense mutation was described in a young girl with isolated micronodular adrenocortical disease (iMAD) [2]. Functional studies showed high levels of cAMP in HEK293 cells transfected with the mutant gene [3]. Subsequently, other novel variants of the PDE8B gene were described in patients with other forms of BAH, mostly primary macronodular adrenal hyperplasia (PMAH) [4] that appear to overall decrease PDE8 enzymatic activity in breaking down cAMP. This is consistent with the overall decreased PDE activity that was found in various cortisol-producing lesions [5]. cAMP-dependent protein kinase, or PKA, is a holoenzyme consisting of a heterotetramer formed by regulatory (RIa, RIb, RIIa, RIIb) and catalytic (Ca, Cb, Cc, PRKX) subunits. Inactivation of 1-a regulatory (RIa) subunit of PKA, encoded by the PRKAR1A gene, is associated with Carney Complex (CNC) and primary pigmented nodular adrenocortical disease (PPNAD), leading to Cushing syndrome (CS) [6–8]. Inactivation of PRKAR1A leads to an increase in PKA activity due to decreased inhibition of
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PKA’s main catalytic subunit (Ca) that is encoded by the PRKACA gene [9]. A gain of function mutation in PRKACA was also recently associated with increased cAMP signaling and high PKA activity in adrenal CS [10], suggesting that increased activity of Ca is associated with abnormalities specific to adrenocortical tissue, whereas inactivating mutations of the RIa result in the full spectrum of lesions, in many tissues, associated with CNC. The guanine nucleotide binding alpha subunit 1 (GNAS1) gene codes for the Gsa protein, which activates cAMP/PKA signaling by activating adenylate cyclase. Activating mutations of the GNAS1 gene can lead to high levels of cAMP, triggering cell proliferation prompting nodule formation and autonomous cortisol [11]. Cases of BAH with somatic mutations of the GNAS1 gene, in the context of McCune–Albright syndrome or sporadically (in PMAH or adrenal adenomas), have been reported [12–14]. Together, these data suggest the importance of cAMP signaling, PDEs, and PKA in predisposition and/or formation of adrenocortical tumors. Here, we present a brief overview on PKA and PDE8B defects and adrenal function and tumors (Fig. 1).
Fig. 1 Cyclic AMP-dependent protein kinase (PKA) controls steroidogenesis: upon binding of a ligand hormone to membrane G proteincoupled receptors (GPCRs), Gsa activates adenylyl cyclase (AC) to generate cAMP from ATP. The cAMP/PKA pathway then promotes steroidogenesis via acute PKA activation of cholesterol import into the mitochondria (not shown) and activation of steroidogenic enzymes. Abbreviations: cAMP, Cyclic adenosine monophosphate; PKA, Protein
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PDE8B: human and mouse studies PDE8 enzymes hydrolyze cAMP with very high affinity; there are two proteins in the family coded by two distinct genes, PDE8A and PDE8B. PDE8A and PDE8B have a relatively restricted tissue expression pattern compared with most other PDEs; however, they are expressed highly in steroidogenic tissues such as the adrenal, ovaries and the testis, and most other endocrine glands, including the pituitary, thyroid, and pancreas [15–17]. PDE8B has been shown to regulate adrenocorticotropinstimulated adrenal zona fasciculata steroidogenesis [18]. A PDE8B missense mutation (p.H305P) was described in a young girl with iMAD [3]. Functional studies showed higher cAMP levels after transfection with the PDE8B mutant than with wild-type PDE8B, indicating an impaired ability of the mutant protein to degrade cAMP [3]. PDE8B expression in the adrenal cortex was also the highest among other PDE genes that bind cAMP, such as PDE1A, PDE4A, PDE4B, PDE4C, PDE4D, PDE7A, and PDE9A [2, 19]. Additional PDE8B mutations were described in patients with PMAH [4]. PMAH and other cortisol-
Kinase A; R, Protein Kinase A regulatory subunits; C, Protein Kinase A catalytic subunits; Ga, G protein alpha subunit; b, G protein beta subunit; c, G protein gamma subunit; PDE8B, phosphodiesterase family 8 molecule B; PRKACA, cAMP-dependent protein kinase A alpha catalytic subunit; PRKAR1A, cAMP-dependent protein kinase A 1-a regulatory subunit
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producing lesions are associated with high cAMP levels and overall decreased PDE activity [5]. Both PDE8A and PDE8B are expressed in human and mouse adrenals. Pde8a in the mouse appears to be expressed from a small population of zona fasciculata cells that lie adjacent to zona glomerulosa. Pde8b, on the other hand, is highly expressed across zona fasciculata [18]. Tsai et al. used the PDE8-selective inhibitor (PF-04957325) on mouse adrenal cells and found that this was sufficient to potentiate basal steroid secretion, while treatment with the non-selective PDE inhibitor, IBMX, resulted in little to no effect on basal adrenal steroidogenesis. Inhibition of Pde8 resulted in an increase of phosphorylation of a number of PKA substrates, suggesting that Pde8b inhibition has immediate effects on PKA activation [18]. Several key molecules associated with steroidogenesis, such as StAR and p450scc (CYP11A), were increased as a result of Pde8b ablation [18]. Mice harboring a loss-of-function Pde8b mutation had higher urinary corticosterone levels, even with lower circulating ACTH [18]. In these mice, LacZ replaced exon 14–15 of the Pde8b gene, thus leading to incomplete ablation of the protein. Interestingly, unlike humans with PDE8B mutations [3], these mice did not develop significant adrenal hyperplasia, and the overall size of the glands was normal [18]. This variance could be due to additional molecular defects that may be required for adrenal hyperplasia to develop, or that these mice were studied at a younger age than what would have been required for them to develop hyperplasia. PRKAR1A: human and mouse studies Tumor-specific loss of heterozygosity (LOH) involving the 17q22-24 chromosomal region harboring PRKAR1A and PRKAR1A-inactivating mutations is observed in tumors of patients with CNC or isolated PPNAD [8, 20], suggesting that PRKAR1A is a likely tumor suppressor gene (TSG) [21]. Heterozygous mutations of PRKAR1A are found in more than 60 % of patients with CNC, and up to 80 % of CNC cases present CS due to PPNAD [6, 7]. Haploinsufficient and tissue-specific knockout (KO) mouse models were engineered to demonstrate Prkar1a tumor suppressor activity and to better understand the mechanisms by which PRKAR1A mutations cause CNC. [22, 23]. Homozygous Prkar1a KO mice could not survive beyond the 10.5 embryonic day of development [22, 23]. Prkar1a-haploinsufficient (Prkar1a?/-) mice, on the other hand, were born normally and survived to a late age; however, they developed tumors in a spectrum of endocrine and non-endocrine tissues that are cAMP responsive; tissue-specific complete ablation of Prkar1a expanded the tissues that could be affected or led to tumors
in the same tissues at an earlier age and with significantly more severe pathology [24–26]. Since neither the Prkar1a KO embryos nor the haploinsufficient mice had any detectable adrenocortical pathology, an adrenal cortex-specific Prkar1a KO mouse was generated using the Akr1b7-Cre mouse line [27]. These animals (referred to as AdKO: Adrenal specific KO) developed early signs of pituitary-independent CS and pathology consistent with defective adrenocortical zonal cell differentiation and hyperplasia. Eventually tumors formed from what appeared to be improper maintenance and expansion of adrenocortical cells with fetal characteristics. These data provided the first evidence that the absence of the R1a subunit of PKA in vivo is sufficient to induce the autonomous adrenal hyper-activity and bilateral hyperplasia observed in human PPNAD [27]. The human and mouse LOH studies and the modeling of CNC tumor formation in transgenic rodents confirmed that PRKAR1A most likely acts as a TSG, albeit a relatively weak one. This was best shown in a study where Prkar1ainsufficient mice were mated with mice that were heterozygous for defects in potent TSGs, such as Tp53 and Rb1. Both crosses led to much more aggressive phenotypes than either the Tp53 or Rb1 heterozygous KO mice had [28]. This experiment showed that Prkar1a deficiency can enhance tumor formation induced by the haploinsufficiency of other, stronger, TSGs. PRKACA: human and mouse studies Overexpression of cAMP-dependent protein kinase alpha catalytic (Ca) subunit (PRKACA) or beta catalytic (Cb) subunit (PRKACB) in cell culture results in significant compensation by an increase in PRKAR1A protein [29]. However, in most tissues other than brain, kidney, and spleen (where there is some Cb expression), PKA activity appears to be Ca dependent [30]. In addition, PRKAR1A haploinsufficiency or expression of a defective (in its binding affinity for the catalytic subunit) PRKAR1A protein leads to increased PRKACA activity [9]. Thus, the recent finding that genetic activation of the PRKACA gene leads to tumor formation was not surprising [31]. However, given the ubiquitous expression of Ca, what was not expected was that germline amplification of the PRKACA resulted in only adrenocortical tumors [32]. Somatic PRKACA mutations resulted in single, adrenocortical, cortisol-producing adenomas; these mutations led to Ca resistance to physiologic suppression by regulatory subunits and resulted in increased PKA activity [10], consistent with older data [33]. These mutations were associated with higher levels of cortisol after dexamethasone testing, with a smaller size of the adrenal mass size compared with non-mutated samples [34, 35].
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Prkaca-/- mice are viable in only very small numbers and in backgrounds other than C57BL/6. Prkaca-/- mice that survive are small, infertile, and die by the 3rd month of life [30]. A cross between Prkaca-/- and Prkar1a-/- mice showed that the absence of Ca can abrogate some of the early heart development effects of PRKAR1A ablation [22]. On the other hand, Prkaca-/- Prkar1a?/- mice showed several bone lesions, such as osteochondromyxoma, cartilaginous metaplasia, chondromas, and osteochondrodysplasia; 100 % of the animals had developed these lesions by the time they were 9 months old. Prkaca-/Prkar1a?/- mice developed an increase in number of these lesions and earlier when compared with Prkar1a?/- mice [36]. These data show that despite ubiquitous Ca expression and PRKACA’s importance for total PKA activity in all tissues (as shown by mouse studies), the human adrenal cortex is exquisitely sensitive to Ca activation either by germline or somatic defects. Thus, small changes in cAMP-dependent PKA activity caused by germline mutations leading to either defects of components of the PKA tetramer (i.e., PRKAR1A) or regulators of cAMP levels (i.e., PDEs) are expected to affect the adrenal cortex without necessarily affecting other tissues, despite the ubiquitous presence and significance of the PKA enzyme.
Conclusion Numerous human and mouse studies point to PDE8B’s involvement in adrenocortical function and regulation of steroidogenesis although the exact mechanism and mode of action remain unclear. The human adrenal cortex appears to be very sensitive to small changes of cAMP levels and, consequently, dosage of cAMP-dependent PKA activity, as demonstrated by both PRKAR1A mutations and PRKACA defects leading, respectively, to PPNAD and various adrenocortical pathologies. With its high expression in the adrenal cortex, highest among the cAMP-specific PDEs, could PDE8B be the PDE that finely titrates cAMP levels in this tissue? Additional studies are needed to find the answer to this question, but the data to date suggest that this certainly could be the case. Acknowledgment This work was supported by the Intramural Research Program of the Eunice Kennedy Shriver National Institute of Child Health and Human Development. No research was conducted involving human articipants and/or animals (we only reviewed the relevant literature) in the present report. Conflict of interest of interest.
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The authors declare that they have no conflict
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Phosphodiesterase 8B and cyclic AMP signaling in the adrenal cortex Leticia Ferro Leal1 • Eva Szarek1 • Fabio Faucz1 • Constantine A. Stratakis1
Received: 2 March 2015 / Accepted: 27 April 2015 Ó Springer Science+Business Media New York (outside the USA) 2015
Abstract Bilateral adrenocortical hyperplasia (BAH) in humans and mice has been recently linked to phosphodiesterase (PDE) 8B (PDE8B) and 11 (PDE11A) defects. These findings have followed the discovery that defects of primary genes of the cyclic monophosphatase (cAMP) signaling pathway, such as guanine nucleotide binding alpha subunit and PRKAR1A, are involved in the pathogenesis of BAH in humans; complete absence of Prkar1a in the adrenal cortex of mice also led to pathology that mimicked the human disease. Here, we review the most recent findings in human and mouse studies on PDE8B, a cAMP-specific PDE that appears to be highly expressed in the adrenal cortex and whose deficiency may underlie predisposition to BAH and possibly other human diseases. Keywords PDE8B PRKAR1A PRKACA cAMP signaling Adrenal
Introduction Adrenal lesions and/or related adrenal hyperplasias have been linked to abnormalities in the adenosine 30 ,50 -cyclic monophosphatase (cAMP)/Protein Kinase A (PKA) signaling pathway, of which several of the phosphodiesterases (PDE) family members play an important regulatory role & Eva Szarek [email protected] 1
Section on Endocrinology and Genetics, Program on Developmental Endocrinology & Genetics (PDEGEN) Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), National Institutes of Health, 10 Center Drive, Building 10, NIH-Clinical Research Center, Room 1-3216, Bethesda, MD 20892, USA
[1]. PDEs exert their functions not only via the cAMPdependent protein kinase (PKA), but also via the cGMPdependent protein kinase (PKG), as well as cyclic nucleotide-gated ion channels (CNGCs). PDEs can be cAMPspecific only, cGMP-specific only, or have dual specificity towards cAMP and cGMP. Any abnormalities in this pathway and/or PDE will result in an imbalance of cAMP/ cGMP. Among the PDEs that are important for adrenocortical function, the relatively recently identified PDE8 family, consisting of two molecules (PDE8A and PDEB), is among the most prominent. PDE8B is highly expressed in the adrenal cortex and a single mutation of this gene was first identified in a patient with a form of bilateral adrenocortical hyperplasia (BAH) [2]: the p.H305P missense mutation was described in a young girl with isolated micronodular adrenocortical disease (iMAD) [2]. Functional studies showed high levels of cAMP in HEK293 cells transfected with the mutant gene [3]. Subsequently, other novel variants of the PDE8B gene were described in patients with other forms of BAH, mostly primary macronodular adrenal hyperplasia (PMAH) [4] that appear to overall decrease PDE8 enzymatic activity in breaking down cAMP. This is consistent with the overall decreased PDE activity that was found in various cortisol-producing lesions [5]. cAMP-dependent protein kinase, or PKA, is a holoenzyme consisting of a heterotetramer formed by regulatory (RIa, RIb, RIIa, RIIb) and catalytic (Ca, Cb, Cc, PRKX) subunits. Inactivation of 1-a regulatory (RIa) subunit of PKA, encoded by the PRKAR1A gene, is associated with Carney Complex (CNC) and primary pigmented nodular adrenocortical disease (PPNAD), leading to Cushing syndrome (CS) [6–8]. Inactivation of PRKAR1A leads to an increase in PKA activity due to decreased inhibition of
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PKA’s main catalytic subunit (Ca) that is encoded by the PRKACA gene [9]. A gain of function mutation in PRKACA was also recently associated with increased cAMP signaling and high PKA activity in adrenal CS [10], suggesting that increased activity of Ca is associated with abnormalities specific to adrenocortical tissue, whereas inactivating mutations of the RIa result in the full spectrum of lesions, in many tissues, associated with CNC. The guanine nucleotide binding alpha subunit 1 (GNAS1) gene codes for the Gsa protein, which activates cAMP/PKA signaling by activating adenylate cyclase. Activating mutations of the GNAS1 gene can lead to high levels of cAMP, triggering cell proliferation prompting nodule formation and autonomous cortisol [11]. Cases of BAH with somatic mutations of the GNAS1 gene, in the context of McCune–Albright syndrome or sporadically (in PMAH or adrenal adenomas), have been reported [12–14]. Together, these data suggest the importance of cAMP signaling, PDEs, and PKA in predisposition and/or formation of adrenocortical tumors. Here, we present a brief overview on PKA and PDE8B defects and adrenal function and tumors (Fig. 1).
Fig. 1 Cyclic AMP-dependent protein kinase (PKA) controls steroidogenesis: upon binding of a ligand hormone to membrane G proteincoupled receptors (GPCRs), Gsa activates adenylyl cyclase (AC) to generate cAMP from ATP. The cAMP/PKA pathway then promotes steroidogenesis via acute PKA activation of cholesterol import into the mitochondria (not shown) and activation of steroidogenic enzymes. Abbreviations: cAMP, Cyclic adenosine monophosphate; PKA, Protein
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PDE8B: human and mouse studies PDE8 enzymes hydrolyze cAMP with very high affinity; there are two proteins in the family coded by two distinct genes, PDE8A and PDE8B. PDE8A and PDE8B have a relatively restricted tissue expression pattern compared with most other PDEs; however, they are expressed highly in steroidogenic tissues such as the adrenal, ovaries and the testis, and most other endocrine glands, including the pituitary, thyroid, and pancreas [15–17]. PDE8B has been shown to regulate adrenocorticotropinstimulated adrenal zona fasciculata steroidogenesis [18]. A PDE8B missense mutation (p.H305P) was described in a young girl with iMAD [3]. Functional studies showed higher cAMP levels after transfection with the PDE8B mutant than with wild-type PDE8B, indicating an impaired ability of the mutant protein to degrade cAMP [3]. PDE8B expression in the adrenal cortex was also the highest among other PDE genes that bind cAMP, such as PDE1A, PDE4A, PDE4B, PDE4C, PDE4D, PDE7A, and PDE9A [2, 19]. Additional PDE8B mutations were described in patients with PMAH [4]. PMAH and other cortisol-
Kinase A; R, Protein Kinase A regulatory subunits; C, Protein Kinase A catalytic subunits; Ga, G protein alpha subunit; b, G protein beta subunit; c, G protein gamma subunit; PDE8B, phosphodiesterase family 8 molecule B; PRKACA, cAMP-dependent protein kinase A alpha catalytic subunit; PRKAR1A, cAMP-dependent protein kinase A 1-a regulatory subunit
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producing lesions are associated with high cAMP levels and overall decreased PDE activity [5]. Both PDE8A and PDE8B are expressed in human and mouse adrenals. Pde8a in the mouse appears to be expressed from a small population of zona fasciculata cells that lie adjacent to zona glomerulosa. Pde8b, on the other hand, is highly expressed across zona fasciculata [18]. Tsai et al. used the PDE8-selective inhibitor (PF-04957325) on mouse adrenal cells and found that this was sufficient to potentiate basal steroid secretion, while treatment with the non-selective PDE inhibitor, IBMX, resulted in little to no effect on basal adrenal steroidogenesis. Inhibition of Pde8 resulted in an increase of phosphorylation of a number of PKA substrates, suggesting that Pde8b inhibition has immediate effects on PKA activation [18]. Several key molecules associated with steroidogenesis, such as StAR and p450scc (CYP11A), were increased as a result of Pde8b ablation [18]. Mice harboring a loss-of-function Pde8b mutation had higher urinary corticosterone levels, even with lower circulating ACTH [18]. In these mice, LacZ replaced exon 14–15 of the Pde8b gene, thus leading to incomplete ablation of the protein. Interestingly, unlike humans with PDE8B mutations [3], these mice did not develop significant adrenal hyperplasia, and the overall size of the glands was normal [18]. This variance could be due to additional molecular defects that may be required for adrenal hyperplasia to develop, or that these mice were studied at a younger age than what would have been required for them to develop hyperplasia. PRKAR1A: human and mouse studies Tumor-specific loss of heterozygosity (LOH) involving the 17q22-24 chromosomal region harboring PRKAR1A and PRKAR1A-inactivating mutations is observed in tumors of patients with CNC or isolated PPNAD [8, 20], suggesting that PRKAR1A is a likely tumor suppressor gene (TSG) [21]. Heterozygous mutations of PRKAR1A are found in more than 60 % of patients with CNC, and up to 80 % of CNC cases present CS due to PPNAD [6, 7]. Haploinsufficient and tissue-specific knockout (KO) mouse models were engineered to demonstrate Prkar1a tumor suppressor activity and to better understand the mechanisms by which PRKAR1A mutations cause CNC. [22, 23]. Homozygous Prkar1a KO mice could not survive beyond the 10.5 embryonic day of development [22, 23]. Prkar1a-haploinsufficient (Prkar1a?/-) mice, on the other hand, were born normally and survived to a late age; however, they developed tumors in a spectrum of endocrine and non-endocrine tissues that are cAMP responsive; tissue-specific complete ablation of Prkar1a expanded the tissues that could be affected or led to tumors
in the same tissues at an earlier age and with significantly more severe pathology [24–26]. Since neither the Prkar1a KO embryos nor the haploinsufficient mice had any detectable adrenocortical pathology, an adrenal cortex-specific Prkar1a KO mouse was generated using the Akr1b7-Cre mouse line [27]. These animals (referred to as AdKO: Adrenal specific KO) developed early signs of pituitary-independent CS and pathology consistent with defective adrenocortical zonal cell differentiation and hyperplasia. Eventually tumors formed from what appeared to be improper maintenance and expansion of adrenocortical cells with fetal characteristics. These data provided the first evidence that the absence of the R1a subunit of PKA in vivo is sufficient to induce the autonomous adrenal hyper-activity and bilateral hyperplasia observed in human PPNAD [27]. The human and mouse LOH studies and the modeling of CNC tumor formation in transgenic rodents confirmed that PRKAR1A most likely acts as a TSG, albeit a relatively weak one. This was best shown in a study where Prkar1ainsufficient mice were mated with mice that were heterozygous for defects in potent TSGs, such as Tp53 and Rb1. Both crosses led to much more aggressive phenotypes than either the Tp53 or Rb1 heterozygous KO mice had [28]. This experiment showed that Prkar1a deficiency can enhance tumor formation induced by the haploinsufficiency of other, stronger, TSGs. PRKACA: human and mouse studies Overexpression of cAMP-dependent protein kinase alpha catalytic (Ca) subunit (PRKACA) or beta catalytic (Cb) subunit (PRKACB) in cell culture results in significant compensation by an increase in PRKAR1A protein [29]. However, in most tissues other than brain, kidney, and spleen (where there is some Cb expression), PKA activity appears to be Ca dependent [30]. In addition, PRKAR1A haploinsufficiency or expression of a defective (in its binding affinity for the catalytic subunit) PRKAR1A protein leads to increased PRKACA activity [9]. Thus, the recent finding that genetic activation of the PRKACA gene leads to tumor formation was not surprising [31]. However, given the ubiquitous expression of Ca, what was not expected was that germline amplification of the PRKACA resulted in only adrenocortical tumors [32]. Somatic PRKACA mutations resulted in single, adrenocortical, cortisol-producing adenomas; these mutations led to Ca resistance to physiologic suppression by regulatory subunits and resulted in increased PKA activity [10], consistent with older data [33]. These mutations were associated with higher levels of cortisol after dexamethasone testing, with a smaller size of the adrenal mass size compared with non-mutated samples [34, 35].
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Prkaca-/- mice are viable in only very small numbers and in backgrounds other than C57BL/6. Prkaca-/- mice that survive are small, infertile, and die by the 3rd month of life [30]. A cross between Prkaca-/- and Prkar1a-/- mice showed that the absence of Ca can abrogate some of the early heart development effects of PRKAR1A ablation [22]. On the other hand, Prkaca-/- Prkar1a?/- mice showed several bone lesions, such as osteochondromyxoma, cartilaginous metaplasia, chondromas, and osteochondrodysplasia; 100 % of the animals had developed these lesions by the time they were 9 months old. Prkaca-/Prkar1a?/- mice developed an increase in number of these lesions and earlier when compared with Prkar1a?/- mice [36]. These data show that despite ubiquitous Ca expression and PRKACA’s importance for total PKA activity in all tissues (as shown by mouse studies), the human adrenal cortex is exquisitely sensitive to Ca activation either by germline or somatic defects. Thus, small changes in cAMP-dependent PKA activity caused by germline mutations leading to either defects of components of the PKA tetramer (i.e., PRKAR1A) or regulators of cAMP levels (i.e., PDEs) are expected to affect the adrenal cortex without necessarily affecting other tissues, despite the ubiquitous presence and significance of the PKA enzyme.
Conclusion Numerous human and mouse studies point to PDE8B’s involvement in adrenocortical function and regulation of steroidogenesis although the exact mechanism and mode of action remain unclear. The human adrenal cortex appears to be very sensitive to small changes of cAMP levels and, consequently, dosage of cAMP-dependent PKA activity, as demonstrated by both PRKAR1A mutations and PRKACA defects leading, respectively, to PPNAD and various adrenocortical pathologies. With its high expression in the adrenal cortex, highest among the cAMP-specific PDEs, could PDE8B be the PDE that finely titrates cAMP levels in this tissue? Additional studies are needed to find the answer to this question, but the data to date suggest that this certainly could be the case. Acknowledgment This work was supported by the Intramural Research Program of the Eunice Kennedy Shriver National Institute of Child Health and Human Development. No research was conducted involving human articipants and/or animals (we only reviewed the relevant literature) in the present report. Conflict of interest of interest.
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The authors declare that they have no conflict
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