Mutation of Prkar1a causes osteoblast neoplasia driven by dysregulation of protein kinase A

Opportunities and Challenges of Kava in Lung Cancer Prevention

Lung cancer is the leading cause of cancer-related deaths due to its high incidence, late diagnosis, and limited success in clinical treatment. Prevention therefore is critical to help improve lung cancer management. Although tobacco control and tobacco cessation are effective strategies for lung cancer prevention, the numbers of current and former smokers in the USA and globally are not expected to decrease significantly in the near future. Chemoprevention and interception are needed to help high-risk individuals reduce their lung cancer risk or delay lung cancer development. This article will review the epidemiological data, pre-clinical animal data, and limited clinical data that support the potential of kava in reducing human lung cancer risk via its holistic polypharmacological effects. To facilitate its future clinical translation, advanced knowledge is needed with respect to its mechanisms of action and the development of mechanism-based non-invasive biomarkers in addition to safety and efficacy in more clinically relevant animal models.

Benign bony tumors of the paranasal sinuses, orbit, and skull base

Benign bony tumors of the skull base and paranasal sinuses are uncommon entities, with an overall higher incidence in males. Benign bony tumors may lead to local expansion with resultant mass effect of potentially critical structures. Some benign bony tumors may undergo malignant transformation. This article reviews the presentation and management of benign bone tumors of the skull base and paranasal sinuses with special consideration to involvement of the adjacent orbit, intracranial and critical neurovascular structures. This review covers tumor incidence, location, gross and histologic appearance as well as radiographic findings, treatment, and recurrence rates. Tumors discussed in this article include osteochondromas, osteomas, osteoid osteomas, aneurysmal bone cysts, fibrous dysplasia, giant cell tumors, cemento-ossifying fibroma, ameloblastic fibro-odontoma, ecchordosis physaliphora, chondromyxoid fibroma, primary chronic osteomyelitis, primary chronic osteomyelitis, osteochondromyxoma, and dense bone islands.

OUP accepted manuscript

Our earlier work demonstrated varying potency of dihydromethysticin (DHM) as the active kava phytochemical for prophylaxis of tobacco carcinogen nicotine-derived nitrosamine ketone (NNK)-induced mouse lung carcinogenesis. Efficacy was dependent on timing of DHM gavage ahead of NNK insult. In addition to DNA adducts in the lung tissues mitigated by DHM in a time-dependent manner, our in vivo data strongly implicated the existence of DNA damage-independent mechanism(s) in NNK-induced lung carcinogenesis targeted by DHM to fully exert its anti-initiation efficacy. In the present work, RNA seq transcriptomic profiling of NNK-exposed (2 h) lung tissues with/without a DHM (8 h) pretreatment revealed a snap shot of canonical acute phase tissue damage and stress response signaling pathways as well as an activation of protein kinase A (PKA) pathway induced by NNK and the restraining effects of DHM. The activation of the PKA pathway by NNK active metabolite 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) at a concentration incapable of promoting DNA adduct was confirmed in a lung cancer cell culture model, potentially through NNAL binding to and activation of the β-adrenergic receptor. Our in vitro and in vivo data overall support the hypothesis that DHM suppresses PKA activation as a key DNA damage-independent mechanistic lead, contributing to its effective prophylaxis of NNK-induced lung carcinogenesis. Systems biology approaches with a detailed temporal dissection of timing of DHM intake versus NNK exposure are warranted to fill the knowledge gaps concerning the DNA damage-driven mechanisms and DNA damage-independent mechanisms to optimize the implementation strategy for DHM to achieve maximal lung cancer chemoprevention.

Genetic Determinants of Inherited Endocrine Tumors: Do They Have a Direct Role in Bone Metabolism Regulation and Osteoporosis?

Endocrine tumors are neoplasms originating from specialized hormone-secreting cells. They can develop as sporadic tumors, caused by somatic mutations, or in the context of familial Mendelian inherited diseases. Congenital forms, manifesting as syndromic or non-syndromic diseases, are caused by germinal heterozygote autosomal dominant mutations in oncogenes or tumor suppressor genes. The genetic defect leads to a loss of cell growth control in target endocrine tissues and to tumor development. In addition to the classical cancer manifestations, some affected patients can manifest alterations of bone and mineral metabolism, presenting both as pathognomonic and/or non-specific skeletal clinical features, which can be either secondary complications of endocrine functioning primary tumors and/or a direct consequence of the gene mutation. Here, we specifically review the current knowledge on possible direct roles of the genes that cause inherited endocrine tumors in the regulation of bone modeling and remodeling by exploring functional in vitro and in vivo studies highlighting how some of these genes participate in the regulation of molecular pathways involved in bone and mineral metabolism homeostasis, and by describing the potential direct effects of gene mutations on the development of skeletal and mineral metabolism clinical features in patients.

Genome-wide association study of bone mineral density trait among three pig breeds

Leg weakness (LW) issues are a great concern for pig breeding industry. And it also has a serious impact on animal welfare. To dissect the genetic architecture of limb-and-hoof firmness in commercial pigs, a genome-wide association study was conducted on bone mineral density (BMD) in three sow populations, including Duroc, Landrace and Yorkshire. The BMD data were obtained by ultrasound technology from 812 pigs (including Duroc 115, Landrace 243 and Yorkshire 454). In addition, all pigs were genotyped using genome-by-sequencing and a total of 224 162 single-nucleotide polymorphisms (SNPs) were obtained. After quality control, 218 141 SNPs were used for subsequent genome-wide association analysis. Nine significant associations were identified on chromosomes 3, 5, 6, 7, 9, 10, 12 and 18 that passed Bonferroni correction threshold of 0.05/(total SNP numbers). The most significant locus that associated with BMD (P value = 1.92e-14) was detected at approximately 41.7 Mb on SSC6 (SSC stands for Sus scrofa chromosome). CUL7, PTK7, SRF, VEGFA, RHEB, PRKAR1A and TPO that are located near the lead SNP of significant loci were highlighted as functionally plausible candidate genes for sow limb-and-hoof firmness. Moreover, we also applied a new method to measure the BMD data of pigs by ultrasound technology. The results provide an insight into the genetic architecture of LW and can also help to improve animal welfare in pigs.

Bone tissue and mineral metabolism in hereditary endocrine tumors: clinical manifestations and genetic bases

Inherited endocrine tumors are neoplasms of endocrine cells, transmitted via autosomal dominant germinal mutations. They present in two different forms: non-syndromic (patient has a single affected endocrine organ during his/her lifetime) or syndromic forms (multiple tumors in endocrine and non-endocrine organs during his/her lifetime).In addition to their common tumoral manifestations, many of these diseases present clinical affection of bone tissues and/or mineral metabolism, both as secondary complications of primary tumors and as primary defects due to genetic mutation. To date, few studies have documented these bone complications, and there are no systematic reviews in this area.We present a revision of medical literature about skeletal and mineral metabolism affections in inherited endocrine tumor syndromes, and studies, in cells and animal models, investigating the direct role of some genes, whose mutations are responsible for the development of endocrine tumors, in the regulation of bone and mineral metabolism.

Celecoxib treatment of fibrous dysplasia (FD) in a human FD cell line and FD-like lesions in mice with protein kinase A (PKA) defects

Osteochondromyxomas (OMX) in the context of Carney complex (CNC) and fibrous dysplasia (FD)-like lesions (FDLL) in mice, as well as isolated myxomas in humans may be caused by inactivation of PRKAR1A, the gene coding for the type 1a regulatory subunit (R1α) of cAMP-dependent protein kinase (PKA). OMXs and FDLL in mice lacking Prkar1a grow from abnormal proliferation of adult bone stromal cells (aBSCs). Prkar1a and Prkaca (coding for Cα) haploinsufficiency leads to COX2 activation and prostaglandin E2 (PGE2) production that, in turn, activates proliferation of aBSCs. Celecoxib is a cyclooxygenase-2 (COX2) inhibitor. We hypothesized that COX-2 inhibition may have an effect in FD and FDLL. In vitro treatment of a human cell line prepared from a FD patient with Celecoxib resulted in decreased PGE2 and cell proliferation. Treatment of mice haploinsufficient for R1α and Cα with 1500 mg/kg Celecoxib led to decreased PGE2 and proliferation and increased apoptosis, with a corresponding gene expression profile, resulting in dramatic reduction of tumor growth. Furthermore, the treatment improved the organization of cortical bone that was adjacent to the tumor. We conclude that, in vitro and in vivo, Celecoxib had an inhibitory effect on FD cell proliferation and in mouse FDLL structure, respectively. We speculate that COX-2 inhibitors offer an attractive alternative to current treatments for benign tumors such as OMX and FD that, apart from tumor suppression, may mechanically stabilize affected bones.

PRKAR1A is a functional tumor suppressor inhibiting ERK/Snail/E-cadherin pathway in lung adenocarcinoma

Protein Kinase cAMP-Dependent Regulatory Type I Alpha (PRKAR1A) is a tissue-specific extinguisher that transduces a signal through phosphorylation of different target proteins. Loss of PRKAR1A was frequently observed in endocrine neoplasia and stromal cell tumors. However, a few cases were seen in epithelial tumors. Previously, we first found that PRKAR1A was downregulated in lung adenocarcinoma patients. Thus, the present study aimed to clarify its clinical implication and biological function as a tumor suppressor in lung adenocarcinoma. The low levels of PRKAR1A transcript were correlated with tumor progression and poor overall survival. The re-expression of PRKAR1A in H1299 cells suppressed the tumor cell proliferation and migration; stable knockdown (KD) of PRKAR1A in A549 cells enhanced this function both in vitro and in vivo. Moreover, KD of PRKAR1A in A549 cells promoted the statistical colonization of circulating tumor cells to the lungs in nude mice. These effects by PRKAR1A were attributed to inhibiting E-cadherin expression. Elevated E-cadherin significantly suppressed the PRKAR1A-KD induced cell proliferation and migration. Most notably, deletion of PRKAR1A inhibited E-cadherin by activating ERK/Snail signaling. In conclusion, PRKAR1A was a potent suppressor, and through the inhibition of PRKAR1A-ERK-Snail-E-cadherin axis could serve as a potential therapeutic target.

Osteochondromyxoma: Review of a rare carney complex criterion

Osteochondromyxoma is an extremely rare bone tumor associated with 1% of Carney complex patients and constitutes one of its 11 diagnostic criteria. This narrative review of osteochondromyxoma is based on a search of all references to the topic in PubMed, Web Of Science, SCOPUS, ScienceDirect, and JSTOR databases. Special attention was focused on case reports, leading to a review encompassing the case reports to date, as well as related animal model studies. This review covers the current understanding of osteochondromyxoma, highlighting its variability while providing consensus on the most common clinical presentation, pathological findings, and genetic features of this rare bone tumor.

Activation of Protein Kinase A in Mature Osteoblasts Promotes a Major Bone Anabolic Response

Protein kinase A (PKA) regulates osteoblast cell function in vitro and is activated by important bone mass modulating agents. We determined whether PKA activation in osteoblasts is sufficient to mediate a bone anabolic response. Thus, a mouse model conditionally expressing a constitutively active PKA (CA-PKA) in osteoblasts (CA-PKA-OB mouse) was developed by crossing a 2.3-kb α1 (I)-collagen promoter-Cre mouse with a floxed-CA-PKA mouse. Primary osteoblasts from the CA-PKA-OB mice exhibited higher basal PKA activity than those from control mice. Microcomputed tomographic analysis revealed that CA-PKA-OB female mice had an 8.6-fold increase in femoral but only 1.16-fold increase in lumbar 5 vertebral bone volume/total volume. Femur cortical thickness and volume were also higher in the CA-PKA-OB mice. In contrast, alterations in many femoral microcomputed tomographic parameters in male CA-PKA-OB mice were modest. Interestingly, the 3-dimensional structure model index was substantially lower both in femur and lumbar 5 of male and female CA-PKA-OB mice, reflecting an increase in the plate to rod-like structure ratio. In agreement, femurs from female CA-PKA-OB mice had greater load to failure and were stiffer compared with those of control mice. Furthermore, the CA-PKA-OB mice had higher levels of serum bone turnover markers and increased osteoblast and osteoclast numbers per total tissue area compared with control animals. In summary, constitutive activation of PKA in osteoblasts is sufficient to increase bone mass and favorably modify bone architecture and improve mechanical properties. PKA activation in mature osteoblasts is, therefore, an important target for designing anabolic drugs for treating diseases with bone loss.

Haploinsufficiency for either one of the type-II regulatory subunits of protein kinase A improves the bone phenotype of Prkar1a+/- mice

Carney Complex (CNC), a human genetic syndrome predisposing to multiple neoplasias, is associated with bone lesions such as osteochondromyxomas (OMX). The most frequent cause for CNC is PRKAR1A deficiency; PRKAR1A codes for type-I regulatory subunit of protein kinase A (PKA). Prkar1a(+/-) mice developed OMX, fibrous dysplasia-like lesions (FDL) and other tumors. Tumor tissues in these animals had increased PKA activity due to an unregulated PKA catalytic subunit and increased PKA type II (PKA-II) activity mediated by the PRKAR2A and PRKAR2B subunits. To better understand the effect of altered PKA activity on bone, we studied Prkar2a and Prkar2b knock out (KO) and heterozygous mice; none of these mice developed bone lesions. When Prkar2a(+/-) and Prkar2b(+/-) mice were used to generate Prkar1a(+/-)Prkar2a(+/-) and Prkar1a(+/-)Prkar2b(+/-) animals, bone lesions formed that looked like those of the Prkar1a(+/-) mice. However, better overall bone organization and mineralization and fewer FDL lesions were found in both double heterozygote groups, indicating a partial restoration of the immature bone structure observed in Prkar1a(+/-) mice. Further investigation indicated increased osteogenesis and higher new bone formation rates in both Prkar1a(+/-)Prkar2a(+/-) and Prkar1a(+/-)Prkar2b(+/-) mice with some minor differences between them. The observations were confirmed with a variety of markers and studies. PKA activity measurements showed the expected PKA-II decrease in both double heterozygote groups. Thus, haploinsufficiency for either of PKA-II regulatory subunits improved bone phenotype of mice haploinsufficient for Prkar1a, in support of the hypothesis that the PRKAR2A and PRKAR2B regulatory subunits were in part responsible for the bone phenotype of Prkar1a(+/-) mice.

Protein kinase A activation enhances β-catenin transcriptional activity through nuclear localization to PML bodies

The Protein Kinase A (PKA) and Wnt signaling cascades are fundamental pathways involved in cellular development and maintenance. In the osteoblast lineage, these pathways have been demonstrated functionally to be essential for the production of mineralized bone. Evidence for PKA-Wnt crosstalk has been reported both during tumorigenesis and during organogenesis, and the nature of the interaction is thought to rely on tissue and cell context. In this manuscript, we analyzed bone tumors arising from mice with activated PKA caused by mutation of the PKA regulatory subunit Prkar1a. In primary cells from these tumors, we observed relocalization of β-catenin to intranuclear punctuate structures, which were identified as PML bodies. Cellular redistribution of β-catenin could be recapitulated by pharmacologic activation of PKA. Using 3T3-E1 pre-osteoblasts as a model system, we found that PKA phosphorylation sites on β-catenin were required for nuclear re-localization. Further, β-catenin's transport to the nucleus was accompanied by an increase in canonical Wnt-dependent transcription, which also required the PKA sites. PKA-Wnt crosstalk in the cells was bi-directional, including enhanced interactions between β-catenin and the cAMP-responsive element binding protein (CREB) and transcriptional crosstalk between the Wnt and PKA signaling pathways. Increases in canonical Wnt/β-catenin signaling were associated with a decrease in the activity of the non-canonical Wnt/Ror2 pathway, which has been shown to antagonize canonical Wnt signaling. Taken together, this study provides a new understanding of the complex regulation of the subcellular distribution of β-catenin and its differential protein-protein interaction that can be modulated by PKA signaling.

PKA signaling drives mammary tumorigenesis through Src

Protein kinase A (PKA) hyperactivation causes hereditary endocrine neoplasias; however, its role in sporadic epithelial cancers is unknown. Here, we show that heightened PKA activity in the mammary epithelium generates tumors. Mammary-restricted biallelic ablation of Prkar1a, which encodes for the critical type-I PKA regulatory subunit, induced spontaneous breast tumors characterized by enhanced type-II PKA activity. Downstream of this, Src phosphorylation occurs at residues serine-17 and tyrosine-416 and mammary cell transformation is driven through a mechanism involving Src signaling. The phenotypic consequences of these alterations consisted of increased cell proliferation and, accordingly, expansion of both luminal and basal epithelial cell populations. In human breast cancer, low PRKAR1A/high SRC expression defines basal-like and HER2 breast tumors associated with poor clinical outcome. Together, the results of this study define a novel molecular mechanism altered in breast carcinogenesis and highlight the potential strategy of inhibiting SRC signaling in treating this cancer subtype in humans.

Knockdown of PRKAR1A, the gene responsible for Carney complex, interferes with differentiation in osteoblastic cells

PRKAR1A is the gene encoding the type 1A regulatory subunit of protein kinase A, and it is the cause of the inherited human tumor syndrome Carney complex. Data from our laboratory has demonstrated that Prkar1a loss causes tumors in multiple cell lineages, including neural crest cells and osteoblasts. We have proposed that one mechanism by which tumorigenesis occurs is through the failure of terminal differentiation. In the present study, we directly test the effects of Prkar1a reduction on osteogenic differentiation in mouse and human cells in vitro. We found that Prkar1a levels noticeably increased during osteoblastic differentiation, indicating a positive correlation between the expression of Prkar1a and osteogenic potential. To validate this hypothesis, we generated stable Prkar1a knockdown in both mouse and human cells. These cells displayed significantly suppressed bone nodule formation and decreased expression of osteoblast markers such as osteocalcin and osteopontin. These observations imply that the antiosteogenic effect of Prkar1a ablation is not species or cell line specific. Furthermore, because Runt-related transcription factor-2 (Runx2) is a key mediator of osteoblast differentiation, we reasoned that the function of this transcription factor may be inhibited by Prkar1a knockdown. Chromatin immunoprecipitation and luciferase assays demonstrated that Prkar1a ablation repressed DNA binding and function of Runx2 at its target genes. Additionally, we determined that this effect is likely due to reductions in the Runx2-cooperating transcription factors forkhead box O1 and activating transcription factor 4. Taken together, this study provides direct evidence that ablation of Prkar1a interferes with signaling pathways necessary for osteoblast differentiation.

cAMP/PKA signaling defects in tumors: genetics and tissue-specific pluripotential cell-derived lesions in human and mouse

In the last few years, bench and clinical studies led to significant new insight into how cyclic adenosine monophosphate (cAMP) signaling, the molecular pathway that had been identified in the early 2000s as the one involved in most benign cortisol-producing adrenal hyperplasias, affects adrenocortical growth and development, as well as tumor formation. A major discovery was the identification of tissue-specific pluripotential cells (TSPCs) as the culprit behind tumor formation not only in the adrenal, but also in bone. Discoveries in animal studies complemented a number of clinical observations in patients. Gene identification continued in parallel with mouse and other studies on the cAMP signaling and other pathways.

New insights into adhesion signaling in bone formation

Mineralized tissues that are protective scaffolds in the most primitive species have evolved and acquired more specific functions in modern animals. These are as diverse as support in locomotion, ion homeostasis, and precise hormonal regulation. Bone formation is tightly controlled by a balance between anabolism, in which osteoblasts are the main players, and catabolism mediated by the osteoclasts. The bone matrix is deposited in a cyclic fashion during homeostasis and integrates several environmental cues. These include diffusible elements that would include estrogen or growth factors and physicochemical parameters such as bone matrix composition, stiffness, and mechanical stress. Therefore, the microenvironment is of paramount importance for controlling this delicate equilibrium. Here, we provide an overview of the most recent data highlighting the role of cell-adhesion molecules during bone formation. Due to the very large scope of the topic, we focus mainly on the role of the integrin receptor family during osteogenesis. Bone phenotypes of some deficient mice as well as diseases of human bones involving cell adhesion during this process are discussed in the context of bone physiology.

The transcriptome that mediates increased cyclic adenosine monophosphate signaling in PRKAR1A defects and other settings

OBJECTIVE

To review current knowledge on the involvement of cyclic adenosine monophosphate (cAMP) and interacting signaling pathways in predisposition to tumor formation in primary pigmented nodular adrenocortical disease (PPNAD), a type of bilateral adrenal hyperplasia (BAH) related to the multiple endocrine neoplasia Carney complex, and also in isolated PPNAD and other BAHs.

METHODS

We review the pertinent literature and discuss genetic defects associated with various endocrine and nonendocrine tumors.

RESULTS

A decade ago, we discovered that PPNAD and the Carney complex are caused by PRKAR1A mutations. PRKAR1A encodes the protein kinase A (PKA) regulatory subunit type IA, an important regulator of cAMP signaling in most cells. Recently, we described PKA or PRKAR1A abnormalities in a variety of other BAHs; in some of these cases, mutations in additional genes of the cAMP signaling pathway, the phosphodiesterases, were identified. Transcriptomic analyses of human lesions or animal models showed that abnormal cAMP/PKA signaling in the adrenal glands, and also in other tissues such as bone, leads to proliferation of tissue-specific pluripotential cells through activation of Wnt signaling.

CONCLUSION

Recent findings indicate the relevance of cAMP signaling in the pathogenesis of adrenocortical disease and point to the Wnt signaling pathway as a potential important mediator of tumorigenesis related to increased cAMP or PKA signaling (or both).

How does cAMP/protein kinase A signaling lead to tumors in the adrenal cortex and other tissues?

The overwhelming majority of benign lesions of the adrenal cortex leading to Cushing syndrome are linked to one or another abnormality of the cAMP signaling pathway. A small number of both massive macronodular adrenocortical disease and cortisol-producing adenomas harbor somatic GNAS mutations. Micronodular adrenocortical hyperplasias are either pigmented (the classic form being that of primary pigmented nodular adrenocortical disease) or non-pigmented; micronodular adrenocortical hyperplasias can be seen in the context of other conditions or isolated; for example, primary pigmented nodular adrenocortical disease usually occurs in the context of Carney complex, but isolated primary pigmented nodular adrenocortical disease has also been described. Both Carney complex and isolated primary pigmented nodular adrenocortical disease are caused by germline PRKAR1A mutations; somatic mutations of this gene that regulates cAMP-dependent protein kinase are also found in cortisol-producing adenomas, and abnormalities of PKA are present in most cases of massive macronodular adrenocortical disease. Micronodular adrenocortical hyperplasias and some cortisol-producing adenomas are associated with phosphodiesterase 11A and 8B defects, coded, respectively, by the PDE11A and PDE8B genes. Mouse models of Prkar1a deficiency also show that increased cAMP signaling leads to tumors in adrenal cortex and other tissues. In this review, we summarize all recent data from ours and other laboratories, supporting the view that Wnt-signaling acts as an important mediator of tumorigenicity induced by abnormal PRKAR1A function and aberrant cAMP signaling.

Protein kinase A regulates caspase-1 via Ets-1 in bone stromal cell-derived lesions: a link between cyclic AMP and pro-inflammatory pathways in osteoblast progenitors

Patients with genetic defects of the cyclic (c) adenosine-monophosphate (AMP)-signaling pathway and those with neonatal-onset multisystem inflammatory disease (NOMID) develop tumor-like lesions of the long bones. The molecular basis of this similarity is unknown. NOMID is caused by inappropriate caspase-1 activity, which in turn activates the inflammasome. The present study demonstrates that NOMID bone lesions are derived from the same osteoblast progenitor cells that form fibroblastoid tumors in mice and humans with defects that lead to increased cAMP-dependent protein kinase A (PKA) signaling. NOMID tumor cells showed high PKA activity, and an increase in their cAMP signaling led to PKA-specific activation of caspase-1. Increased PKA led to inflammation-independent activation of caspase-1 via over-expression of the proto-oncogene (and early osteoblast factor) Ets-1. In NOMID tumor cells, as in cells with defective PKA regulation, increased prostaglandin E2 (PGE2) led to increased cAMP levels and activation of Wnt signaling, like in other states of inappropriate PKA activity. Caspase-1 and PGE2 inhibition led to a decrease in cell proliferation of both NOMID and cells with abnormal PKA. These data reveal a previously unsuspected link between abnormal cAMP signaling and defective regulation of the inflammasome and suggest that caspase-1 and PGE2 inhibition may be therapeutic targets in bone lesions associated with defects of these two pathways.

PRKAR1A and the evolution of pituitary tumors

Carney complex (CNC) is an inherited tumor predisposition associated with pituitary tumors, including GH-producing pituitary adenomas and rare reports of prolactinomas. This disease is caused by mutations in PRKAR1A, which encodes the type 1A regulatory subunit of the cAMP-dependent protein kinase, PKA. Loss of PRKAR1A causes enhanced PKA signaling, which leads to pituitary tumorigenesis. Mutations in the gene have not been detected in sporadic pituitary tumors, but there is some data to suggest that non-genomic mechanisms may cause loss of protein expression. Unlike CNC patients, mice heterozygous for Prkar1a mutations do not develop pituitary tumors, although complete knockout of the gene in the Pit1 lineage of the pituitary produces GH-secreting pituitary adenomas. These data indicate that complete loss of Prkar1a/PRKAR1A is able to cause pituitary tumors in mice and men. The pattern of tumors is likely related to the signaling pathways employed in specific pituitary cell types.

Prkar1a is an osteosarcoma tumor suppressor that defines a molecular subclass in mice

Some cancers have been stratified into subclasses based on their unique involvement of specific signaling pathways. The mapping of human cancer genomes is revealing a vast number of somatic alterations; however, the identification of clinically relevant molecular tumor subclasses and their respective driver genes presents challenges. This information is key to developing more targeted and personalized cancer therapies. Here, we generate a new mouse model of genomically unstable osteosarcoma (OSA) that phenocopies the human disease. Integrative oncogenomics pinpointed cAMP-dependent protein kinase type I, alpha regulatory subunit (Prkar1a) gene deletions at 11qE1 as a recurrent genetic trait for a molecularly distinct subclass of mouse OSA featuring RANKL overexpression. Using mouse genetics, we established that Prkar1a is a bone tumor suppressor gene capable of directing subclass development and driving RANKL overexpression during OSA tumorigenesis. Finally, we uncovered evidence for a PRKAR1A-low subset of human OSA with distinct clinical behavior. Thus, tumor subclasses develop in mice and can potentially provide information toward the molecular stratification of human cancers.

Neural crest-specific loss of Prkar1a causes perinatal lethality resulting from defects in intramembranous ossification

The cranial neural crest (CNC) undergoes complex molecular and morphological changes during embryogenesis in order to form the vertebrate skull, and nearly three quarters of all birth defects result from defects in craniofacial development. The molecular events leading to CNC differentiation have been extensively studied; however, the role of the cAMP-dependent protein kinase [protein kinase A (PKA)] during craniofacial development has only been described in palate formation. Here, we provide evidence that strict PKA regulation in postmigratory CNC cells is essential during craniofacial bone development. Selective inactivation of Prkar1a, a regulatory subunit of the PKA holoenzyme, in the CNC results in perinatal lethality caused by dysmorphic craniofacial development and subsequent asphyxiation. Additionally, aberrant differentiation of CNC mesenchymal cells results in anomalous intramembranous ossification characterized by formation of cartilaginous islands in some areas and osteolysis of bony trabeculae with fibrous connective tissue stabilization in others. Genetic interaction studies revealed that genetic reduction of the PKA catalytic subunit C(alpha) was able to rescue the phenotype, whereas reduction in Cbeta had no effect. Overall, these observations provide evidence of the essential role of proper regulation of PKA during the ossification of the bones of the skull. This knowledge may have implications for the understanding and treatment of craniofacial birth defects.

Alternate protein kinase A activity identifies a unique population of stromal cells in adult bone

A population of stromal cells that retains osteogenic capacity in adult bone (adult bone stromal cells or aBSCs) exists and is under intense investigation. Mice heterozygous for a null allele of prkar1a (Prkar1a(+/-)), the primary receptor for cyclic adenosine monophosphate (cAMP) and regulator of protein kinase A (PKA) activity, developed bone lesions that were derived from cAMP-responsive osteogenic cells and resembled fibrous dysplasia (FD). Prkar1a(+/-) mice were crossed with mice that were heterozygous for catalytic subunit Calpha (Prkaca(+/-)), the main PKA activity-mediating molecule, to generate a mouse model with double heterozygosity for prkar1a and prkaca (Prkar1a(+/-)Prkaca(+/-)). Unexpectedly, Prkar1a(+/-)Prkaca(+/-) mice developed a greater number of osseous lesions starting at 3 months of age that varied from the rare chondromas in the long bones and the ubiquitous osteochondrodysplasia of vertebral bodies to the occasional sarcoma in older animals. Cells from these lesions originated from an area proximal to the growth plate, expressed osteogenic cell markers, and showed higher PKA activity that was mostly type II (PKA-II) mediated by an alternate pattern of catalytic subunit expression. Gene expression profiling confirmed a preosteoblastic nature for these cells but also showed a signature that was indicative of mesenchymal-to-epithelial transition and increased Wnt signaling. These studies show that a specific subpopulation of aBSCs can be stimulated in adult bone by alternate PKA and catalytic subunit activity; abnormal proliferation of these cells leads to skeletal lesions that have similarities to human FD and bone tumors.

Cyclic nucleotide signaling in polycystic kidney disease

Increased levels of 3'-5'-cyclic adenosine monophosphate (cAMP) stimulate cell proliferation and fluid secretion in polycystic kidney disease. Levels of this molecule are more sensitive to inhibition of phosphodiesterases (PDEs), whose activity far exceeds the rate of cAMP synthesis by adenylyl cyclase. Several PDEs exist, and here we measured the activity and expression of PDE families, their isoforms, and the expression of downstream effectors of cAMP signaling in the kidneys of rodents with polycystic kidney disease. We found a higher overall PDE activity in kidneys from mice as compared with rats, as well as a higher contribution of PDE1, relative to PDE4 and PDE3, to total PDE activity of kidney lysates and lower PDE1, PDE3, and PDE4 activities in the kidneys of cystic as compared with wild-type mice. There were reduced amounts of several PDE1, PDE3, and PDE4 proteins, possibly due to increased protein degradation despite an upregulation of their mRNA. Increased levels of cGMP were found in the kidneys of cystic animals, suggesting in vivo downregulation of PDE1 activity. We found an additive stimulatory effect of cAMP and cGMP on cystogenesis in vitro. Cyclic AMP-dependent protein kinase subunits Ialpha and IIbeta, PKare, the transcription factor CREB-1 mRNA, and CREM, ATF-1, and ICER proteins were upregulated in the kidneys of cystic as compared with wild-type animals. Our study suggests that alterations in cyclic nucleotide catabolism may render cystic epithelium particularly susceptible to factors acting on Gs-coupled receptors. This may account, in part, for increased cyclic nucleotide signaling in polycystic kidney disease and contribute substantially to disease progression.

Regulation of actin function by protein kinase A-mediated phosphorylation of Limk1

Proper regulation of the cAMP-dependent protein kinase (protein kinase A, PKA) is necessary for cellular homeostasis, and dysregulation of this kinase is crucial in human disease. Mouse embryonic fibroblasts (MEFs) lacking the PKA regulatory subunit Prkar1a show altered cell morphology and enhanced migration. At the molecular level, these cells showed increased phosphorylation of cofilin, a crucial modulator of actin dynamics, and these changes could be mimicked by stimulating the activity of PKA. Previous studies of cofilin have shown that it is phosphorylated primarily by the LIM domain kinases Limk1 and Limk2, which are under the control of the Rho GTPases and their downstream effectors. In Prkar1a(-/-) MEFs, neither Rho nor Rac was activated; rather, we showed that PKA could directly phosphorylate Limk1 and thus enhance the phosphorylation of cofilin. These data indicate that PKA is crucial in cell morphology and migration through its ability to modulate directly the activity of LIM kinase.

Targeted deletion of Prkar1a reveals a role for protein kinase A in mesenchymal-to-epithelial transition

Dysregulation of protein kinase A (PKA) activity, caused by loss of function mutations in PRKAR1A, is known to induce tumor formation in the inherited tumor syndrome Carney complex (CNC) and is also associated with sporadic tumors of the thyroid and adrenal. We have previously shown that Prkar1a(+/-) mice develop schwannomas reminiscent of those seen in CNC and that similar tumors are observed in tissue-specific knockouts (KO) of Prkar1a targeted to the neural crest. Within these tumors, we have previously described the presence of epithelial islands, although the nature of these structures was unclear. In this article, we report that these epithelial structures are derived from KO cells originating in the neural crest. Analysis of the mesenchymal marker vimentin revealed that this protein was markedly down-regulated not only from the epithelial islands, but also from the tumor as a whole, consistent with mesenchymal-to-epithelial transition (MET). In vitro, Prkar1a null primary mouse embryonic fibroblasts, which display constitutive PKA signaling, also showed evidence for MET, with a loss of vimentin and up-regulation of the epithelial marker E-cadherin. Reduction of vimentin protein occurred at the posttranslational level and was rescued by proteasomal inhibition. Finally, this down-regulation of vimentin was recapitulated in the adrenal nodules of CNC patients, confirming an unexpected and previously unrecognized role for PKA in MET.