PDE8A runs interference to limit PKA inhibition of Raf-1

Cyclic nucleotide phosphodiesterases as drug targets

Cyclic nucleotides are synthesized by adenylyl and/or guanylyl cyclase, and downstream of this synthesis, the cyclic nucleotide phosphodiesterase families (PDEs) specifically hydrolyze cyclic nucleotides. PDEs control cyclic adenosine-3',5'monophosphate (cAMP) and cyclic guanosine-3',5'-monophosphate (cGMP) intracellular levels by mediating their quick return to the basal steady state levels. This often takes place in subcellular nanodomains. Thus, PDEs govern short-term protein phosphorylation, long-term protein expression, and even epigenetic mechanisms by modulating cyclic nucleotide levels. Consequently, their involvement in both health and disease is extensively investigated. PDE inhibition has emerged as a promising clinical intervention method, with ongoing developments aiming to enhance its efficacy and applicability. In this comprehensive review, we extensively look into the intricate landscape of PDEs biochemistry, exploring their diverse roles in various tissues. Furthermore, we outline the underlying mechanisms of PDEs in different pathophysiological conditions. Additionally, we review the application of PDE inhibition in related diseases, shedding light on current advancements and future prospects for clinical intervention. SIGNIFICANCE STATEMENT: Regulating PDEs is a critical checkpoint for numerous (patho)physiological conditions. However, despite the development of several PDE inhibitors aimed at controlling overactivated PDEs, their applicability in clinical settings poses challenges. In this context, our focus is on pharmacodynamics and the structure activity of PDEs, aiming to illustrate how selectivity and efficacy can be optimized. Additionally, this review points to current preclinical and clinical evidence that depicts various optimization efforts and indications.

Disruption of the pro-oncogenic c-RAF-PDE8A complex represents a differentiated approach to treating KRAS-c-RAF dependent PDAC

Pancreatic ductal adenocarcinoma (PDAC) is considered the third leading cause of cancer mortality in the western world, offering advanced stage patients with few viable treatment options. Consequently, there remains an urgent unmet need to develop novel therapeutic strategies that can effectively inhibit pro-oncogenic molecular targets underpinning PDACs pathogenesis and progression. One such target is c-RAF, a downstream effector of RAS that is considered essential for the oncogenic growth and survival of mutant RAS-driven cancers (including KRASMT PDAC). Herein, we demonstrate how a novel cell-penetrating peptide disruptor (DRx-170) of the c-RAF-PDE8A protein-protein interaction (PPI) represents a differentiated approach to exploiting the c-RAF-cAMP/PKA signaling axes and treating KRAS-c-RAF dependent PDAC. Through disrupting the c-RAF-PDE8A protein complex, DRx-170 promotes the inactivation of c-RAF through an allosteric mechanism, dependent upon inactivating PKA phosphorylation. DRx-170 inhibits cell proliferation, adhesion and migration of a KRASMT PDAC cell line (PANC1), independent of ERK1/2 activity. Moreover, combining DRx-170 with afatinib significantly enhances PANC1 growth inhibition in both 2D and 3D cellular models. DRx-170 sensitivity appears to correlate with c-RAF dependency. This proof-of-concept study supports the development of DRx-170 as a novel and differentiated strategy for targeting c-RAF activity in KRAS-c-RAF dependent PDAC.

cAMP-PKA/EPAC signaling and cancer: the interplay in tumor microenvironment

Cancer is a complex disease resulting from abnormal cell growth that is induced by a number of genetic and environmental factors. The tumor microenvironment (TME), which involves extracellular matrix, cancer-associated fibroblasts (CAF), tumor-infiltrating immune cells and angiogenesis, plays a critical role in tumor progression. Cyclic adenosine monophosphate (cAMP) is a second messenger that has pleiotropic effects on the TME. The downstream effectors of cAMP include cAMP-dependent protein kinase (PKA), exchange protein activated by cAMP (EPAC) and ion channels. While cAMP can activate PKA or EPAC and promote cancer cell growth, it can also inhibit cell proliferation and survival in context- and cancer type-dependent manner. Tumor-associated stromal cells, such as CAF and immune cells, can release cytokines and growth factors that either stimulate or inhibit cAMP production within the TME. Recent studies have shown that targeting cAMP signaling in the TME has therapeutic benefits in cancer. Small-molecule agents that inhibit adenylate cyclase and PKA have been shown to inhibit tumor growth. In addition, cAMP-elevating agents, such as forskolin, can not only induce cancer cell death, but also directly inhibit cell proliferation in some cancer types. In this review, we summarize current understanding of cAMP signaling in cancer biology and immunology and discuss the basis for its context-dependent dual role in oncogenesis. Understanding the precise mechanisms by which cAMP and the TME interact in cancer will be critical for the development of effective therapies. Future studies aimed at investigating the cAMP-cancer axis and its regulation in the TME may provide new insights into the underlying mechanisms of tumorigenesis and lead to the development of novel therapeutic strategies.

Treatment of Experimental Autoimmune Encephalomyelitis with an Inhibitor of Phosphodiesterase-8 (PDE8)

After decades of development, inhibitors targeting cyclic nucleotide phosphodiesterases (PDEs) expressed in leukocytes have entered clinical practice for the treatment of inflammatory disorders, with three PDE4 inhibitors being in clinical use as therapeutics for psoriasis, psoriatic arthritis, chronic obstructive pulmonary disease and atopic dermatitis. In contrast, the PDE8 family that is upregulated in pro-inflammatory T cells is a largely unexplored therapeutic target. We have previously demonstrated a role for the PDE8A-Raf-1 kinase complex in the regulation of myelin oligodendrocyte glycoprotein peptide 35–55 (MOG_35–55) activated CD4⁺ effector T cell adhesion and locomotion by a mechanism that differs from PDE4 activity. In this study, we explored the in vivo treatment of experimental autoimmune encephalomyelitis (EAE), a model for multiple sclerosis (MS) induced in mice immunized with MOG using the PDE8-selective inhibitor PF-04957325. For treatment in vivo, mice with EAE were either subcutaneously (s.c.) injected three times daily (10 mg/kg/dose), or were implanted subcutaneously with Alzet mini-osmotic pumps to deliver the PDE8 inhibitor (15.5 mg/kg/day). The mice were scored daily for clinical signs of paresis and paralysis which were characteristic of EAE. We observed the suppression of the clinical signs of EAE and a reduction of inflammatory lesion formation in the CNS by histopathological analysis through the determination of the numbers of mononuclear cells isolated from the spinal cord of mice with EAE. The PDE8 inhibitor treatment reduces the accumulation of both encephalitogenic Th1 and Th17 T cells in the CNS. Our study demonstrates the efficacy of targeting PDE8 as a treatment of autoimmune inflammation in vivo by reducing the inflammatory lesion load.

The Role of PDE8 in T Cell Recruitment and Function in Inflammation

Inhibitors targeting cyclic nucleotide phosphodiesterases (PDEs) expressed in leukocytes have entered clinical practice to treat inflammatory disorders, with three PDE4 inhibitors currently in clinical use as therapeutics for psoriasis, psoriatic arthritis, atopic dermatitis and chronic obstructive pulmonary disease. In contrast, the PDE8 family that is upregulated in pro-inflammatory T cells is a largely unexplored therapeutic target. It was shown that PDE8A plays a major role in controlling T cell and breast cancer cell motility, including adhesion to endothelial cells under physiological shear stress and chemotaxis. This is a unique function of PDE8 not shared by PDE4, another cAMP specific PDE, employed, as noted, as an anti-inflammatory therapeutic. Additionally, a regulatory role was shown for the PDE8A-rapidly accelerated fibrosarcoma (Raf)-1 kinase signaling complex in myelin antigen reactive CD4⁺ effector T cell adhesion and locomotion by a mechanism differing from that of PDE4. The PDE8A-Raf-1 kinase signaling complex affects T cell motility, at least in part, via regulating the LFA-1 integrin mediated adhesion to ICAM-1. The findings that PDE8A and its isoforms are expressed at higher levels in naive and myelin oligodendrocyte glycoprotein (MOG)_35–55 activated effector T (Teff) cells compared to regulatory T (Treg) cells and that PDE8 inhibition specifically affects MOG_35–55 activated Teff cell adhesion, indicates that PDE8A could represent a new beneficial target expressed in pathogenic Teff cells in CNS inflammation. The implications of this work for targeting PDE8 in inflammation will be discussed in this review.

Cyclic adenosine monophosphate (cAMP) signaling in melanocyte pigmentation and melanomagenesis

The second messenger cyclic adenosine monophosphate (cAMP) regulates numerous functions in both benign melanocytes and melanoma cells. cAMP is generated from two distinct sources, transmembrane and soluble adenylyl cyclases (tmAC and sAC, respectively), and is degraded by a family of proteins called phosphodiesterases (PDEs). cAMP signaling can be regulated in many different ways and can lead to varied effects in melanocytes. It was recently revealed that distinct cAMP signaling pathways regulate pigmentation by either altering pigment gene expression or the pH of melanosomes. In the context of melanoma, many studies report seemingly contradictory roles for cAMP in tumorigenesis. For example, cAMP signaling has been implicated in both cancer promotion and suppression, as well as both therapy resistance and sensitization. This conundrum in the field may be explained by the fact that cAMP signals in discrete microdomains and each microdomain can mediate differential cellular functions. Here, we review the role of cAMP signaling microdomains in benign melanocyte biology, focusing on pigmentation, and in melanomagenesis.

Mitogen-activated protein kinase kinase kinase 8 (MAP3K8) mediates the LH-induced stimulation of progesterone synthesis in the porcine corpus luteum

Progesterone (P4) synthesized by the corpus luteum (CL) plays a key role in the establishment and maintenance of pregnancy. The LH signal is important for luteinisation and P4 synthesis in pigs. In a previous study, we demonstrated that mitogen-activated protein kinase kinase kinase 8 (MAP3K8) regulates P4 synthesis in mouse CL, but whether the function and mechanism of MAP3K8 in the pig is similar to that in the mouse is not known. Thus, in the present study we investigated the effects of MAP3K8 on porcine CL. Abundant expression of MAP3K8 was detected in porcine CL, and, in pigs, MAP3K8 expression was higher in mature CLs (or those of the mid-luteal phase) than in regressing CLs (late luteal phase). Further functional studies in cultured porcine luteal cells showed that P4 synthesis and the expression of genes encoding the key enzymes in P4 synthesis are significantly reduced when MAP3K8 is inhibited with the MAP3K8 inhibitor Tpl2 kinase inhibitor (MAP3K8i, 10μM). After 12-24h treatment of luteal cells with 100ngmL-1 LH, MAP3K8 expression and P4 secretion were significantly upregulated. In addition, the 10μM MAP3K8 inhibitor blocked the stimulatory effect of LH on P4 synthesis and extracellular signal-regulated kinase (ERK) 1/2 phosphorylation in porcine luteal cells. The LH-induced increases in MAP3K8 phosphorylation and expression, ERK1/2 phosphorylation and P4 synthesis were all blocked when protein kinase A was inhibited by its inhibitor H89 (20 μM) in porcine luteal cells. In conclusion, MAP3K8 mediates the LH-induced stimulation of P4 synthesis through the PKA/mitogen-activated protein kinase signalling pathway in porcine CL.

Targeting B-Raf inhibitor resistant melanoma with novel cell penetrating peptide disrupters of PDE8A - C-Raf

Background

Recent advances in the treatment of melanoma that involve immunotherapy and B-Raf inhibition have revolutionised cancer care for this disease. However, an un-met clinical need remains in B-Raf inhibitor resistant patients where first-generation B-Raf inhibitors provide only short-term disease control. In these cases, B-Raf inhibition leads to paradoxical activation of the C-Raf – MEK – ERK signalling pathway, followed by metastasis. PDE8A has been shown to directly interact with and modulate the cAMP microdomain in the vicinity of C-Raf. This interaction promotes C-Raf activation by attenuating the PKA-mediated inhibitory phosphorylation of the kinase.

Methods

We have used a novel cell-penetrating peptide agent (PPL-008) that inhibits the PDE8A – C-Raf complex in a human malignant MM415 melanoma cell line and MM415 melanoma xenograft mouse model to investigate ERK MAP kinase signalling.

Results

We have demonstrated that the PDE8A – C-Raf complex disruptor PPL-008 increased inhibitory C-Raf-S259 phosphorylation and significantly reduced phospho-ERK signalling. We have also discovered that the ability of PPL-008 to dampen ERK signalling can be used to counter B-Raf inhibitor-driven paradoxical activation of phospho-ERK in MM415 cells treated with PLX4032 (Vemurafenib). PPL-008 treatment also significantly retarded the growth of these cells. When applied to a MM415 melanoma xenograft mouse model, PPL-008C penetrated tumour tissue and significantly reduced phospho-ERK signalling in that domain.

Conclusion

Our data suggests that the PDE8A-C-Raf complex is a promising therapeutic treatment for B-Raf inhibitor resistant melanoma.

Electronic supplementary material

The online version of this article (10.1186/s12885-019-5489-4) contains supplementary material, which is available to authorized users.

PDE8 controls CD4+ T cell motility through the PDE8A-Raf-1 kinase signaling complex

The levels of cAMP are regulated by phosphodiesterase enzymes (PDEs), which are targets for the treatment of inflammatory disorders. We have previously shown that PDE8 regulates T cell motility. Here, for the first time, we report that PDE8A exerts part of its control of T cell function through the V-raf-1 murine leukemia viral oncogene homolog 1 (Raf-1) kinase signaling pathway. To examine T cell motility under physiologic conditions, we analyzed T cell interactions with endothelial cells and ligands in flow assays. The highly PDE8-selective enzymatic inhibitor PF-04957325 suppresses adhesion of in vivo myelin oligodendrocyte glycoprotein (MOG) activated inflammatory CD4⁺ T effector (Teff) cells to brain endothelial cells under shear stress. Recently, PDE8A was shown to associate with Raf-1 creating a compartment of low cAMP levels around Raf-1 thereby protecting it from protein kinase A (PKA) mediated inhibitory phosphorylation. To test the function of this complex in Teff cells, we used a cell permeable peptide that selectively disrupts the PDE8A-Raf-1 interaction. The disruptor peptide inhibits the Teff-endothelial cell interaction more potently than the enzymatic inhibitor. Furthermore, the LFA-1/ICAM-1 interaction was identified as a target of disruptor peptide mediated reduction of adhesion, spreading and locomotion of Teff cells under flow. Mechanistically, we observed that disruption of the PDE8A-Raf-1 complex profoundly alters Raf-1 signaling in Teff cells. Collectively, our studies demonstrate that PDE8A inhibition by enzymatic inhibitors or PDE8A-Raf-1 kinase complex disruptors decreases Teff cell adhesion and migration under flow, and represents a novel approach to target T cells in inflammation.

Local cAMP signaling in disease at a glance

The second messenger cyclic AMP (cAMP) operates in discrete subcellular regions within which proteins that synthesize, break down or respond to the second messenger are precisely organized. A burgeoning knowledge of compartmentalized cAMP signaling is revealing how the local control of signaling enzyme activity impacts upon disease. The aim of this Cell Science at a Glance article and the accompanying poster is to highlight how misregulation of local cyclic AMP signaling can have pathophysiological consequences. We first introduce the core molecular machinery for cAMP signaling, which includes the cAMP-dependent protein kinase (PKA), and then consider the role of A-kinase anchoring proteins (AKAPs) in coordinating different cAMP-responsive proteins. The latter sections illustrate the emerging role of local cAMP signaling in four disease areas: cataracts, cancer, diabetes and cardiovascular diseases.