The phosphodiesterase PDE4D7 has been well characterised in recent years as a prognostic biomarker for prostate cancer (PCa), with its diminished expression throughout PCa progression correlating with advanced disease and aggressive phenotype ​(Henderson et al., 2014; Böttcher et al., 2016; Alves de Inda et al., 2018)​. The transition of PCa from androgen sensitive to castration resistant prostate cancer (CRPC) occurs due to alterations in androgen receptor (AR) signalling and/or androgen biosynthesis, rendering cells resistant to androgen deprivation therapy ​(Karantanos, Corn and Thompson, 2013; Fujita and Nonomura, 2019). ​This led on to the generation of powerful androgen receptor signalling inhibitors, such as enzalutamide. However, acquired resistance to androgen signalling inhibition (ARSi) eventually occurs, alongside further progression to the most aggressive form, neuroendocrine prostate cancer (NEPC), in which cells become AR-negative and hence targeting AR signalling is no longer a feasible option. Incidence rates of patients presenting with AR-negative tumours are rising due to the increased use of potent ARSi, representing an expanding cohort of high-risk patients for whom treatment options are extremely limited ​(Chen et al., 2022)​. Given that downregulated PDE4D7 appears to play a significant role in PCa progression to CRPC, we sought to investigate the molecular mechanisms associated with PDE4D7 expression. 

Using shRNA-mediated knockdown of PDE4D7, a stable LNCaP cell line was generated which replicated clinical observations associated with diminished PDE4D7 expression and successfully phenocopied the transition from androgen sensitive to androgen insensitive PCa in an in vitro model. Using next generation RNA sequencing and gene set enrichment analysis (GSEA), signalling pathways influenced by PDE4D7 knockdown were identified as key molecular pathways implicated in PCa tumourigenesis (Figure 1). Importantly, the androgen response pathway was identified as the most enriched pathway in wild-type (WT) LNCaP cells which was subsequently depleted upon PDE4D7 knockdown, coinciding with loss of AR and AR-response gene expression at both mRNA and protein level. PDE4D7 knockdown-mediated loss of AR expression coincided with resistance to ARSi via enzalutamide, highlighting the influence of PDE4D7 expression on the AR signalling pathway and mediating therapeutic response to ARSi. Furthermore, the neuroendocrine differentiation (NED) pathway was enriched upon PDE4D7 knockdown, overall implicating PDE4D7 loss in the development of an NEPC phenotype characterised by AR-negativity and NED.  

Depletion of PDE4D7 also promoted the enrichment of pathways associated with development of an aggressive phenotype, such as epithelial-mesenchymal transition (EMT) which is associated with enhanced migratory and metastatic potential in cancer ​(Huang, Hong and Wei, 2022)​. This coincided with enhanced proliferation of PDE4D7 knockdown cells, alongside a metabolic shift to favour glycolysis over oxidative phosphorylation which aids to support metabolism of highly proliferative cancer cells ​(Ahmad, Cherukuri and Choyke, 2021)​, further highlighting PDE4D7 loss in conferring an aggressive PCa phenotype. The DNA damage response (DDR) was also influenced by PDE4D7 expression, with low PDE4D7 resulting in enhanced DNA damage repair via homologous recombination and decreased sensitivity to chemotherapeutic agents. Therapeutic resistance to PARP inhibition was also observed upon PDE4D7 knockdown, likely due to the enhanced ability to repair DNA damage via homologous recombination and thus lack of reliance on the back up PARP signalling pathway ​(Rimar et al., 2017)​. Ultimately, PDE4D7 plays an important role in mediating the DDR, with PDE4D7 loss conferring enhanced ability for PCa cells to repair DNA damage and evade apoptosis.  

Importantly, PDE4D7 re-expression was shown to rescue many knockdown-influenced pathways, such as NED and EMT, restore growth characteristics, and heighten sensitivity to PARP inhibition and chemotherapeutics via docetaxel. Additionally, given the development of ARSi resistance alongside the increasing progression of CRPC cells to an NEPC phenotype, research has focussed on restoration of the AR and AR signalling to subsequently re-sensitise these cells to ARSi. Our findings revealed that, not only does PDE4D7 loss confer an NEPC state, re-introduction of PDE4D7 also led to slight restoration of AR and AR-target gene expression. PDE4D7 re-expression, alongside pharmacological PDE4 activation, re-sensitised PDE4D7 knockdown LNCaPs to ARSi via enzalutamide. 

Overall, augmentation of PDE4D7 expression and/or activity is capable of hindering aggressive growth characteristics as well as restoring therapeutic sensitivity in resistant tumours, ultimately representing a novel therapeutic strategy for patients with high-risk CRPC and NEPC for whom today treatment options are extremely limited.