Investigating the interaction between phosphodiesterase 4D7 (PDE4D7) and DHX9 in the progression of prostate cancer


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Tara Busiau1,Jane Finlay1,Joanne Edwards1,Ralf Hoffman2,George Baillie1
1University of Glasgow,2Philips Diagnostics

Abstract

Background

In recent years, the involvement of phosphodiesterase 4D7 (PDE4D7) in prostate cancer (PC) progression has been validated extensively. More specifically, PDE4D7 is the highly expressed in early stage androgen sensitive PC samples, and undergoes a dramatic reduction in mRNA transcript abundance in late stage androgen insensitive cancer. The significance of this downregulation is underscored by the finding that PDE4D7 contributes a major fraction of cAMP degrading activity in PC . Interestingly, PDE4D7 expression positively correlates with primary tumour development. As such, PDE4D7 has the potential to be a novel target for use as a biomarker in PC. Although there is a change in expression from early to late stage disease, the molecular events that are regulated by this event are still to be discovered. Recent work in the Baillie lab has identified that DHX9, a DNA/RNA helicase, as  a PDE4D7 interacting protein. The role of DHX9 in the progression of different cancers, including breast and colorectal cancers has been identified recently. Interestingly, the DHX9 gene maps to the PC susceptibility locus on the 25th band of the long arm of chromosome 1 (1q25). 

Method

The interaction between PDE4D7 and DHX9 was confirmed using immunoprecipitation, proximity ligation assays with confocal micrscopy. Binding sites were determined using peptide array technology. Cell penetrating peptides were generated in order to confirm the binding sites, as well as investigate how the loss of interaction may affect cell growth and DHX9 activity.

Results

Using the newly designed cell penetrating peptide, the binding sites between PDE4D7 and DHX9 were confirmed. Interestingly PDE4D7 binds to DHX9 within its helicase core domain, suggesting it may have a role in regulating DHX9 function. Using XCelligence, treatment of LNCaP cells with the peptide leads to a change in growth rate. Early DHX9 functional assays suggests that the disruption of the PDE4D7-DHX9 complex leads to a decreased DHX9 activity.

Conclusion

The interaction between these two proteins could regulate DHX9, with the loss of PDE4D7 expression leading to an altered helicase activity and contributing to PC progression.