Cone photoreceptor phosphodiesterase PDE6H inhibition regulates cancer cell growth and metabolism, replicating the dark retina response

BACKGROUND

PDE6H encodes PDE6γ', the inhibitory subunit of the cGMP-specific phosphodiesterase 6 in cone photoreceptors. Inhibition of PDE6, which has been widely studied for its role in light transduction, increases cGMP levels. The purpose of this study is to characterise the role of PDE6H in cancer cell growth.

METHODS

From an siRNA screen for 487 genes involved in metabolism, PDE6H was identified as a controller of cell cycle progression in HCT116 cells. Role of PDE6H in cancer cell growth and metabolism was studied through the effects of its depletion on levels of cell cycle controllers, mTOR effectors, metabolite levels, and metabolic energy assays. Effect of PDE6H deletion on tumour growth was also studied in a xenograft model.

RESULTS

PDE6H knockout resulted in an increase of intracellular cGMP levels, as well as changes to the levels of nucleotides and key energy metabolism intermediates. PDE6H knockdown induced G1 cell cycle arrest and cell death and reduced mTORC1 signalling in cancer cell lines. Both knockdown and knockout of PDE6H resulted in the suppression of mitochondrial function. HCT116 xenografts revealed that PDE6H deletion, as well as treatment with the PDE5/6 inhibitor sildenafil, slowed down tumour growth and improved survival, while sildenafil treatment did not have an additive effect on slowing the growth of PDE6γ'-deficient tumours.

CONCLUSIONS

Our results indicate that the changes in cGMP and purine pools, as well as mitochondrial function which is observed upon PDE6γ' depletion, are independent of the PKG pathway. We show that in HCT116, PDE6H deletion replicates many effects of the dark retina response and identify PDE6H as a new target in preventing cancer cell proliferation and tumour growth.

Abstract LB-B07: Photoreceptor phosphodiesterase 6H expression regulates HCT116 proliferation and cell cycle progression

Phosphodiesterase 6 (PDE6) protein complex, which is responsible for the turnover of the second messenger cyclic guanosine monophosphate (cGMP) and is involved in visual signal transduction of light stimuli, is highly expressed in certain cancer cell lines. Despite having been discovered in the outer segment of rods and cones of the retina, PDE6 protein is also detected in brain, adrenal glands, lungs, and gonads. Genes encoding the catalytic subunits of the enzyme are known to be upregulated in certain breast, non-small cell lung cancer and medulloblastoma cells. PDE6H, which encodes the cone-specific inhibitory gamma subunit of PDE6 protein, is part of the gene signature of achromatopsia and congenital stationary night blindness while its relation to cancer remains unclear. We recently identified PDE6H as a cell cycle controller in HCT116 colorectal cancer cell line, as part of an siRNA screen in which DNA content measurement via flow cytometry was undertaken to monitor cell cycle distribution. The screen was conducted with a custom library of metabolic genes commonly amplified in primary tumours, as well as those known to be involved in the key regulatory points of metabolic pathways.

We have shown that PDE6H knockdown and pharmacological inhibition of PDE6 protein hindered the activity of the protein complex, causing an accumulation of cGMP in various colorectal and breast cancer cell lines. This inhibition resulted in a G1 cell cycle arrest and disruption of G1/S checkpoint mechanism. PDE6H knockdown inhibited cell proliferation and cellular oxidative phosphorylation. It also altered mitochondrial morphology and induced apoptosis. Our results suggest that the gamma subunit is important for the function of photoreceptor phosphodiesterase PDE6 complex and PDE6H inhibition can be explored as an anti-proliferative strategy.

Citation Format: Ceren Yalaz, Syed Haider, Jianzhou Chen, Remko Prevo, Mick Woodcock, Adrian L. Harris, Ruth J. Muschel. Photoreceptor phosphodiesterase 6H expression regulates HCT116 proliferation and cell cycle progression [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr LB-B07.

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