Fig. 4.

The cGMP/PKG-independent inhibition of proliferation by PDE-inhibitors is a common feature of colon cancer cell lines. HCT116 and Caco2 colon cancer cells were either untreated or treated with the PDE5 inhibitor sildenafil (Sil), the PDE9 inhibitor PF-04447943 (PF), and the PDE10a inhibitor TAK-063 (TAK) as indicated. The ability of the drugs to increase cGMP in (A) HCT116-PKG2 and (C) Caco2-PKG2 was measured by immunoblotting for VASP phosphorylation at Ser239 (VASP-239P) using 8Br-cGMP as a positive control. The effect of 50 µM of the PDE inhibitors on proliferation and viability was measured in both parental and PKG2-expressing (B) HCT116 and (D) Caco2 colon cancer cells. Cells were treated with 50 µM of Sil, PF, or TAK for 3 days. Cell death was measured using trypan blue exclusion. Data show means, and error bars are S.D. ***P < 0.0001, **P < 0.005.