Phosphorylation of TRPC6 channels at Thr⁶⁹ is required for anti-hypertrophic effects of phosphodiesterase 5 inhibition

Activation of Ca²⁺ signaling induced by receptor stimulation and mechanical stress plays a critical role in the development of cardiac hypertrophy. A canonical transient receptor potential protein subfamily member, TRPC6, which is activated by diacylglycerol and mechanical stretch, works as an upstream regulator of the Ca²⁺ signaling pathway. Although activation of protein kinase G (PKG) inhibits TRPC6 channel activity and cardiac hypertrophy, respectively, it is unclear whether PKG suppresses cardiac hypertrophy through inhibition of TRPC6. Here, we show that inhibition of cGMP-selective PDE5 (phosphodiesterase 5) suppresses endothelin-1-, diacylglycerol analog-, and mechanical stretch-induced hypertrophy through inhibition of Ca²⁺ influx in rat neonatal cardiomyocytes. Inhibition of PDE5 suppressed the increase in frequency of Ca²⁺ spikes induced by agonists or mechanical stretch. However, PDE5 inhibition did not suppress the hypertrophic responses induced by high KCl or the activation of protein kinase C, suggesting that PDE5 inhibition suppresses Ca²⁺ influx itself or molecule(s) upstream of Ca²⁺ influx. PKG activated by PDE5 inhibition phosphorylated TRPC6 proteins at Thr⁶⁹ and prevented TRPC6-mediated Ca²⁺ influx. Substitution of Ala for Thr⁶⁹ in TRPC6 abolished the anti-hypertrophic effects of PDE5 inhibition. In addition, chronic PDE5 inhibition by oral sildenafil treatment actually induced TRPC6 phosphorylation in mouse hearts. Knockdown of RGS2 (regulator of G protein signaling 2) and RGS4, both of which are activated by PKG to reduce Gα_q-mediated signaling, did not affect the suppression of receptor-activated Ca²⁺ influx by PDE5 inhibition. These results suggest that phosphorylation and functional suppression of TRPC6 underlie prevention of pathological hypertrophy by PDE5 inhibition.