Involvement of PKA, but not Epac, in flavonoid accumulation in cell cultures of Hypericum androsaemum

Hypericum androsaemum L. is a perennial herb found in damp or shady places throughout Europe. Leaf infusions have been used traditionally for its diuretic, cholagogue, and hepatoprotective properties.

Cell cultures established from hypocotyl-derived callus of H. androsaemum were reported [1] to accumulate small amounts of flavonoids, with the highest levels occurring on the 14th day of the growth cycle.

Later on [2], it was found that treatment of 11-day-old cultures for 72h with 15 mM CaCl₂ or 5 µM calcium ionophore A23187 induced a substantial increase in the accumulation of flavonoids and in the activity of phenylalanine ammonia-lyase (PAL, a key enzyme of phenolic metabolism).

Because Ca²⁺ and cAMP are known to interact with each other and influence several metabolic processes, similar experiments were conducted using 100 µM db-cAMP, a membrane-permeable cAMP analog. The treatment with this cAMP modulator also induced a marked increase in both PAL activity and flavonoid content of cells measured on day 14.

To understand the mechanism by which cAMP exert these effects, cell cultures were then treated with either 50 µM Sp-cAMPS, an activator of cAMP-dependent protein kinase A (PKA), or 50 µM 8-pCPT-2'-O-Me-cAMP, an activator of exchange protein directly activated by cAMP (Epac). Addition of the PKA agonist to control cultures mimicked the Ca²⁺-induced enhancement of flavonoid accumulation and PAL activity, whereas the Epac activator had no significant effect on the levels of these parameters.

Taken together, these results suggest that PKA, not Epac, plays a role in regulating flavonoid accumulation in H. androsaemum cell cultures.

References:

[1] Paranhos A. Effect of calcium on enzyme activities and phenolic accumulation in Hypericum androsaemum cell cultures. Planta Med 2006; 72: 1060 – 1061

[2] Paranhos A. Effects of calcium and W-7 on flavonoid accumulation in cell cultures of Hypericum androsaemum L. Planta Med 2008; 74: 1160 – 1160