Selective inhibition of cyclic AMP-dependent protein kinase by amphiphilic triterpenoids and related compounds

A set of plant- and animal-derived amphiphilic triterpenoids have been shown to be potent and selective inhibitors of the catalytic subunit of rat liver cyclic AMP-dependent protein kinase (cAK). Thus plant-derived 18α-and 18α-glycyrrhetinic acid, ursolic acid, oleanolic acid and betulin and animal-derived lithocholic acid, 5-cholenic acid and lithocholic acid methyl ester are inhibitors of cAK with IC₅₀ values (concentrations for 50% inhibition) in the range 4-20 μM. These compounds are ineffective or relatively ineffective as inhibitors of various other eukaryote signal-regulated protein kinases namely wheat embryo Ca²⁺ -dependent protein kinase (CDPK), avian calmodulin-dependent myosin light chain kinase (MLCK) and rat brain Ca²⁺- and phospholipid-dependent protein kinase C (PKC). These naturally occurring triterpenoids have a common structural motif involving polar residues located at opposite ends of an otherwise non-polar triterpenoid nucleus. A variety of triterpenoids not possessing this structural motif are relatively inactive as inhibitors of cAK and of CDPK, PKC and MLCK. The terpenoid amphiphilic compound crocetin is also a potent and relatively selective inhibitor of cAK (IC₅₀value for cAK 3.0μM). 12-Hydroxystearic acid and 10-hydroxydecanoic acid do not inhibit CDPK, PKC or MLCK but are selective inhibitors of cAK (IC₅₀ values 127 and 138 μM, respectively), consistent with a simple model for amphiphile inhibition of cAK involving two polar groups separated by a non-polar region. However, laurylgallate and 15-pentadecanolide are also potent and selective inhibitors of cAK (IC₅₀ values 1.5 and 20 μM, respectively) although the structures of both of these compounds involve a large non-polar portion associated with only one polar region. Crocetin and the plant-derived amphiphilic triterpenoids described here are the most potent non-aromatic plant-derived inhibitors of cAK yet found.