Selective inhibitors of calmodulin-dependent phosphodiesterase and other enzymes

Cholesterol efflux to apoA-I in ABCA1-expressing cells is regulated by Ca2+-dependent calcineurin signaling

ATP-binding cassette transporter A1 (ABCA1) is required for the lipidation of apolipoprotein A-I (apoA-I), although molecular mechanisms supporting this process remain poorly defined. In this study, we focused on the role of cytosolic Ca(2+) and its signaling and found that cytosolic Ca(2+) was required for cholesterol efflux to apoA-I. Removing extracellular Ca(2+) or chelating cytosolic Ca(2+) were equally inhibitory for apoA-I lipidation. We provide evidence that apoA-I induced Ca(2+) influx from the medium. We further demonstrate that calcineurin activity, the downstream target of Ca(2+) influx, was essential; inhibition of calcineurin activity by cyclosporine A or FK506 completely abolished apoA-I lipidation. Furthermore, calcineurin inhibition abolished apoA-I binding and diminished JAK2 phosphorylation, an established signaling event for cholesterol efflux to apoA-I. Finally, we demonstrate that neither Ca(2+) manipulation nor calcineurin inhibition influenced ABCA1's capacity to release microparticles or to remodel the plasma membrane. We conclude that this Ca(2+)-dependent calcineurin/JAK2 pathway is specifically responsible for apoA-I lipidation without directly modifying ABCA1 activity.

Molecular properties of cyclic nucleotide phosphodiesterase isozymes

Mammalian cells contain multiple molecular forms of cyclic nucleotide phosphodiesterase that differ in substrate specificity and kinetic and regulatory properties. Calcium/calmodulin and cyclic GMP are important regulators of the hydrolysis of cyclic AMP by either stimulating or inhibiting the activity of distinct forms of phosphodiesterase. Several isozymes of cyclic nucleotide phosphodiesterase have been purified to apparent homogeneity. Although some sequence homology is observed the isozymes appear genetically distinct by immunological criteria. Cyclic AMP- and calmodulin-dependent protein kinases can phosphorylate these enzymes and alter their kinetic and regulatory properties. Both tissue specificity and pharmacological selectivity of isozymes have been demonstrated for several drugs. In certain cases, e.g. cardiac muscle, the selective inhibition of a high affinity cAMP phosphodiesterase activity in a specific subcellular fraction correlates with pharmacologic responses. The results from molecular and pharmacologic studies of cyclic nucleotide phosphodiesterases have indeed expanded the role this system of isoenzymes exerts in the regulation of cellular function.