PMA has been shown to phosphorylate and translocate MARCKS to lysosome in rat hepatocytes.36

Studies in most other cell types suggest a role of MARCKS in the exocytosis and exocytotic insertion of membrane proteins. Thus, the phosphorylation of MARCKS has been implicated in neurotransmitter release,37 glucose-induced secretion in isolated rat pancreatic islets,38 the release of adrenocorticotropin in ovine anterior pituitary cells,39 thrombin-induced serotonin release from platelets,40 insulin-induced Glut4 translocation to the PM in rat skeletal muscle cells,41 and mucin secretion in bronchial epithelial cells.15 However, MARCKS Epigenetics Compound Library cell assay phosphorylation, most likely by PKCϵ, has also been suggested to be involved in basolateral fluid-phase endocytosis in T84 cells.19 MARCKS phosphorylation has also been

suggested to be involved in an abnormal endocytic pathway in Alzheimer disease.42 On the basis of these studies and the results of the present study, we suggest that MARCKS phosphorylation leads to endocytic retrieval Erastin manufacturer of MRP2 in hepatocytes. To our knowledge, this is the first study implicating MARCKS phosphorylation in membrane transporter retrieval in hepatocytes. The precise intracellular mechanisms by which MARCKS regulates endocytosis and exocytosis have not been fully elucidated.12, 43 The finding that MARCKS can bind directly to actin and crosslinks it to PM44 has led to the suggestion that actin is essential to the overall functioning

of MARCKS. The binding of MARCKS to the membrane requires the electrostatic interaction of basic (serine) residues of MARCKS in its effector domain with acidic lipids of the membrane and the hydrophobic insertion of myristate into the core of the membrane. Both of these interactions are necessary for significant membrane binding.12, 43, 44 When the serine residues in the effector domain of MARCKS are phosphorylated by PKC or replaced by alanine as in PD-MARCKS, the electrostatic interaction between MARCKS and the acidic lipids is abolished, and this results in the dissociation of MARCKS from the membrane. Because of the proximity of MARCKS phosphorylation sites to the actin binding site,45 MARCKS phosphorylation buy Paclitaxel also results in the release of actin and a local softening (disruption) of the actin cytoskeleton with increased plasticity and endocytosis.11, 12 Thus, it can be speculated that by binding and tethering actin, unphosphorylated MARCKS stabilizes MRP2 in the membrane, as has been suggested for other actin crosslinking proteins, radixin46, 47 and Na+/H+ exchanger regulatory factor 1.48 Consistent with this hypothesis is a recent study in rats showing that taurochenodeoxycholate-induced retrieval of MRP2 is associated with changes in the actin cytoskeleton.49 Because three serine residues are replaced by alanine in PD-MARCKS, such a mechanism can also explain decreased PM-MRP2 in cells transfected with PD-MARCKS (Fig.