Role for the PIP2-binding protein myristoylated alanine-rich C-kinase substrate in vascular tissue: A novel therapeutic target for cardiovascular disease

Abstract

In vascular smooth muscle cells (VSMCs) and vascular endothelial cells (VECs), phosphatidylinositol 4,5-bisphosphate (PIP₂) acts as a substrate for phospholipase C (PLC)- and phosphoinositol 3-kinase (PI3K)-mediated signaling pathways and an unmodified ligand at ion channels and other macromolecules, which are key processes in the regulation of cell physiological and pathological phenotypes. It is envisaged that these distinct roles of PIP₂ are achieved by PIP₂-binding proteins, which act as PIP₂ buffers to produce discrete pools of PIP₂ that permits targeted release within the cell. This review discusses evidence for the expression, cell distribution, and role of myristoylated alanine-rich C-kinase substrate (MARCKS), a PIP₂-binding protein, in cellular signaling and function of VSMCs. The review indicates the possibilities for MARCKS as a therapeutic target for vascular disease involving dysfunctional cell proliferation and migration, endothelial barrier permeability, and vascular contractility such as atherosclerosis, systemic and pulmonary hypertension, and sepsis.

Keywords: MARCKS; PIP2; contractility; migration; permeability; proliferation; vascular endothelial cells; vascular smooth muscle cells.

FIGURE 1

Molecular structure of MARCKS. Diagrammatic representation of MARCKS, showing that it is composed of three domains, an N‐terminal myristoylated domain which links to the plasma membrane, an MH‐2 domain, and a 25‐amino acid (aa) effector domain (ED) containing a +13 charge that produces electrostatic interactions with three PIP₂ molecules in the intracellular leaflet of the plasma membrane. PKC‐dependent phosphorylation of serine residues or Ca²⁺‐CaM binding within the ED leads to the dispersion of the electrostatic interactions, with the release of PIP₂ into the local environment that can bind to local transmembrane proteins and translocation of MARCKS from the plasma membrane into the cytoplasm.

FIGURE 2

Proposed role of MARCKS in cell cycle and proliferation of VSMCs. MARCKS binds to and stabilizes KIS, which enables KIS to phosphorylate p27^kip1 at serine 10. Phosphorylated p27^kip1 is translocated from the nucleus to the cytoplasm, releasing a cell‐cycle brake leading to cell‐cycle progression and proliferation.

FIGURE 3

Proposed role of MARCKS in regulating contractility of VSMCs. Stimulation of G‐protein‐coupled receptors induces inhibition of MARCKS, likely via activation of PKC‐ and/or Ca²⁺‐CaM‐dependent pathways, which leads to the translocation of MARCKS from the plasma membrane to the cytoplasm and release of PIP₂ that interacts with and stimulates VGCCs, leading to Ca²⁺ influx and contraction. This model suggests that in unstimulated VSMCs MARCKS inhibits the contractile process.

FIGURE 4

Proposed role of MARCKS in regulating movement of VECs and endothelial barrier permeability. (A) Stimulation of the insulin receptor leads to phosphorylation of MARCKS, which causes translocation of MARCKS from the plasma membrane into the cytosol and the release of PIP₂ in the local environment where it binds to N‐WASP, leading to interactions with Arp 2/3 and increased actin arrangement and remodeling leading to cell movement. (B) Stimulation of AT‐1 and P2Y1 receptors leads to H2O2 production (via different Rac1 and NOX‐mediated pathways), which induces phosphorylation of MARCKS that is associated with increased actin arrangement, remodeling, and barrier permeability.