Targeting the mevalonate cascade as a new therapeutic approach in heart disease, cancer and pulmonary disease

Abstract

The cholesterol biosynthesis pathway, also known as the mevalonate (MVA) pathway, is an essential cellular pathway that is involved in diverse cell functions. The enzyme 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase (HMGCR) is the rate-limiting step in cholesterol biosynthesis and catalyzes the conversion of HMG-CoA to MVA. Given its role in cholesterol and isoprenoid biosynthesis, the regulation of HMGCR has been intensely investigated. Because all cells require a steady supply of MVA, both the sterol (i.e. cholesterol) and non-sterol (i.e. isoprenoid) products of MVA metabolism exert coordinated feedback regulation on HMGCR through different mechanisms. The proper functioning of HMGCR as the proximal enzyme in the MVA pathway is essential under both normal physiologic conditions and in many diseases given its role in cell cycle pathways and cell proliferation, cholesterol biosynthesis and metabolism, cell cytoskeletal dynamics and stability, cell membrane structure and fluidity, mitochondrial function, proliferation, and cell fate. The blockbuster statin drugs ('statins') directly bind to and inhibit HMGCR, and their use for the past thirty years has revolutionized the treatment of hypercholesterolemia and cardiovascular diseases, in particular coronary heart disease. Initially thought to exert their effects through cholesterol reduction, recent evidence indicates that statins also have pleiotropic immunomodulatory properties independent of cholesterol lowering. In this review we will focus on the therapeutic applications and mechanisms involved in the MVA cascade including Rho GTPase and Rho kinase (ROCK) signaling, statin inhibition of HMGCR, geranylgeranyltransferase (GGTase) inhibition, and farnesyltransferase (FTase) inhibition in cardiovascular disease, pulmonary diseases (e.g. asthma and chronic obstructive pulmonary disease (COPD)), and cancer.

Keywords: Asthma; Farnesyl transferase inhibitors; Fibrosis; Geranylgeranyl transferase inhibitors; Rho GTPase; Statins.

Figure 1. Overview of the cholesterol biosynthesis pathway

(A) Farnesol or related isoperinoids regulate Ras farnesylation and other GTPases like Rho and Ras, resulting in GTPase activation and p53-mediated induction of apoptosis and cell growth regulation. (B) Inhibition of squalene synthase (SQS) decreases raft-associated cholesterol levels, thus attenuates cancer cell proliferation and also induces death of cancer cell. (C) Suppression of cholesterol biosynthesis from lanosterol leads to inhibition of cell cycle progression and also cell differentiation. (D) AEBS ligands are associated with zymosterol and 7-dehydrocholesterol, which can induce cancer cell differentiation and death through the production of reactive oxygen species (ROS) and oxysterols. Suppression of ROS production by antioxidants leads to cell survival through an autophagic process by induction of AEBS ligands. (E) Cholesteryl esters of fatty acids (CEFA) is the product of intracellular cholesterol esterification that is catalyzed by the Acyl-coA:Cholesterol Acyl Transferase (ACAT) using cholesterol and fatty acyl-coenzyme A esters (RCoA). CEFA is the major lipid found in foam cells which plays important role in atherosclerosis. CEFA is also implicated in the stimulation of cancer cell proliferation, invasiveness and mitogenesis.

Figure 2. Effect of different inhibitors on the mevalonate pathway

Statins, bisphosphonates, FTIs and GGTIs are different classes of drugs which have various inhibitory effects on the MVA pathway. Statins block the conversion of HMG-CoA to MVA by suppressing the HMGCR and thereby inhibits Rac geranylgeranylation and Ras farnesylation. Statins also attenuate reactive oxygen species (ROS) derived from NADPH oxidase. Bisphosphonates (BPs) inhibit the IPP isomerase and isoprenoid biosynthesis downstream by targeting FPP synthase and indirectly interfering with protein isoprenylation. FTIs and GGTIs are prenyltransferase inhibitors. The formation of a covalent bond between the isoprenoids FPP and GGPP and the GTPases (e.g. Ras, Rho and Rac) by prenyl transferases is targeted by FTIs and GGTIs. Other inhibitors such as squalene synthase inhibitors (SQSIs) and oxidosqualene cyclase inhibitors (OSCIs) target the synthesis of the cholesterol precursor squalene and lanosterol synthesis, respectively.

Figure 3. The Rho GTPase molecular switch

When cell is in resting state, Rho GTPases exist mostly in the cytosol, in inactive (GDP-bound) form, in complexes with Rho GDI. Following an activation signal, Rho GTPases are targeted to the membrane by post-translational modification of their COOH termini with lipid moieties (i.e. farnesyl, geranyl-geranyl, palmitoyl and methyl) by geranyl-geranyltransferases (GGTases). This reaction allows the activated small GTPases (in the GTP-bound form) to interact with the cell membranes, where they exert their function. Cycling between an inactive GDP-bound and an active GTP-bound is regulated by guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). GEFs release guanosine diphosphate (GDP) from Rho GTPases promoting the binding of guanosine triphosphate (GTP) and activation of Rho GTPases. In the GTP-bound form, Rho proteins undergo a conformational change which allows them to interact with effector proteins and initiate a downstream cellular response. GTPase activating protein (GAP) converts GTP-bound form of Rho GTPases to inactive GDP-GTPases by hydrolyzing GTP into GDP and terminate the signal transduction.