Laura Trapani

Regulation and deregulation of cholesterol homeostasis: The liver as a metabolic “power station”

Cholesterol plays several structural and metabolic roles that are vital for human biology. It spreads along the entire plasma membrane of the cell, modulating fluidity and concentrating in specialized sphingolipid-rich domains called rafts and caveolae. Cholesterol is also a substrate for steroid hormones. However, too much cholesterol can lead to pathological pictures such as atherosclerosis, which is a consequence of the accumulation of cholesterol into the cells of the artery wall. The liver is considered to be the metabolic power station of mammalians, where cholesterol homeostasis relies on an intricate network of cellular processes whose deregulations can lead to several life-threatening pathologies, such as familial and age-related hypercholesterolemia. Cholesterol homeostasis maintenance is carried out by: biosynthesis, via 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) activity; uptake, through low density lipoprotein receptors (LDLr); lipoprotein release in the blood; storage by esterification; and degradation and conversion into bile acids. Both HMGR and LDLr are transcribed as a function of cellular sterol amount by a family of transcription factors called sterol regulatory element binding proteins that are responsible for the maintenance of cholesterol homeostasis through an intricate mechanism of regulation. Cholesterol obtained by hepatic de novo synthesis can be esterified and incorporated into apolipoprotein B-100-containing very low density lipoproteins, which are then secreted into the bloodstream for transport to peripheral tissues. Moreover, dietary cholesterol is transferred from the intestine to the liver by high density lipoproteins (HDLs); all HDL particles are internalized in the liver, interacting with the hepatic scavenger receptor (SR-B1). Here we provide an updated overview of liver cholesterol metabolism regulation and deregulation and the causes of cholesterol metabolism-related diseases. Moreover, current pharmacological treatment and novel hypocholesterolemic strategies will also be introduced.

Potential role of nonstatin cholesterol lowering agents

Although statins, 3β‐hydroxy‐3β‐methylglutaryl coenzyme A reductase (HMGR) inhibitors, have revolutionized the management of cardiovascular diseases by lowering serum low density lipoproteins, many patients suffer from their side effects. Whether the statin side effects are related to their intrinsic toxicity or to the decrease of HMGR main isoprenoid end products, which are essential compounds for cell viability, is still debated. In addition to HMGR, the key and rate limiting step of cholesterol synthesis, many enzymes are involved in this multi‐step pathway whose inhibition could be taken into account for a “nonstatin approach” in the management of hypercholesterolemia. In particular, due to their unique position downstream from HMGR, the inhibition of squalene synthase, farnesyl diphosphate farnesyltransferase (FDFT1), squalene epoxidase (SQLE), and oxidosqualene cyclase:lanosterol synthase (OSC) should decrease plasma levels of cholesterol without affecting ubiquinone, dolichol, and isoprenoid metabolism. Thus, although FDFT1, SQLE and OSC are little studied, they should be considered as perspective targets for the development of novel drugs against hypercholesterolemia. Here, structure–function relationships of FDFT1, SQLE, and OSC are reviewed highlighting the advantages that the downstream inhibition of HMGR could provide when compared to the statin‐based therapy.

Cholesterol metabolism is altered in Rett syndrome: a study on plasma and primary cultured fibroblasts derived from patients.

Rett (RTT) syndrome is a severe neurological disorder that affects almost exclusively females. Several detectable mutations in the X-linked methyl-CpG-binding protein 2 gene (MECP2) are responsible for the onset of the disease. MeCP2 is a key transcription regulator involved in gene silencing via methylation-dependent remodeling of chromatin. Recent data highlight that lipid metabolism is perturbed in brains and livers of MECP2-null male mice. In addition, altered plasma lipid profile in RTT patients has been observed. Thus, the aim of the work is to investigate the protein network involved in cholesterol homeostasis maintenance on freshly isolated fibroblasts and plasma from both RTT and healthy donors. To this end, protein expression of 3-hydroxy-3methyl glutaryl Coenzyme A reductase (HMGR), sterol regulatory element binding proteins (SREBPs), low density lipoprotein receptor (LDLr) and scavenger receptor B-1 (SRB-1) was assessed in cultured skin fibroblasts from unaffected individuals and RTT patients. In addition, lipid profile and the abundance of proprotein convertase subtilisin/kexin type 9 (PCSK9) were analyzed on plasma samples. The obtained results demonstrate that the main proteins belonging to cholesterol regulatory network are altered in RTT female patients, providing the proof of principle that cholesterol metabolism may be taken into account as a new target for the treatment of specific features of RTT pathology.

3-hydroxy 3-methylglutaryl coenzyme A reductase inhibition impairs muscle regeneration.

Skeletal muscle has the ability to regenerate new muscle fibers after injury. The process of new muscle formation requires that quiescent mononuclear muscle precursor cells (myoblasts) become activated, proliferate, differentiate, and fuse into multinucleated myotubes which, in turn, undergo further differentiation and mature to form functional muscle fibers. Previous data demonstrated the crucial role played by 3‐hydroxy 3‐methylglutaryl coenzyme A reductase (HMGR), the rate‐limiting enzyme of cholesterol biosynthetic pathway, in fetal rat myoblast (L6) differentiation. This finding, along with epidemiological studies assessing the myotoxic effect of statins, HMGR inhibitors, allowed us to speculate that HMGR could be strongly involved in skeletal muscle repair. Thus, our research was aimed at evaluating such involvement: in vitro and in vivo experiments were performed on both mouse adult satellite cell derived myoblasts (SCDM) and mouse muscles injured with cardiotoxin. Results demonstrate that HMGR inhibition by the statin Simvastatin reduces SCDM fusion index, fast MHC protein levels by 60% and slow MHC by 40%. Most importantly, HMGR inhibition delays skeletal muscle regeneration in vivo. Thus, besides complaining of myopathies, patients given Simvastatin could also undergo an impairment in muscle repair. J. Cell. Biochem. 113: 2057–2063, 2012.

Age- and sex-related differences in extra-hepatic low-density lipoprotein receptor.

To determine whether differences in LDLr behavior in extra‐hepatic tissues and whether extra‐hepatic receptors could differentially contribute to cholesterol homeostasis under physiological conditions, we evaluated the presence and regulation of LDLr from both a gender and an aging perspective. We used the brain cortex, the gastrocnemius, and the heart ventricle of 3‐ and 12‐month‐old male and female rats. We observed a protein decrease of total LDLr in 12‐month‐old female rat brains that was completely restored by 17‐β estradiol treatment. In the gastrocnemius, LDLr accumulates in the skeletal muscle in both male and female aged rats as a precursor probably due to a glycosylation impairment. In the heart, no modifications were observed in either older rats or rats of a specific gender. These data highlight a tissue‐specific dysregulation of LDLr that is age‐ and gender‐dependent. J. Cell. Physiol. 226: 2610–2616, 2011.

Effects of myosin heavy chain (MHC) plasticity induced by HMGCoA-reductase inhibition on skeletal muscle functions

The rate‐limiting step of cholesterol biosynthetic pathway is catalyzed by 3‐hydroxy‐3‐methylglu‐taryl coenzyme reductase (HGMR), whose inhibitors, the statins, widely used in clinical practice to treat hypercholesterolemia, often cause myopathy, and rarely rhabdomyolysis. All studies to date are limited to the definition of statin‐induced myotoxicity omitting to investigate whether and how HMGR inhibition influences muscle functions. To this end, 3‐mo‐old male rats (Rattus norvegicus) were treated for 3 wk with a daily intraperitoneal injection of simvastatin (1.5 mg/kg/d), and biochemical, morphological, mechanical, and functional analysis were performed on extensor digitorum longus (EDL) muscle. Our results show that EDL muscles from simvastatin‐treated rats exhibited reduced HMGR activity; a 15% shift from the fastest myosin heavy‐chain (MHC) isoform IIb to the slower IIa/x; and reduced power output and unloaded shortening velocity, by 41 and 23%, respectively, without any change in isometric force and endurance. Moreover, simvastatin‐treated rats showed a decrease of maximum speed reached and the latency to fall off the rotaroad (~–30%). These results indicate that the molecular mechanism of the impaired muscle function following statin treatment could be related to the plasticity of fast MHC isoform expression.—Trapani, L., Melli, L., Segatto, M., Trezza, V., Campolongo, P., Jozwiak, A., Swiezewska, E., Pucillo, L.P., Moreno, S., Fanelli, F., Linari, M., Pallottini, V. Effects of myosin heavy chain (MHC) plasticity induced by HMGCoA‐reductase inhibition on skeletal muscle functions. FASEB J. 25, 4037–4047 (2011). www.fasebj.org

3-Hydroxy 3-methylglutaryl coenzyme A reductase increase is essential for rat muscle differentiation

3‐Hydroxy 3‐methylglutaryl coenzyme A reductase (HMG‐CoAR) is the key and rate‐limiting enzyme of cholesterol biosynthetic pathway. Although HMG‐CoAR activity has already been related to the differentiation of some cellular lines there are no studies that analyze the role of HMG‐CoAR, and the pathway it is involved with in a fully characterized muscle differentiation model. Thus, the aim of this work is to evaluate such role and delineate the pathway involved in foetal rat myoblasts (L6) induced to differentiate by insulin—a standard and feasible model of the myogenic process. The results obtained by biochemical and morphological approaches demonstrate that (i) HMG‐CoAR increase is crucial for differentiation induction, (ii) p21waf, whose increase is a necessary requisite for differentiation to occur, rises downstream HMG‐CoAR activation, (iii) the main role of p38/MAPK as key regulator also for HMG‐CoAR. Pathologies characterized by muscle degeneration might benefit from therapeutic programmes committed to muscle function restoration, such as modulation and planning myoblast differentiation. Thus, the important role of HMG‐CoAR in muscular differentiation providing new molecular basis for the control of muscle development can help in the design of therapeutic treatment for diseases characterized by the weakening of muscular fibers and aging‐related disorders (sarcopenia). J. Cell. Physiol. 220: 524–530, 2009.

Cholesterol Homeostasis Failure in the Brain: Implications for Synaptic Dysfunction and Cognitive Decline

Cholesterol is one of the most important molecules in cell physiology because of its involvement in several biological processes: for instance, it determines both physical and biochemical properties of cell membranes and proteins. Disruption to cholesterol homeostasis leads to coronary heart disease, atherosclerosis and metabolic syndrome. Strong evidence suggests that cholesterol also has a crucial role in the brain as various neurological and neurodegenerative disorders, including Alzheimers, Huntingtons and Parkinson diseases are associated with disruptions to cholesterol homeostasis. Here, we summarize the current knowledge about the role cholesterol plays at synaptic junctions and the pathological consequences caused by disruptions in the homeostatic maintenance of this compound.

Regulation of cholesterol biosynthetic pathway in different regions of the rat central nervous system

In this study, we investigated the regulatory network of the key and rate‐limiting enzyme of cholesterol biosynthetic pathway, the 3‐hydroxy 3‐methylglutaryl coenzyme A reductase (HMGR) in different brain regions, to add new insight about lipid metabolism and physiology in the central nervous system (CNS).

Short- and long-term regulation of 3-hydroxy 3-methylglutaryl coenzyme A reductase by a 4-methylcoumarin

Dyslipidemia is one of the most significant risk factors for cardiovascular diseases. Cholesterol homeostasis is regulated by both the receptor-mediated endocytosis of Low Density Lipoproteins by LDL receptors and de novo cholesterol synthesis via the rate-limiting enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase. Although statins, 3-hydroxy-3-methylglutaryl coenzyme A reductase substrate competitors, have revolutionized the management of cardiovascular diseases by lowering serum LDL, their side effects range from myalgia to rhabdomyolysis. Treatment with antioxidant compounds could represent an efficient alternative in the modulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity. Indeed it has already been demonstrated that the rise in reactive oxygen species levels causes the complete dephosphorylation and, in turn activation of the enzyme. Many coumarins and their derivatives have the special ability to scavenge reactive oxygen species or show a lipid lowering potential. Here we evaluated whether the coumarin, 4-methylesculetin could exert both the ability to scavenge ROS and to modulate 3-hydroxy-3-methylglutaryl coenzyme A reductase in HepG2 cell line where the enzyme activity dysregulation induced by reactive oxygen species has already been reported. The antioxidant property of 4-methylesculetin led to the reduction of 3-hydroxy-3-methylglutaryl coenzyme A reductase activation state through the increase of the enzyme phosphorylation. In addition, this coumarin showed the ability to modulate 3-hydroxy-3-methylglutaryl coenzyme A reductase protein levels both by transcriptional and degradational events independent of its antioxidant activity.