Current Molecular Pharmacology - Volume 10, Issue 1, 2017

Inhibition of hydroxymethylglutaryl-coenzyme A (HMG-CoA) reductase by pharmaceuticals, commonly referred to as statins, has proven to be an effective and efficient way in reducing cholesterol levels in patients. As a result of this intervention, mevalonate production, formed during cholesterol synthesis, is inhibited. Mevalonate is the precursor to a variety of crucial downstream products, including those involved with the mitochondrial electron transport chain, and localized activation of small GTPases. Statins have also been observed to induce changes of the immune system, favouring a reduced proinflammatory phenotype. However, near complete cessation of mevalonate and its downstream products have severe pro-inflammatory consequences as evident by patients suffering from mevalonate kinase deficiency who have increased inflammasome activity. It is evident that mevalonate production is a pivotal component of normal homeostatic cell processing, especially in maintaining a muted inflammatory response.

The cholesterol biosynthesis pathway, also referred to as the mevalonate (MVA) pathway, is responsible for the biosynthesis of two key isoprenoids: farnesyl pyrophosphate (FPP) and geranylgeranyl pyrophosphate (GGPP). Post-translational modification of small GTPases by FPP and GGPP has captured much attention due to their potential contribution to cancer, cardiovascular and neurodegenerative diseases. The enzyme 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase (HMGCR) catalyzes the conversion of HMG-CoA to MVA, and is the rate-limiting step in the biosynthesis of cholesterol. Statins are HMGCR inhibitors that are used extensively in the treatment of hypercholesterolemia. Inhibitors of the MVA pathway exhibit anti-tumor effects and may reduce cancer incidence and cancer-related mortality in humans. In this review, we will focus on the mevalonate cascade and its regulation in cholesterol metabolism as well as polymorphisms of the MVA cascade in cancer development, infectious and cardiovascular disease (CVD).

The mevalonic acid synthesis pathway, cholesterol, and lipoproteins play fundamental roles in lung physiology and the innate immune response. Recent literature investigating roles for cholesterol synthesis and trafficking in host defense against respiratory infection was critically reviewed. The innate immune response and the cholesterol biosynthesis/trafficking network regulate one another, with important implications for pathogen invasion and host defense in the lung. The activation of pathogen recognition receptors and downstream cellular host defense functions are critically sensitive to cellular cholesterol. Conversely, microorganisms can co-opt the sterol/lipoprotein network in order to facilitate replication and evade immunity. Emerging literature suggests the potential for harnessing these insights towards therapeutic development. Given that >50% of adults in the U.S. have serum cholesterol abnormalities and pneumonia remains a leading cause of death, the potential impact of cholesterol on pulmonary host defense is of tremendous public health significance and warrants further mechanistic and translational investigation.

Current evidence suggests that persisting and/or exaggerated inflammation in the lungs initiated by smoking, and up-regulated through genetic susceptibility, may result in lung remodelling and impaired repair. The mevalonate pathway, through its modifying effects on innate immune responsiveness, may be involved in these processes providing a plausible pathogenic link between the development of chronic obstructive pulmonary disease (COPD) and lung cancer. The mevalonate pathway, mediates these effects through important intra-cellular signalling molecules called guanine phosphate transferases (GTPases) such as Rho-A. Smoke exposure activates cell surface proteins which, through the mediating influence of GTPases, then modify the activation of NFΚB and its downstream effects on genes underlying innate immunity, neutrophilic inflammation and carcinogenesis. The mevalonate pathway is readily and substantially modified by inhibition of the enzyme 3-hydroxy-3-methyl-glutaryl-Coenzyme A (HMGCo-A) reductase. This enzyme controls the rate limiting step of the mevalonate pathway and is subject to inhibition by statin drugs and small chain fatty acids derived from high dietary fibre intake. Thus inhibiting the mevelonate pathway, and dampening the innate immune response to smoking, may play a critical role in modifying pulmonary inflammation and lung remodelling. Such an action might slow the progression of COPD and reduce the tendency to the development of lung cancer. This review examines the pre-clinical and clinical data suggesting that HMGCoA-reductase inhibition and it’s modification of the mevalonate pathway, may have a chemo-preventive effect on lung cancer, particularly in patients with COPD where pulmonary inflammation is increased and the risk of lung cancer is greatest.

Immunomodulatory effects of statins in vitro and in experimental models of asthma and COPD could potentially be relevant to the treatment of chronic airway diseases. This article provides an overview of the evidence from the key clinical studies on the effects of statins on clinical outcomes and inflammatory biomarkers in asthma and COPD. Future directions for clinical studies of statins in asthma and COPD are discussed. A small number of randomized controlled trials (RCTs) in adults with mild to moderate asthma suggest that short-term statin treatment does not improve lung function or symptom control, except for a possible improvement in quality of life and symptoms in smokers with asthma. Several observation studies report that statin use in asthma is associated with a reduced risk of asthma-related emergency department visits, oral corticosteroid dispensing or hospital admissions. Statins treatment in asthma may have modest local anti-inflammatory effects in the airways. There is a need for a large RCT examining the effects of statins on reducing exacerbations, particularly in severe asthma. In COPD, observational studies suggest that statin use is associated with reduced morbidity and/or mortality, whereas a large RCT concluded that simvastatin did not reduce exacerbations in patients with COPD that had no cardiovascular indication for statin treatment. It is possible that a subgroup of COPD patients with cardiovascular indications for statins and/or systemic inflammation may obtain clinical benefit from statin treatment. Understanding the immunomodulatory effects of statins in asthma and COPD may lead to the development of novel targeted interventions.