Effect of statin treatment on preterm labour

Abstract

Preterm labour (PTL) is defined as labour before 37 completed weeks of gestation.

Despite advances in medical research, PTL remains a major clinical problem. Preterm birth (PTB) rates range from approximately 5-18% worldwide. Importantly, PTB is the leading cause of childhood morbidity and mortality. PTL is difficult to predict and the aetiology is poorly understood but infection and inflammation are believed to be major factors. It has been suggested that the presence of intrauterine infection or inflammation may initiate the pathological, preterm activation of the inflammatory cascade associated with term labour. Therefore, PTL therapeutics should aim to inhibit these inflammatory pathways. Statins, 5-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors, are potent inhibitors of cholesterol biosynthesis, which act on the mevalonate pathway. In addition to their lipid-lowering effects, statins also have anti-inflammatory and anti-contraction properties. The hypothesis of this thesis was that statins will prevent PTB by reducing inflammation. The aims of this thesis were firstly to investigate the effect of the statins, simvastatin and pravastatin, on inflammation and contractility in a pregnant human myometrial cell line. Secondly, to determine whether simvastatin and/or pravastatin can prevent PTB or improve neonatal outcome in a lipopolysaccharide (LPS)-induced mouse model of PTB.

Myometrial cells were either co-treated with LPS and simvastatin/pravastatin, pretreated with simvastatin/pravastatin or treated with simvastatin/pravastatin post-LPS stimulation. The effect of statin treatment on the mRNA expression and the release of inflammatory mediators was then investigated. Simvastatin treatment reduced LPS-induced inflammation by both lowering the expression of pro-inflammatory mediators and increasing the expression of anti-inflammatory mediators. Pravastatin treatment did not alter the expression of inflammatory mediators following LPS stimulation.

The effect of simvastatin on the contraction of myometrial cells was investigated by embedding the cells in rat tail collagen to form gels. As these are smooth muscle cells, basal contraction was observed causing the gel size to reduce. When LPS was introduced, this caused the gels to contract further than the vehicle treated gels.

Simvastatin attenuated the contraction of the myometrial cells, both alone and in the presence of LPS. These effects were reversed by the addition of mevalonate pathway metabolites, mevalonate and geranylgeranyl pyrophosphate (GG-PP) but not by farnesyl pyrophosphate (F-PP). Simvastatin also lowered levels of phosphorylated myosin light chain (pMLC) in the myometrial cells, which is essential for smooth muscle contraction. Again, this effect was abolished by mevalonate and GG-PP but not F-PP. It is hypothesised that simvastatin attenuated myometrial cell contraction by inhibiting Rho isoprenylation by GG-PP, preventing Rho-associated kinase (ROCK) activation, which then prevented the phosphorylation of MLC.

A mouse model of intrauterine LPS-induced PTB was utilised to investigate the effect of statin treatment on PTB and fetal survival. Mice received an intraperitoneal injection of pravastatin (10μg) or simvastatin (20μg or 40μg) on gestational day (D)16.

This was followed by ultrasound-guided intrauterine injection of LPS (1μg) on D17 and another pravastatin/simvastatin treatment two hours later. When mice were treated with LPS, 77.8% of mice delivered preterm. When mice received LPS and 20μg simvastatin, 50% delivered preterm. However, when mice were treated with LPS and 40μg simvastatin, 40% delivered preterm, more pups were born alive and uterine pro-inflammatory mRNA expression was downregulated. Conversely, pravastatin did not prevent PTB or improve the percentage of live born pups.

In summary, simvastatin treatment exerted anti-inflammatory and anti-contraction effects on human myometrial cells in vitro. The anti-contractile properties were likely due to the inhibition of the Rho/ROCK pathway. Furthermore, in our LPS-induced mouse model of PTB, fewer mice delivered preterm with simvastatin treatment, simvastatin attenuated LPS-induced pup mortality and reduced uterine inflammatory gene expression. These results suggest that statin therapy may be a novel treatment for PTL.