Review article: inhibition of methanogenic archaea by statins as a targeted management strategy for constipation and related disorders

Abstract

Background: Observational studies show a strong association between delayed intestinal transit and the production of methane. Experimental data suggest a direct inhibitory activity of methane on the colonic and ileal smooth muscle and a possible role for methane as a gasotransmitter. Archaea are the only confirmed biological sources of methane in nature and Methanobrevibacter smithii is the predominant methanogen in the human intestine.

Aim: To review the biosynthesis and composition of archaeal cell membranes, archaeal methanogenesis and the mechanism of action of statins in this context.

Methods: Narrative review of the literature.

Results: Statins can inhibit archaeal cell membrane biosynthesis without affecting bacterial numbers as demonstrated in livestock and humans. This opens the possibility of a therapeutic intervention that targets a specific aetiological factor of constipation while protecting the intestinal microbiome. While it is generally believed that statins inhibit methane production via their effect on cell membrane biosynthesis, mediated by inhibition of the HMG-CoA reductase, there is accumulating evidence for an alternative or additional mechanism of action where statins inhibit methanogenesis directly. It appears that this other mechanism may predominate when the lactone form of statins, particularly lovastatin lactone, is administered.

Conclusions: Clinical development appears promising. A phase 2 clinical trial is currently in progress that evaluates the effect of lovastatin lactone on methanogenesis and symptoms in patients with irritable bowel syndrome with constipation. The review concludes with an outlook for the future and subsequent work that needs to be done.

Figure 1

Taxonomy of Methanobrevibacter smithii. Only two methanogenic species have so far been isolated from the human colon: M. smithii is the predominant methanogen in the human gut. Methanosphaera stadtmanae, family Methanobacteriaceae, is less abundant. Note the Methanobrevibacter species in bovine and sheep rumen and mammalian faeces. The official taxonomy does not necessarily correlate with current molecular phylogenetics. Data from the Integrated Taxonomic Information System (ITIS)52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64 with isolate information from the Global Catalogue of Microorganisms.65

Figure 2

Methanobrevibacter smithii cell wall and cell membrane determine susceptibility to antibiotics and statins. The cell wall (violet) is composed of pseudomurein (and not murein as in bacteria) which makes archaea resistant to lysozyme and many antibiotics that interfere with cell wall synthesis. The cell membrane (ochre) consists of a lipid bilayer or monolayer the backbone of which composed of isoprene units that are linked to glycerol by ether bonds. In contrast, the lipid bilayer of bacteria consists of a fatty acid backbone that is linked to glycerol by an ester bond. The presence of statin‐sensitive isoprene units in the cell membrane of archaea allows statins to selectively interfere with the growth of archaea while leaving the cell membrane of bacteria unaffected. While bacteria do not use isoprene units in their cell membrane they are still required elsewhere. These bacterial isoprene units are, however, synthesised by a pathway (MEP) that is not inhibited by statins. See also Figure 3.

Figure 3

Most bacteria are not affected by statins. Most bacteria produce isoprenoids by the MEP pathway (red) that is not affected by HMGR‐inhibitors (statins), most other organisms including humans and archaea use the MVA pathway (blue). MEP: 2‐C‐methyl‐D‐erythritol 4‐phosphate. MVA: mevalonic acid. Humans and archaea utilise the HMGR‐I isoform (inhibited by statins) for isoprenoid biosynthesis. GAP, D‐glyceraldehyde 3‐phosphate; DXP, 1‐deoxy‐D‐xylulose 5‐phosphate; CDP‐ME, 4‐diphosphocytidyl‐2‐C‐methyl‐d‐erythritol; MEcPP, 2‐C‐methyl‐D‐erythritol 2,4‐cyclodiphosphate; HMBPP, 4‐hydroxy‐3‐methylbut‐2‐enyl diphosphate; IPP, isopentenyl diphosphate; DMAPP, dimethylallyl diphosphate. HMG‐CoA, 3‐hydroxy‐3‐methylglutaryl‐CoA; MVP, 5‐phosphomevalonate; MVPP, 5‐diphosphomevalonate. The alternative steps described in archaea are shown in green (IP, isopentenyl phosphate). Enzymes catalysing other reactions besides those in the MEP pathway are between brackets. Steps catalysed by different types of enzymes (shown within parentheses) are highlighted. Enzyme acronyms are given in the paper by Perez‐Gil and Rodriguez‐Concepcion 36 from which this illustration has been adapted.

Figure 4

Methanogenesis pathway and potential role of lovastatin. Coenzymes F₄₃₀ and F₄₂₀ play crucial roles in the methanogenesis pathway (see also Figure 5). In silico molecular docking of the methanogenic enzyme F₄₂₀‐dependent NADP oxidoreductase (fno) showed that both lovastatin and mevastatin had higher affinities for the F₄₂₀ binding site on fno than did F₄₂₀ itself. As such, lovastatin may act as inhibitor of fno. Fno is not shown in the diagram because it catalyses an alternative pathway of methanogenesis that utilises alcohol and methanol as substrates. However, if inhibition of fno by lovastatin can be experimentally confirmed, it is likely that other enzymes, such as those in the hydrogen‐CO ₂‐methanogenesis pathway depicted here that require the coenzyme F₄₂₀ would also be inhibited by lovastatin (as suggested in the diagram). The carbon atom reduced is highlighted in green. MF, methanofuran; MP, methanopterin; CoM, coenzyme M; Fd, ferredoxin; CoB, coenzyme B. This figure was adapted from reference.66

Figure 5

Coenzymes in the methanogenesis pathway. The final step in the methanogenesis pathway is catalysed by the key enzyme methyl‐coenzyme M reductase (Mcr). In its active site, Mcr contains a unique active group, a nickel porphinoid (left panel), called coenzyme F₄₃₀. Coenzyme F₄₂₀ (right panel) participates in two earlier steps in the methanogenesis pathway (see Figure 4) and is also responsible for the characteristic fluorescence of methanogens. In silico protein–ligand docking experiments suggest that lovastatin may have higher affinity for the F₄₂₀ binding site than F₄₂₀ itself.

Figure 6

Lovastatin lactone may have a different target in archaea than the hydroxyacid. Simvastatin and lovastatin are the commercially available statins that come in the lactone form. Their cholesterol‐lowering effect and the impairment of archaeal membrane synthesis through inhibition of HMGR requires activation, i.e. the lactone ring needs to be opened to result in the hydroxyacid form. As can be seen, the stereochemistry of lovastatin lactone and hydroxyacid is significantly different. Recent evidence suggests that methanogenesis is preferentially inhibited by the lactone form of lovastatin. This and other evidence would suggest that lovastatin may have a different or an additional target other than HMGR. A possible target for the lactone form are enzymes in the methanogenesis pathway that have F₄₂₀ as coenzyme. See Figure 5. The 3‐D was model generated with CORINA (http://www.molecular-networks.com/).