Acyl homoserine lactone – signal or weapon or both? Implications for cancer treatment

Carmichael A. J. C. BDS

Abstract

The phenomenon of cell signalling or quorum sensing in bacterial populations was first suggested in 1970 by Nealson et al. Knowledge of its various forms has since developed rapidly as more accurate measuring and compound identification techniques have become available. The production of small, low molecular weight signalling molecules able to diffuse freely between the cellular and external environment continues until a minimum threshold level is reached indicative of an optimum population size – the quorum. It is seen as a way of controlling overgrowth that would lessen the efficiency of the population. These small molecules are of the general description of acyl homoserine lactones.

The recent exciting discovery of homocysteine lactones as a unique feature of aggressively dividing cancer cells suggests some parallels. The ability of homocysteine lactone to combine with dehydroascorbate to form the toxic compound 3-mercaptoproprionaldehyde results in selective cancer cell apoptosis. This finding raises the question as to whether the apparent signalling compound acyl homoserine lactone is also a bacterial weapon in view of the ascorbate present in all normal host cells and the similar possibility of formation of lethal or damaging compounds. It may be a major factor in the mechanism of initial bacterial attack on host cells. If the ‘weapon’ hypothesis is taken further the homocysteine lactone produced by the proliferating cancer cells could be a significant factor in the unselective breakdown of adjoining host cells during invasive tumour growth.

Article summary

Contrasts the conserved similarities of bacterial, human and malignant cell defences with the offensive mechanisms of bacteria and invasive carcinoma cells. Postulates that perceived bacterial signalling may be offensive. Suggests that a lipid soluble form of ascorbate may be effective against malignant growth.

Keywords: ascorbate, quorum-sensing, ulcer, cancer, lactone

Article focus

• The conserved similarities in bacterial signalling / weaponry and invasive carcinoma attack
• The potential for use of a simple food additive as potentiator of normal host cell defence.

Key message

Conserved mechanisms in bacteria and human cells are similar enough to provide a possible answer to treatment of diffuse or non-resectable cancer

Strength and limitation

• A new approach focussing on supporting basic inbuilt cell defence mechanisms with a safe, low-cost substance
• Potential life and cost savings
• Case support for the hypothesis
• Further study required

Discussion

Acyl homoserine lactones (AHSL) are small molecules produced by bacteria in circumstances of active growth under stress or competition (1) and their production appears to have a measurable relationship to the optimum concentration of the bacterial population in different circumstances (2). This relationship has been proposed as a feedback or signalling method to provide a measure of the optimum population of the particular bacteria in varying situations of stress level, nutrition, environment and level of competition present – the quorum.

It has been demonstrated that in a benign environment of adequate nutrition and no competitor strains, no measurable amounts of AHSL were produced by bacterial strains which had previously shown the ability to do so (1).

Although there is acknowledged difficulty in proving a negative conclusion it is suggested that the choice of production of AHSL or none is available to the bacteria and is influenced by their environmental conditions (1)(2)(3). This may reflect variations in virulence of the same strains dependent on the conditions. For example it might in some part explain the ability of MRSA organisms to cause no reaction on the skin surface where they are normally resident but to be aggressive when in a different sub-surface environment.

As long ago as 1982 M. E. Poydock (4)(5) demonstrated the effectiveness of dehydroascorbate against aggressive cancers in mice but the phenomenon was ignored for lack of explanation or because it was simply not believed.

The recent publication of the reasons for the selective apoptosis of cancer cells when dehydroascorbate is given (6) centres on the production within the tumour cells of homocysteine lactone (HCyL). This thiolactone has been shown to be singular to actively proliferating tumour cells (7) and in this respect the tumour cells resemble bacteria producing the similar AHSL whilst rapidly proliferating in a stressful environment.

The reaction of dehydroascorbate and HCyL within or near the tumour cell results in production of small amounts of a toxic substance, 3-mercaptoproprionaldehyde, which kills the cell (8,9).

Most mammalian normal cells contain ascorbic acid – Vitamin C – and it is transported across the cell membrane as dehydroascorbate via a glucose pathway (10). Compounds capable of donation of a thio-radical are found in normal cell constituents (11). Indeed, both AHSL and HCyL are formed from methionine. A plant precursor of ascorbic acid is galactono-1, 4-lactone.

Removal of the acyl group from AHSL to leave homoserine lactone (HSL) is a necessary prerequisite of formation of the toxic compound 3-hydroxypropionaldehyde (15) as a result of the reaction of dehydroascorbate and HSL. Acylases are produced by many bacteria and can be expected to be found in areas of multi-species infection. Deacylation is one of the means by which competing bacterial populations seek to gain advantage over their competitors by swamping the competing signals or employing host dehydroascorbate to destroy their rivals’ signal (or weapon) production (30).

There are suggestions of using these signalling or weapon systems as a means of bacterial control (11)(12)(13).

It is reasonable to propose that the production and dissemination of AHSL by bacteria could be construed to be a weapon in a mirror of the process in tumour cells whereby host cell metabolic damage, suspension of mitosis or apoptosis is the result. The use of the acyl group can be seen as a measure for protection of the bacterial signal against the ascorbate present in the host cells.

Host cell damage could result in an initial ulcerative lesion or abscess providing an attraction for opportunistic secondary infection or a source of nutrients for invasive proliferation by the primary bacterial attacker. The propionaldehyde produced from the reaction of dehydroascorbate and homoserine lactone is an inhibitor of cell metabolism and distorts normal processing (31) resulting in a backup of chemical detritus such as aldehydes. Aldehyde blockage is wasteful of nutrients and energy and inhibits cell function (32). If its production can be reduced or halted the processes can return to normal as the blockage is cleared. If the attack is powerful enough then apoptosis occurs.

It has been recently demonstrated that microglial metabolism in Alzheimer’s Disease is similarly damaged by herpes simplex virus (33) and that the virus ends up entombed in extra-cellular misfolded protein waste products known as amyloid (34). It is interesting that the process of formation of such extra-cellular waste follows the path of neurons and associated microglia from the gut to the brain as the virus spreads over a period of years (36). It is an attempt by the host cells to neutralise and eliminate an aggressor using the effect of the aggressor on cell metabolism to the best advantage. In this research (35) the virus is shown incorporated irretrievably into the mis-folded protein. As a result the normal cell protein breakdown is disrupted for lack of nutrients and energy and the cell malfunctions. In the nervous system these blockages of normal cell metabolism lead to long-term dysfunction such as Parkinson’s or Alzheimer’s Diseases. The changes are reversible in the early stages but end with eventual apoptosis of cells after long periods of disruption.

In the mirror of this process the production of HCyL and its emission from the invasive tumour cell into the area of contact with host tissue would provide an opportunity for formation of the toxic 3-mercaptopropionaldehyde around and within the cell membrane and subsequent apoptosis of the host cell. This toxicity would also affect the invading tissue but its greater speed of cell division would eventually result in the advantage in favour of the invader. The immediate interface between host and invasive tissue is acidic and oxygen deprived which are not good conditions for host cells and may advantage the invader (16)(17). A supply of nutrients such as glucose from the demolition of the well vascularised host tissue is a great benefit to the poorly vascularised invasive tissue (18). These factors are a plausible explanation for the apparently unselective invasive tissue damage caused by malignant tumours with the key differences of rapid growth and HCyL formation separating their behaviour from that of more benign growths. The mechanism of this change from benign to malignant may be simply the genetic changes needed for production of HCyL within the cells. It is reasonable to speculate that this might be influenced or induced by bacterial, fungal, viral or environmental involvement and in the presence of host genetic susceptibility. Archaea, a very resilient, sulphur tolerant genus of protobacteria, nocardia, various fungal infections, common viruses and Pseudomonas aeruginosa are candidates for investigation in this context (29).

The processes can be summarised:

Bacterial Weapon                                                                                                    Host Defence

• Acylhomoserine lactone                                                                Dehydroascorbate
• Effect: (12)(13)(14) host cell damage                                         : invader cell damage / apoptosis
• Outcome: abscess or ulcer /  secondary infection                   : recovery

Tumour Weapon                                                                                                     Host defence

• Homocysteine lactone                                                                  Dehydroascorbate
• Effect: indiscriminate apoptosis invasion / proliferation     :apoptosis of tumour cells with host cell replacement
• Outcome: invasion / rapid growth / vital organ damage      :tumour retreat with possible recovery

NB. This is a balancing act

• If dehydroascorbate present in insufficient amounts tumour advances
• Tumour retreats if dehydroascorbate present in sufficient amounts

These considerations lead to a suggestion that ascorbate in some form might be a potent cure for cancer either by itself or in combination with other agents (27)(28).

In view of the difficulty of supplying dehydroascorbate itself (requiring injection into the target) it is proposed that 6-O-palmitoyl-l-ascorbate (E-304) (PA) (known in the USA and Canada as ascorbyl palmitate) is a suitable candidate for consideration as an orally administered anti-cancer agent. It is lipid soluble and therefore hydrophobic and can be taken dissolved in olive oil, fish oil or butter – olive oil is in itself beneficial (19)(20). A daily intake of 1,000 mg PA gives the recommended maximum daily intake of 400 mg of ascorbate although some practitioners suggest much higher intakes. On breakdown it supplies palmitic acid and the ascorbic acid which on oxidation produces dehydroascorbate, the form in which ascorbate is transported across cell membranes by a glucose transport system. It is thus available in the immediate vicinity of the host cell in its most potent form.

PA is widely used as E-304 in products ranging from infant formula milk to beefburgers. It has been extensively studied and used in the cosmetics, agricultural and the food industries as an anti-oxidant and even as a component of edible anti-oxidant salad packaging now in development. It is available in nanopackages (21)(22) or simply as a powder. Safe human consumption in therapeutic quantities up to 1.6 g/day is well-established.

Palmitoyl ascorbate has known anti-angiogenic properties which contribute to its attraction as an anti-cancer agent (24). Anti-angiogenesis has been shown to be a valuable addition to the armoury of cancer treatments (25)(26). The formation of new vessels in an actively invasive tumour is less robust than in healthy tissue and restriction of it is a potent restraint on tumour development.

The lipophilic nature of PA is beneficial in the lipid-rich environment of the interface between invasive tissue and host. Increasing the concentration of lipid soluble ascorbate in the host cell / invasive cell interface in both bacterial and tumour invasion will act as a booster to the defence and result in reduced host cell damage whilst increasing the more fragile invader cell casualties.

A recent interesting suggestion is the use of porphyrins and ascorbate in tandem as a potential anti-cancer therapy (27). The chemical structure of porphyrin resembles a polymer of AHSL or HCyL and could block their formation or activity. Chemical variations of PA have been shown to have strong and selective effects on the hyaluronidases in bacterial and tumour populations leading to reduction (28). There is a pressing need for further investigation of this potentially useful treatment for invasive carcinomas.

Andrew Carmichael

No conflicts of interest arise from this work

This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors

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