Fasting & Cancer

 Does fasting help cancer? It depends.

Two forms of fasting- one, comprised of eschewing food but taking unrestricted water intake, and the other, called Fasting Mimicking Diet (FMD) have been shown to be useful.

How does it work?

Glucose, amino acids and certain hormones or hormone like substances are thought to encourage cancer growth, including insulin, IGF-1, and leptin. Think of these as anabolic pathways, which provide the nutrients and the drivers for cancer growth and survival. Turn them off, and the cancer cells are disdavantaged.

Note that I haven't mentioned fatty acids, non-intuitive though it may seem. Cancer cells are highly dependent on anaerobic metabolism, epitomised by the Warburg effect, which involves hijacking the glycolytic pathway (normally anaerobic) even in the presence of oxygen, and producing ATP therefrom. This is seen in fully 70-80% of cancer cells.

Conversely fatty acids are metabolised inside mitochondria by beta-oxidation and therefore is a highly aerobic process, not normally utilised by cancer cells.

The corollary is that cancer fighting diets must contain very little carb or protein and any calories, limited albeit in amount, must come from fat. This is exactly what happens with FMD, which consists of 300-1100 calories perday, derived from broths, soups, juices and nutty bars, with some herbal teas thrown in for taste.

It is thought that by abrogating the anabolic hormones mentioned above, you shut down two key canonical intracellular pathways- the PI3K-AKT-MTOR pathhway and the cAMP-Protein Kinase A pathway. Both are proliferatogenic and indispensable for cancer cells.

However, the claim that FMD alone will cure cancer is inaccurate. This diet works well in tandem with chemotherapy and radiotherapy, and must be administered during the peri-chemotherapy period, usually 48 hours before and 24 hours after. It stops the cancer cells from finding "escape" pathways to circumvent chemotherapeutic agents. But the piece-de-resistance of such diets is that it protects normal, healthy cells from being destroyed by chemo, while the cancer cells perish. 

That is to say, the normal cells display Differential Stress Resistance (DSR)  to cancer cells. At the onset of fasting, normal, non-cancerous cells go into a sort of hibernant, low metabolic state, which reduces their vulnerability to chemotherapeutic agents. As they are no longer actively taking up nutients, metabolising or dividing, they become relatively immune to chemo and radiotherapy.

You might well ask- does this work for all cancers? 

Unfortunately not. It's particularly effective for breast cancer, which is ER/PR positive, but not for ER/PR negative,  HER-2 positive cases. Thus, it potentiates the action of both Tamoxifen, without causing endomtrial hypertrophy, and Fulvestrant.

Similar benefits are seen for prostate and colon cancer.

If a relatively fat rich diet works, what about the ketogenic diet (4:1 fat:carb+protein in terms of weight)? After all, the blood ketone levels can rise by >0.5 mmol/L with FMDs.

Neurosurgeons here will know that ketogenic diets have considerable benefits for some intractable childhood epilepsies and has been advocated for certain gliomas/glioblastomas. However, in general, ketogenic diet does not work for cancers, and can worsen prognosis in melanomas. Hence, best avoided.

A few caveats. Most of the data for these findings were from animal, mainly murine studies and from yeasts. Human data is limited.

Secondly, some cancer patients are cachexic at treatment. They would be at risk from FMD like diets.

Thirdly, trying to achieve these metabolic benefits pharmacologically does not seem to work. The MTOR inhibitor Rapamycin (Sirolimus) is not generally useful for cancers.

Fourth, chronic calory restricted diets don't seem to work for cancer. FMD must coincide with the chemotherapy cycles. This is therefore intermittent fasting.

The Purinergic Pathway in Inflammation, Heart Disease & Cancer

 So you drink a cup of coffee because you are tired. It relieves your fatigue and headache? How does it do it? 

Or for that matter, how do methotexate and sulfasalazine relieve inflammation in rheumatoid arthritis or inflammatory bowel disease? How do clopidogrel and dipyridamole work? And what are the most important cellular markers of effective adoptive cell transfer therapy?

The answer to all these questions lies in the all-important purinergic system. This is the dance between the nucleotides ATP and ADP on one hand and adenosine on the other. They often have diametrically opposite effects in health and disease.

ATP is of course the currency of energy in eukaryotic cells. However, here we are referring to extracellular ATP, acting in a paracrine fashion on specific receptors. Such extracellular ATP may be released from necrotic cells, or leave apoptotic cells through pannexin channels or exit inflammatory cells such as neutrophils through connexin channels- connexin 37 & 44, specifically. They can also be part of vesicles, released from cells through exocytosis.

Whatever the origin of ATP, or the closely related dinucleotide ADP, formed from ATP, they act through two groups of cell surface receptors- the first, called P2Y receptors, are metabotropic receptors, i.e. they directly respond to the nucleotides- these are G-protein coupled receptors, and the second, called P2X receptors, are ionotropic receptors, i.e. ligand gated ion channels that open in response to inward flux of calcium (mostly) or sodium, or outward flux of potassium.

On the other side of the spectrum, sit P1 receptors, which ligate extracellular adenosine. These are also GPCRs. There are 4 types, adenosine receptor A1 (also called ADORA1), A2A(ADORA2A), A2B(ADORA2B) and A3(ADORA3). Of these, A2A and A2B are coupled to Gs and lead to increased levels of cellular cAMP, resulting in profound immunosuppression. A1 & A3 inhibit the formation of cAMP through Gi/o and are therefore generally immune promoting. While A1, A2A and A3 are high affinity adenosine receptors, A2B has low affinity, and is only stimulated under pathologic conditions such as high prevailing levels of adenosine in a hypoxic tumour microenvironment.

How is adenosine formed extracellularly? It is principally formed by the action of 2 sequential cell surface enzymes called CD39 and CD73. CD39 is an ectonucleoside triphosphate diphosphohydrolase, which converts ATP and ADP to AMP. CD73 is an ecto-5'-nuleotidase, that converts AMP to adenosine.

Adenosine can sometimes be generated by other enzymes from ATP & AMP, namely alkaline phosphatase, which has been described as a "promiscuous" enzyme.

Extracellular adenosine is short lived and pushed inside the cell through a couple of channels called Equilibrative Nucleoside Transporters 1&2 , also called ENT1 & ENT2.

Extracellular ATP is pro-inflammatory. It activates the NLRPR3 inflammasome in neutrophils and monocytes. The resulting Caspase1 cleaves pro-IL1 and pro-IL18 into their active forms. 

Adenosine, on the other hand is anti-inflammatory. It reduces inflammation through the A2A receptors present on neutrophils and lymphocytes, by increasing levels of cAMP. Remember, A2A is a high affinity receptor for adenosine.

Methotrexate and sulfasalazine owe their anti-inflammatory effect at least partly due to the fact that they stimulate CD73, which increases the formation of extracellular adenosine from AMP.

What of the A1, A2B and A3 receptors for adenosine? These have all been exploited pharmacologically. The heart blocking effect of adenosine in terminating SVT is exerted through the A1 receptor, while its vasodilating effect in cardiac stress testing is due to its action on A2B receptor on vascular endothelial cells. Stimulation of A3 receptors in non-pigmented cells in the anterior chamber of the eye leads to increased production of aqueous humour, and can be useful for treating sicca symptoms.

Dipyridamole inhibits the ENT1 & 2 channels, thus leading to increased levels of  extracellular adenosine. Hence its use in pharmacological cardiac stress testing.

As adenosine is formed from ATP and ADP, their levels vary inversely with each other. Thus, in inflammatory bowel disease, tissue hypoxia leads to increased production of HIF, which, acting as a transcription factor, stabilises the promoters for CD73 and A2B. A similar transcription factor called Sp1 binds to and stabilises the promoter for CD39. The net result is an increase in extracellular adenosine and a reduction in ATP. This leads to reduction in inflammation, both due to a fall in extracellular ATP levels and stimulation of A2A receptors on neutrophils and lymphocytes by adenosine. The latter also stimulates A2B receptors and maintains epithelial integrity, presumably by promoting healing through augmented blood flow.

The purinergic system, in particular ADP, plays an important role in the function of platelets. Thus, ADP stimulates P2Y1 receptors on platelets and through the G-protein Gq, activates phospholipase C. The downstream effect of this is change in the shape of platelets through the actin cytoskeleton. Similarly ADP activates the P2Y12 receptor, which, through the intermediation of the Gi G-protein switches off adenylyl cyclase, decreases cAMP and activates the GPIIa/IIIb receptor, thus facilitating the binding of platelets to fibrinogen, resulting in platelet aggregation.

Clopidogrel is a P2Y12 inhibitor. It is a prodrug and needs to be activated in the liver. This particular stage can be affected in certain mutations and thus reduce the efficacy of clopidogrel in affected subjects.

Stimulation of A2A and A2B receptors leads to platelet inhibition, explaining the efficacy of dipyridamole in prevention of ischaemic stroke.

While extracellular adenosine is regarded as a safety signal in ischaemia and reperfusion, where it reduces inflammation and tissue damage, it can have the opposite effect in cancer. In general, while its immunosuppressive effect on T-lymphocytes reduces autoimmunity, it can be a hindrance in fighting cancerous cells. In a recent paper in Nature, the investigators found that the subset of cancer sufferers who had the highest benefit from adoptive T cell therapy had a higher proportion of CD8+CD39-CD69- T-cells in the infusate. It is possible that increased expression of CD39 on T-cells leads to production of extracellular adenosine, immunosuppression and T-cell exhaustion, although this is yet to be confirmed. In general, adenosine is found in higher quantities in hypoxic tumour microenvironments, although this may be consequence rather than cause of tumour survival.

Finally, to the salutary effects of that cup of coffee. It is thought that caffeine reduces fatigue by inhibiting cerebral adenosine A2A receptors.