Gut Microbial Metabolism Drives Transformation of Msh2-Deficient Colon Epithelial Cells

The great question posed in Belcheva et al.’s study “What is the nature of the interaction between our microbiota, colorectal cancer and inflammation?” has produced results that can help RECONCILE CONFLICTING EVIDENCE regarding the fiber-colorectal cancer question [CRC = colorectal cancer].

Generally speaking, epidemiological studies on the matter have only provided hints for generating hypotheses that are oftentimes more confused than the questions they’re trying to answer. In Good Calories, Bad Calories, Taubes’ position on fiber is echoed in a New England Journal of Medicine randomized controlled trial that he refers to. It concludes that “adopting a diet that is low in fat and high in fiber, fruits, and vegetables does not influence the risk of recurrence of colorectal adenomas […& that…] two previous trials [Toronto Polyp Prevention Trial & Australian Poly Prevention Project] also found that dietary changes had no effect on the overall risk of recurrence of colorectal adenomas”. This is damning evidence – damning evidence against adopting a modern day, grain-derived, sugar heavy & fat devoid ‘fibrous’ diet.

Ass cancer or Chipotle - that is the question.

Ass cancer or Chipotle – that is the question.

Buyer beware. Yes, yes, yes – the authors do unfortunately disenchant themselves by stating “Nor should we overlook the abundant data indicating that a diet low in saturated fats and rich in fruits, vegetables, and whole-grains has a favorable influence on the risk of chronic disease and mortality”. That statement is incorrect and frankly stupid in 2014. Let’s not waste time over it.

Now that we know consuming lots of fiber according to the ‘modern eating model’ is pretty useless – not only for preventing CRC – let’s do away with a whole load of variables and look at cellular mechanisms up-close to see what different kinds of murine (mouse) epithelial colon cells think of fibre. Let’s dive into Belcheva et al.’s study . Their message is essentially positive about butyrates’ effects on CRC, but cautions (at least in principle) against blind application.

Genes often affected in CRC

  • Adenomatous Polyposis Coli (APC) genes = tumor suppressing
  • DNA mismatch repair (MMR) genes (self-explanatory name)

The researchers approached their awesome question, keeping in mind the “role of inflammation in producing a niche for specific microbes to elicit their oncogenic effects” all the while recognizing that “the etiology of most CRCs does not have an inflammatory component”. Furthermore, “a comprehensive meta-analysis found a positive association of total carbohydrate intake with CRC”.

Bullshit aside - where's the CRC?

Bullshit aside – where’s the CRC?

To TEST their hypothesis, they make:

  • a mouse model [APCMin/+ (multiple intestinal neoplasia)] of human adenomatous polyposis
  • & cells with a “Mutation in or inactivation (via silencing) of MMR genes, such as MutS homolog 2 (MSH2)

They give an APCMin/+ mouse MSH-deficient cells and observe the growth of many more polyps.

Naturally, the question “Does the microbiota affect the #/growth of polyps?” arises. Well, lets see what happens when you take their microbiota away: “Polyp incidence is not reduced in germ-free APCMin/+ mice (Dove et al., 1997), indicating that gut microbes have little to no role in disease progression in this background.”

Ok – so what about antibiotics?

  • oral antibiotics dramatically reduced CRC specifically in APCMin/+MSH2-/- mice”

OK – since carbs “feed” bacteria and antibiotics “kill” ‘em, what’s up with carbs on the #/growth of polyps?

  • “a diet reduced in carbohydrates can phenocopy this effect and show that gut microbes stimulated CRC development through the production of carbohydrate-derived metabolites such as butyrate

INTERESTING. That which feeds our microscopic partners going back eons & eons is also implicated in feeding CRC. Hhhmm…if it’s not on already, tightly fasten your skeptic helmet.

The researchers elaborate further:

  • “The fact that antibiotic treatment led to a reduction in grade 1 polyps, which include aberrant crypt foci, argues that gut microbiota in APCMin/+MSH2-/- mice act at an early stage in the formation of CRC, perhaps even as a tumor initiator […with the caveat that…] not all members of the gut microbiota contribute equally to CRC development in this animal model”.

A hint as to possible mechanisms (or the elimination thereof):

  • “the mutation frequencies were similar between colon epithelial cells from untreated or antibiotic-treated MSH2-/- mice”

But how does the mice microbiota contribute to CRC formation?

  • “gut microbiota induce CRC through a mechanism that is independent of both inflammation and DNA damage.” So we know what mechanisms AREN’T responsible but still don’t know which one(s) IS/ARE.

Carbs as a function of microbial depletion – what happens in that paradigm?

They checked by setting up “Three-week-old APCMin/+MSH2-/- mice [who] were given a normal or low-carbohydrate diet (Table S1). Approximately 58% of the calories provided with the normal diet derived from carbohydrates, compared to 7%

2 things surfaced:

  • “Strikingly, the low-carbohydrate diet reduced polyp numbers in the small intestines and colons of APCMin/+MSH2-/- mice”
  • no additive effect on polyp number by combining the low-carbohydrate diet and the antibiotic treatment (Figures 3B & 3C) suggesting that both treatments function by the same mechanism

It is reasonable to think: maybe the decreased metabolite levels (i.e. poop from fermenting fiber/resistant starch) resulting from carb restriction is the deterministic factor in tipping the balance towards CRC? The authors think so:

  • “reducing either gut microbiota or dietary carbohydrates resulted in the reduction in both Ki-67 and β-catenin expression in APCMin/+MSH2-/- mice led us to hypothesize that bacterial metabolites might fuel the aberrant hyperproliferation of colon epithelial cells in these mice”.

So now we need a better mouse model to tease out the (potential) effect of metabolites (i.e. butyrate) on polyps (as a CRC proxy marker):

  • Previous mouse model = inadequate. It was MMR-deficient AND DDR-deficient (MMR also works via a ‘DNA Damage Response’ pathway)
  • New mouse model, succinctly named MSH2G674D/G674D = adequate ===because===> they have a functional DDR mechanism but CANNOT carry out other MMR functions (this is called controlling for variables).
    • ==> ‘what mechanisms do what’ can now be narrowed down via a process of elimination.

MSH2G674D/G674D mice revealed “the only SCFA that was statistically reduced by all antibiotic treatments and by the low-carbohydrate diet was butyrate”…Specifically, these treatments led to the reduction of three families within Firmicutes, namely Clostridiaceae, Lachnospiraceae, and Ruminococcaceae (Figure 6C), that are known to produce butyrate”.

From their controlled 2nd mouse model they remark on the plausible mechanism of butyrate’s **apparent** proliferative properties in murine epithelial colon cells

  • “butyrate modulates canonical Wnt signaling (Lazarova et al., 2004) and has been shown in some studies to promote CRC (Freeman, 1986; Lupton, 2004)”
  • Canonical Wnt signaling = regulates gene transcription, cell size and calcium levels inside the cell; all majorly important factors determining if cells live or die…so yes, of prime interest in cancer.

This is the CRUX of the study ===> “To TEST whether butyrate directly affects polyp formation in APCMin/+MSH2-/- mice, antibiotic-treated mice were fed a diet enriched in tributyrin, a stable form of butyrate that breaks down into three butyrate molecules in the gastrointestinal tract

  • 50 mM and 0.5mM of sodium butyrate, which represent concentrations of butyrate found in the distal part of the colon (Donohoe et al., 2012a), stimulated proliferation of colon epithelial cells in APCMin/+MSH2-/- mice but NOT in controls
  • AND “high concentrations of sodium butyrate (i.e., 10 and 100mM) did NOT increase colon epithelial cell proliferation in APCMin/+MSH2-/- mice

KEY POINT: I’m underlining “in APCMin/+MSH2-/- mice” like it’s going out of style because the study should not be extrapolated as saying

  • ‘butyrate = horrible for humans via mouse proxy’
  • but rather that ‘if certain cell repair mechanisms are non-functional, butyrate (through no fault of its own) can still fuel dastardly clever murine mutant epithelial colon cells
    • In technical speak: “because deregulated β-catenin signaling is a marker of early neoplastic changes of intestinal epithelium (Van der Flier et al., 2007), our results suggest that MSH2-deficient colonic epithelial cells are highly predisposed to transformation
  • And consequently, the authors say the “gut microbiota plays a key role in CRC by providing metabolites such as butyrate that ’fuel’ the transformation of murine APCMin/+MSH2-/- colonic epithelial cells” [into an “aberrant hyperproliferation phenotype”]

So, we’re left with:

  • Our study supports the carbohydrate-cancer link by showing that a diet reduced in carbohydrates resulted in reduced polyp formation in APCMin/+MSH2-/- mice
  • YET…”butyrate has been shown to modulate canonical Wnt signaling, and depending on the status of β-catenin activity, colon epithelial cells respond differently to butyrate (Lazarova et al., 2004)” ==> i.e. the butyrate paradox. It is not a paradox, just that the dose-response curve is not linear but rather U-shaped (as so often is the case) & dependent on the particular cellular metabolic milieu. OF COURSE this matters.

What to conclude? Let us think about these 2 points:

  • The authors’ conclusion of “a diet reduced in carbohydrates as well as alterations in the intestinal microbial community could be beneficial to those individuals that are genetically predisposed to CRC
  • A paper referenced by Belcheva et al.’s group states that “The Warburg effect dictates the mechanism of butyrate-mediated histone acetylation and cell proliferation”] and explains how butyrate metabolism is impaired in cancer cells. It remarks on the fact that “butyrate has opposing effects on cell growth: it inhibits cancer cell proliferation as an HDAC inhibitor but stimulates the proliferation of noncancerous cells (and cancerous cells when the Warburg effect is blocked) by being oxidized as an energy source
    • More details about 2 histone-base mechanism butyrate uses in its **seeming** flip-flops:
      • Colonocytes near the base of crypts receive tiny amounts (uMs) of available butyrate and so makes us of the acetyl-CoA/HAT mechanism for histone acetylation (inhibiting aberrant proliferation). The “acetyl-CoA/HAT mechanism involves metabolism of butyrate in the mitochondria followed by the subsequent ACL-catalyzed production of acetyl-CoA.”
      • Luminal colonocytes however receive more available butyrate (maybe in mM quantities) and thus uses another histone-based mechanism (for inhibiting aberrant proliferation), HDACinhibition, where higher levels of butyrate surpass the oxidative capacity of the cell, causing it to accumulate butyrate within.
  • NO FLIP-FLOP, just AWESOMENESS: “our transcriptome profiling results indicate that they upregulate different targets, with the former (acetyl-CoA/HAT) enriched for cell-proliferation genes and the latter (HDAC-inhibition) being enriched for apoptotic genes
  • THE GOLD: “These changes in gene expression are consistent with the lower doses of butyrate stimulating cell proliferation, while higher doses inhibit proliferation and increase apoptosis. These findings lead to a model whereby butyrate facilitates the normal turnover of the colonic epithelium by promoting colonocyte proliferation in the bottom half of each crypt while increasing apoptosis in those cells that exfoliate into the lumen

I conclude that our bodies really like fat as a fuel. Being the opportunists that we are, we do have long-term bacterial friends that can give us that oh-so-good fatty-fat-fat we love if we give them carbohydrate. Redundancy & mechanistic diversity are true evolutionary treats.

Taken together, it seems like a good idea to feed your colon cells plenty of butyrate and lower your total ‘sugar burden’. You can ingest sources of fiber that will not (usually) induce mutant cell genotypes. Not all sources are equal. Your colon cells like a high-fat diet and so do other cells in your human body. Fiber does and probably should play a role in a high-fat diet. How big a role? No idea. You probably can include quantities that satisfy your taste as a starting point.

Lastly, ketogenic diets are NOT synonymous with fiber poor diets. They fact that people implement them as such says more about their lack of understanding than what a ketogenic really is or can actually be when done properly.

TL;DR A varied and nutrient dense ketogenic diet combining evolutionarily concordant foods (for the most part), including vegetation feeding colon cells butyrate (& other metabolites) by bacterial proxy seems not only reasonable, safe and tasty but also a good strategy for avoiding god damn ass cancer.

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