Secondary bile acids: an underrecognized cause of colon cancer

Abstract

Bile acids were first proposed as carcinogens in 1939. Since then, accumulated evidence has linked exposure of cells of the gastrointestinal tract to repeated high physiologic levels of bile acids as an important risk factor for gastrointestinal cancers. High exposure to bile acids may occur in a number of settings, but most importantly, is prevalent among individuals who have a high dietary fat intake.

A rapid effect on cells of high bile acid exposure is the generation of reactive oxygen species and reactive nitrogen species, disruption of the cell membrane and mitochondria, induction of DNA damage, mutation and apoptosis, and development of reduced apoptosis capability upon chronic exposure. Here, we review the substantial evidence of the mechanism of secondary bile acids and their role in colon cancer.

Introduction

Bile Acids (BA) are normal components of the lumenal contents of the gastrointestinal (GI) tract, where they enable absorption of lipids, cholesterol, and fat-soluble vitamins. In essence, they act as a physiologic detergent and regulator of intestinal epithelial homeostasis in the gastrointestinal tract[1].

However, BAs, specifically lithocholic acid (LCA) - a secondary BA - also constitute a rare example of toxic endobiotics[2]. In fact, BAs were first proposed as a potential tumor-promoting agent in 1939[3].

At high physiologic concentrations, BAs can cause oxidative/nitrosative stress, DNA damage, apoptosis, and mutation[4]. Furthermore, frequently repeated and prolonged exposure of tissues to high physiological levels of BAs can lead to the generation of genomic instability, development of apoptosis resistance and, ultimately, cancer[4].

And since BAs are normal components of the luminal contents of the GI tract, finding the exact mechanism of their carcinogenic effect has become intriguing. Several factors have been found to increase levels of BAs: most importantly, a high dietary fat intake.

Our aim is to explain the correlation between the concentration of fecal secondary BAs -mainly deoxycholic acid (DOC) and LCA - and the colorectal cancer incidence that was highlighted by several epidemiological studies but whose molecular mechanism remain far from clear.

Furthermore, BAs were also found to be etiologic agents of other GI tract cancers, namely that of the esophagus[5], stomach[6], small intestine[7], liver[8], pancreas[9] and biliary tract[10].

Review

Biochemistry and physiology of secondary bile acids in the body

Primary BAs (cholic acid and chenodeoxycholic acid) are derived from cholesterol by a sequence of enzymatic reactions occurring mainly in the liver. Synthesis of a full complement of BAs requires 17 individual enzymes and occurs in multiple intracellular compartments that include the cytosol, endoplasmic reticulum (ER), mitochondria, and peroxisomes[11]. After synthesis, these BAs are conjugated with glycine or taurine and then excreted and stored in the gall bladder. In humans, BAs are largely re-absorbed in the terminal ileum by an active transport mechanism, but less than 5% of the BA pool enters the colon per day[12]. The BAs that enter the colon are metabolized by bacterial flora, where the primary BAs (cholic and chenodeoxycholic acid) are converted into the secondary BAs, DOC and LCA, respectively.

DOC is partly absorbed in the colon and enters the enterohepatic circulation, where it is conjugated in the liver and secreted in the bile[13]. LCA is fairly insoluble and little of it is reabsorbed[13]. Thus, the circulating BA pool (conjugated when it leaves the gallbladder, and then de-conjugated by action of bacterial enzymes after it enters the colon) is composed of about 30 to 40% each of cholic acid and chenodeoxycholic acid, about 20 to 30% of DOC, and less than 5% of LCA[14].

BAs are amphipathic and many of their properties are related to their amphipathic nature[15]. Their amphipathic nature enables them to get involved in emulsification and digestion of dietary fats; yet, levels above those that are physiologic are potentially membrane damaging.

Factors that change secondary bile acids levels

Almost 40 years ago, Berg was the first to make the observation that CRC (colorectal cancer) risk was higher among descendants of individuals in low-risk populations after moving to developed countries and converting to a Western-type diet that is rich in red meat and saturated fatty acids[16]. This was also consistent with a prospective cohort study that investigated 55,487 Danish middle-aged men and women and suggested that adherence to recommendations for five lifestyle factors (physical activity, smoking, waist circumference, alcohol intake and diet) may indeed considerably reduce CRC risk[17].

Prolonged, high consumption of red meat and saturated fatty acids were found to increase CRC risk. In their case control study, Bayerdorffer et al.[18] confirmed that DOC (Doxycholic acid) levels were significantly higher in the sera of patients with colorectal adenomas. Later, Bayerdorffer et al.[19] found that this positive association between DCA in the serum and colorectal adenomas was highest in the unconjugated fraction, which originates directly from the colon. High fat diets stimulate bile discharge, hence they increase the concentration of BAs above physiologic levels[20]. Population-based studies have shown that subjects who consume high-fat and high-beef foods display elevated levels of fecal secondary BAs, mostly DOC and LCA, as do patients diagnosed with colonic carcinomas[12, 13]. Results of such studies, however, are not very coherent due to difficulties in accurately measuring secondary BA levels in their different forms. Such incoherencies transpired because only levels of free LCA were measured, when most of LCA is in sulphated form. The increase in DOC and LCA reflects increased production of BAs in order to emulsify the increased level of dietary fat. Consequently, elevated secondary BA levels would alter the growth of intestinal epithelium, thus acting as tumor promoters[21]. Add to that, nicotine from smoking can interact synergistically in colon cells to increase oxidative stress and DNA damage[22].

Conversely, diets rich in vegetables and fruits are linked to a decreased CRC incidence. Dietary fibers (from vegetables and fruits) can bind to LCA and aid in its excretion in stool[23]; as such, fibers can protect against colon cancer. Not only fibers play a protective role, but vitamin D and high dietary Calcium supplementation also inhibits colon carcinogenesis induced by either high-fat diets or intrarectal instillation of LCA[24].

LCA activates vitamin D receptor, which may activate a feed-forward catabolic pathway that leads to the detoxification of LCA. Whereas, high dietary calcium leads to the formation of insoluble calcium soaps, this in turn decreases the concentration of free BAs in the intestinal lumen that ultimately may protect against formation of colon cancer[25].

Secondary bile acids and the plasma membrane

An essential constituent of plasma membrane is cholesterol, which rigidifies the membrane and is an important structural component of membrane microdomains[26]. Due to the fact that BAs are cholesterol derivatives with detergent properties, BAs may alter the stability of the membrane lipid bilayer[27]. In fact, BAs with increased hydrophobicity have a greater capacity to perturb the structure of, or partly digest, cell membranes[28]. Secondary BAs (DOC and LCA) also increased paracellular permeability in a dose-related manner, with LCA exerting more potent effects than DOC[29]. When present in high concentrations, secondary BAs, cause unspecific cell membrane damage resulting in focal destruction of intestinal epithelium (Payne, 2008[4]). Worse yet, subsequent repair mechanisms involving inflammatory reactions and hyperproliferation of undifferentiated cells would then increase the risk of transition into a precancerous state. Hyperproliferation of the colorectal mucosa is regarded as an early step in colorectal tumorigenesis[30].

In the colonic epithelium, high secondary BA concentrations induce cell proliferation by activating epidermal growth factor receptors (EGFRs) and post-EGFR/ERK (extracellular signal-regulated kinase) signaling[31]. In addition, BA-induced hyperproliferation can occur through the activity of protein kinase C (PKC), which can be activated downstream of the EGFR or by membrane perturbations[32].

Bile acids and nuclear receptors

Nuclear receptors (NRs) are transcription factors that act as sensors of dietary and endogenous molecules, translating nutritional and hormonal stimuli into transcriptional programs[33]. Recently, it has become apparent that nuclear BA receptors FXR, vitamin D receptor (VDR) and pregnane X receptor (PXR)/steroid xenobiotic receptor (SXR) play an important role in protecting against carcinogenic effects of BAs by activating transcriptional programs aimed at coordinating the control of BA uptake, detoxification, and basolateral secretion[34]. FXR, a member of the nuclear receptor superfamily, responds to BAs as physiological ligands[35]. FXR has a key role in activating pathways that maintain BA homeostasis. FXR protects against intestinal tumorigenesis, possibly by a mechanism involving induction of apoptosis[36].

Vitamin D deficiency is a known major risk factor for colorectal cancer[

Abbreviations

BA:

bile acids

CRC:

colorectal cancer

DOC:

deoxycholic acid

EGFRs:

epidermal growth factor receptors

ER:

endoplasmic reticulum ERK, extracellular signal-regulated kinase

GI:

gastrointestinal

LCA:

lithocholic acid

NR:

nuclear receptors

PKC:

protein kinase C

PXR:

pregnane X receptor

RNS:

reactive nitrogen species

ROS:

reactive oxygen species

SXR:

steroid xenobiotic receptor

VDR:

vitamin D receptor.