A hypothesis on inflammation and cholestasis (LONG)

[Guest guest]

O.K., here's a hypothesis about inflammation and cholestasis. I'd

like to apologize that this is very long and very complicated.

I've mentioned previously that there is a link between inflammatory

bowel disease, particularly ulcerative colitis and primary

sclerosing cholangitis (PSC), and the pregnane X receptor (PXR)

[also known as the steroid and xenobiotic receptor (SXR)]. PXR/SXR

acts together with another receptor called the retinoid X receptor-

alpha (RXRa). The following is an outline of a hypothesis which

explains how inflammation associated with inflammatory bowel disease

might cause an inhibition of RXRa, leading to impaired expression of

PXR/SXR responsive genes, and susceptibility to cholestasis.

The importance of the pregnane X receptor

The pregnane X receptor (PXR) controls the expression of a number of

genes involved in bile acid transport and detoxification. PXR is a

sensor of toxic bile acids that protects against liver toxicity

(Staudinger et al., 2001). " The nuclear pregnane X receptor (PXR;

NR1I2) is an integral component of the body's defense mechanism

against chemical insult (chemoprotection). PXR is activated by a

diverse array of lipophilic chemicals, including xenobiotics and

endogenous substances, and regulates the expression of cytochromes

P450, conjugating enzymes, and transporters involved in the

metabolism and elimination of these potentially harmful chemicals

from the body. Among the chemicals that bind and activate PXR is the

toxic bile acid lithocholic acid; activation of PXR, in turn,

protects against the severe liver damage caused by this bile acid.

Thus, PXR serves as a physiological sensor of lithocholic acid and

perhaps other bile acids and coordinately regulates genes involved

in their detoxification. " (Kliewer and Willson, 2002). Lithocholic

acid is capable of inducing bile duct obstruction and destructive

cholangitis in mice (Fickert et al., 2006).

One of the key transport proteins that PXR regulates is the MDR1

protein. The human multidrug resistance 1 P-glycoprotein, MDR1,

encoded by the gene ABCB1 (localized on chromosome 7), is highly

expressed in intestinal epithelial cells, where it constitutes a

barrier against xenobiotics. P-glycoprotein is an ATP-dependent

efflux pump that contributes to the protection of the body from

environmental toxins (Schwab et al., 2003a). It transports a huge

variety of structurally diverse compounds. P-glycoprotein is

involved in limiting absorption of xenobiotics from the gut lumen,

and in biliary and urinary excretion of its substrates. P-

glycoprotein can be inhibited or induced by xenobiotics, thereby

contributing to variable drug disposition and drug interactions.

Recently, several polymorphisms have been identified in the MDR1

(ABCB1) gene, some of which can affect P-glycoprotein expression and

function (Schwab et al., 2003a). Certain of the mutations appear to

influence susceptibility to inflammatory bowel disease, including

ulcerative colitis (Brant et al., 2003; Schwab et al., 2003b; Ho et

al., 2005). Evidence for linkage at chromosome 7q has been reported

for both Crohn's disease and ulcerative colitis, and the gene for

MDR1 (ABCB1) is located within this region. Mice defective in the

MDR1 gene (mdr1a-/-) spontaneously develop colitis (Wilk et al.,

2005).

Langmann et al. (2004) have reported that the expression of the MDR1

(ABCB1) gene is down-regulated in ulcerative colitis. This down-

regulation of MDR1 (together with other defense genes) appears to be

due to down-regulation of the transcription factor, pregnane X

receptor (PXR), and may contribute to the pathophysiology of

ulcerative colitis (Langmann et al., 2004). Genetic variants in the

PXR gene have recently been associated with inflammatory bowel

disease (especially ulcerative colitis) (Dring et al., 2006).

Recently, genetic variants in PXR (SXR) have also been shown to be

associated with the severity of PSC (Karlsen et al., 2006).

Rifampin (rifampicin) is an activator of PXR (SXR) and is often used

to control pruritus in liver diseases such as PSC (Khurana and

Singh, 2006). It has been suggested that rifampin could have

complementary effects to ursodeoxycholic acid (UDCA) in treating

cholestatic liver diseases (Marschall et al., 2005).

Rifampin " enhances bile acid detoxification as well as bilirubin

conjugation and export systems, whereas UDCA stimulates the

expression of transporters for canalicular and basolateral bile acid

export as well as the canalicular phospholipid flippase. These

independent but complementary effects may justify a combination of

both agents for the treatment of cholestatic liver diseases "

(Marschall et al., 2005).

Certain polychlorinated biphenyls (PCBs) have been shown to inhibit

human PXR (SXR) (Tabb et al., 2004). Based on the above more recent

evidence, such inhibition of PXR by PCBs might be expected to

predispose individuals to inflammatory bowel disease and/or bile

acid toxicity?

How does UC result in down-regulation of PXR: NF-kB inhibition of

RXRa?

How does ulcerative colitis result in the down-regulation of PXR

described by Langmann et al. (2004)? Aside from the afore-mentioned

genetic mechanism [i.e. mutation in the PXR gene itself (Dring et

al., 2006)], evidence suggests that persistent inflammation in

inflammatory bowel diseases up-regulates both tumor necrosis factor-

alpha and nuclear factor-kappaB (NF-kB) (Schreiber et al., 1998).

It has recently been shown that NF-kB directly (or indirectly) then

down-regulates PXR (Zhou et al., 2006; Gu et al., 2006). Activation

of PXR, on the other hand, down-regulates NF-kB, perhaps explaining

why PXR activators, such as rifampin, also have anti-inflammatory

effects (Zhou et al., 2006). This was recently reviewed by Xie and

Tian (2006): " It has long been appreciated that inflammation and

infection reduce drug metabolism and that exposure to drug

metabolism-inducing xenobiotics can impair immune function. A new

study reveals the mutual repression between the xenobiotic nuclear

receptor PXR/SXR and NF-kappaB signaling pathways, providing a

molecular mechanism linking xenobiotic metabolism and inflammation

(Zhou et al., 2006). "

The effect of NK-kB on PXR may be indirect, and may actually involve

a key " partner " of PXR, the retinoid X receptor-alpha (RXRa). Gu et

al. (2006) have shown that NF-kB binds to RXRa, and prevents the PXR-

RXRa complex from functioning, thus inhibiting expression of genes

regulated by PXR. Because RXRa is a " partner " for a number of other

receptors involved in lipid, bone, and bile acid metabolism

(including the farnesoid X receptor (FXR), the constitutive

androstane receptor (CAR), the vitamin D receptor (VDR), and the

peroxisome proliferator-activated receptor-alpha (PPARa) (Cai et

al., 2002)), it will be of interest to see if the binding of NF-kB

to RXRa also blocks the expression of genes regulated by these other

receptors (Gu et al., 2006; Xie and Tian, 2006). Nevertheless, it is

becoming clear that chronic inflammation (via persistent activation

of NF-kB) could exacerbate cholestasis by blocking bile acid

detoxification and transport (Xie and Tian, 2006).

If NF-kB directly binds to RXRa and prevents it from functioning (Gu

et al., 2006), could this help explain why males are more prone to

PSC than females. It has been shown that the " expression of CYP450

genes is differentially expressed in male and female hepatocyte

RXRalpha-deficient mice; male mice have reduced expression of

cytochrome P450 (CYP) CYP4A, CYP3A, and CYP2B mRNAs, but females do

not exhibit such phenotypes " (Cai et al., 2003). It has been

proposed that testosterone has a negative impact on retinoid

signaling when the level of RXRa is low, which may in turn reduce

the expression of the CYP450 genes (Cai et al., 2003).

It has been assumed that the main ligand (activator) of RXRa is 9-

cis retinoic acid, derived from vitamin A. Could vitamin A

deficiency lead to a susceptibility to RXRa inhibition by NF-kB

during inflammation? Vitamin A deficiency certainly causes increased

inflammation during colitis in experimental animal models (Reifen et

al., 2002). Vitamin A deficiency is commonly associated with

inflammatory bowel disease (Bousvaros et al., 1998) and PSC

(nsen et al., 1995).

Is docosahexaenoic acid (DHA) an alternative ligand/activator of

RXRa?

Recent studies suggest that the omega-3 fatty acid, docosahexaenoic

acid (DHA), can also serve as a ligand/activator for RXRa (de

Urquiza et al., 2000; Lengqvist et al., 2004; Fan et al., 2003).

Could this explain why DHA has been shown to protect against bile

duct injury (resembling sclerosing cholangitis) in mice that are

deficient in the cystic fibrosis transmembrane conductance regulator

(cftr) when given colitis (Blanco et al., 2004)? It has been

proposed that DHA protects against bile-duct injury in these mice

because it up-regulates PPARa (Pall et al., 2006), but this effect

might also be attributed to activation of RXRa by DHA. The up-

regulation of PPARa (or the PPARa-RXRa complex) by DHA is thought to

be involved in down-regulating NF-kB, contributing to the anti-

inflammatory effects of omega-3 fatty acids (Calder, 2002; Calder,

2006). This may explain the beneficial effects of omega-3 fatty

acids in trauma, burn injury, and sepsis (Calder, 2006).

Interestingly all of these conditions can lead to sclerosing

cholangitis (Schmitt et al., 1997; Engler et al., 2003; Benninger et

al., 2005). Parenteral nutrition-associated liver disease in infants

with short bowel syndrome has been shown to be reversed by fish oils

supplementation (Gura et al., 2006). Deficiency of omega-3 fatty

acids (DHA and eicosapentaenoic acid (EPA)) has been linked to

susceptibility to inflammatory and autoimmune diseases (Simopoulos,

2002). Perhaps DHA deficiency (like vitamin A deficiency) also leads

to susceptibility to inflammation mediated (i.e. NF-kB mediated)

inhibition of RXRa-PXR and hence susceptibility to cholestasis?

Results from the ongoing trial of DHA in PSC are eagerly awaited.

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Dave

(father of (21); PSC 07/03; UC 08/03)