Sterolins ABCG5 and ABCG8: regulators of whole body dietary sterols
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ABCG5 and ABCG8 are two ATP-binding cassette half-transporters that belong to the G family members. They were identified as proteins that are mutated in a rare human disorder, sitosterolemia, and their identification led to the completion of the physiological pathways by which dietary cholesterol, as well as noncholesterol sterols, traffics in the mammalian body. These proteins are likely to function as heterodimers, and current evidence suggests that these proteins are responsible for the majority of sterol secretions into bile, thus may define the long sought-after biliary sterol transporters. This review will focus on some of the backgrounds of this physiology, the genetics and regulation of these genes, as well as our current understanding of their functions. This review will also highlight the current limitations in our knowledge gap.
KeywordsCholesterol Sitosterol Sterols Intestinal transport Biliary secretion Atherosclerosis Diet
The history of how mammals can distinguish between dietary noncholesterol sterols and cholesterol is intertwined with the history of cholesterol itself; whether cholesterol could be synthesized by the body or was wholly absorbed from the diet, whether the body degraded cholesterol, what determines its absorption and biliary secretion, and whether cholesterol was involved in the process of atherosclerosis are all questions that have involved or continue to involve noncholesterol sterols[53, 54]. Cows, for example, eat only foods that contain plant sterols, and yet their bodies contain cholesterol but not plant sterols. Investigations of such observations led to the discovery that plant sterols were excluded by the body, but could compete with bulk cholesterol for entry into the micelles formed during digestion, thus preventing dietary absorption of cholesterol [6, 14, 21, 34, 50, 55, 59]. Into this milieu of understanding, two key landmark observations led to a revolution in our current knowledge of how whole-body sterol balance may be achieved. The context of the first landmark event was a knowledge that plant sterols were poorly absorbed relative to cholesterol , and that very high plasma cholesterol levels in humans were caused by a dominant genetic defect in the low-density lipoprotein receptor (and was associated with patients who developed accumulations of sterols in their tendons, called xanthomas). In a classic paper that should epitomize clinical investigation, Bhattacharyya and Connor described a new disease, named β-sitosterolemia, after identification of two sisters who had tendon xanthomas but did not have elevated plasma cholesterol, and who had very high amounts of plasma plant sterols, the major species of which was sitosterol, hence the name . The ∝ conformer of sitosterol is not normally present in nature, and thus it is probably not necessary to use the term ‘β’ preceding sitosterol. In one single publication, these authors showed that a likely single gene defect led to the disruption of the intestinal processes that keeps noncholesterol sterols out, led to tendon xanthomas, an ominous sign of systemic atherosclerosis, and was key to understanding how dietary noncholesterol sterols were (not) absorbed. The second key observation was the identification of Niemann-Pick C1 Like 1 (NPC1L1), the ‘cholesterol receptor’, as the key molecule in determining entry of sterols into enterocytes [1, 31]. The latter will not be discussed in this review, although an overview is provided for better physiological context.
Once the clinical description of sitosterolemia had been reported (clinical and physiological features reviewed in ), the next breakthrough was the localization of the sitosterolemia locus, STSL, to chromosome 2p21 to a very narrow region which led to the identification of the genetic defect in sitosterolemia . The surprise was that the STSL locus comprised of not one but two highly homologous genes; complete mutations in either of the two genes are necessary to cause sitosterolemia [2, 37, 39]. These genes encode for the two ABC ‘G’ family half-transporters, ABCG5 and ABCG8, also known as sterolin-1 and sterolin-2, respectively.
Genetics of ABCG5 and ABCG8
Regulation of ABCG5 and ABCG8
Factors involved in regulation of the STSL locus
Mode of action
Pregnenolone α carbonitrile
Streptozotocin-induced DM + insulin
Bile duct ligation
Function of ABCG5 and ABCG8
Sterolins are critically involved in regulating the whole-body retention of noncholesterol plant sterols, and that these proteins are also key to secreting cholesterol into the biliary lumen (and likely the intestinal lumen) [30, 32, 35, 46, 64, 65, 66]. The genetic disease of sitosterolemia attests to their role in keeping noncholesterol sterols (plant sterols as well as shellfish sterols, etc.) out of the human body .
From genetic manipulations in mice, it is clear that these proteins are key to secreting cholesterol into bile. The liver is the dominant organ for whole-body sterol balance and uses the biliary system for sterol loss, whether as direct sterol secretion or by breakdown of cholesterol into bile acids and secretion into bile. Over-expression of human ABCG5 and ABCG8 in mice led to a supersaturation of cholesterol in bile , whereas knockout of either Abcg5/Abcg8 or Abcg8 genes led to a failure to secrete sterols [30, 65]. However, knockout of Abcg5 led to a sterol-poor bile, but sterol secretion seemed to be restored upon Lxr activation . Lxr activation in Abcg5/Abcg8 double knockout did not lead to a stimulation of biliary sterol secretion [65, 68]. There are no data reported for Abcg8 knockout in response to Lxr agonists. From both genetic data, as well as in vitro experimental data, ABCG5 and ABCG8 are likely to function as obligate heterodimers. It is also unlikely that they can heterodimerize with other ABCG family members. Thus, this observation, as well as the difference in the accumulations of polymorphisms in ABCG5 gene relative to ABCG8, is perplexing. Studies in humans have shown that there is preferential excretion of sitosterol compared to cholesterol when these are directly infused into their veins . Studies in knockout mice confirm that this response, at least for biliary sterol secretion, is mediated by Abcg5/Abcg8 and is increased by Lxr activation . While it seems that sterols in general seem to be substrates for ABCG5/ABCG8, limited studies indicate that oxysterols are not substrates . The full spectrum of substrates for ABCG5/ABCG8 is not known. Finally, despite their role in secreting sterols into the lumen, the exact mechanism of action remains a matter of controversy. One hypothesis argues that ABCG5/ABCG8 may act as ‘extruders’, exposing sterols in the outer leaflet of the membrane for facilitated extraction into the lumen by sterol acceptors , such as bile acid:phospholipid complexes, whereas others have proposed that sterolins may act as ‘flippases’, akin to the flipping of phospholipids from the inner to the outer leaflet of the apical membranes . In the absence of robust in vitro assays, it is not possible to discern if either of these models is valid. Finally, it should be noted that some sterol secretions continue, both in man, as well as mice deficient in sterolins, suggesting that there are other pathways whereby sterols can gain entry into the biliary tract, albeit at very low levels. Moreover, the ‘rate-limiting’ step for biliary sterol secretion has not been identified. ABCG5/ABCG8 may not be rate-limiting, because parents of sitosterolemic patients have no biochemical phenotype under physiological conditions, although they must have half-normal functioning sterolins. In this context, a recent study of ABCG5/ABCG8 expression in liver transplant patients found no correlation between mRNA expression of these transporters and biliary cholesterol secretion (sitosterol secretion was not reported) .
There are now robust data to support the contention that sterolins (ABCG5 and ABCG8) are the major sterol transporters active at the apical surface of hepatocytes and enterocytes. Evidence in support of this comes from the genetic basis of sitosterolemia, from the development of animal models that specifically manipulate their function, and from experiments that show to alter biliary sterol secretion. The STSL genetic locus is highly conserved from fish, toad, chicken, rodent, cow, dog, ape, and man, preserving not only the exon–intron organization but also shows a remarkably high degree of polypeptide conservation, raising the possibility that the true function of these proteins may still remain to be defined. A number of questions remain to be fully determined. One of these is defining fully the mode of transcriptional control; another is how much post-transcriptional regulation may occur as well as how these putative transporters ‘pump’ sterols out of the cell. Finally, the conservation of the polypeptides suggests a conservation in function. Thus, it is not clear what the selective advantage would be in this remarkable evolution, unless these proteins evolved to limit the entry of as yet unknown dietary sterol toxins.
We would like to thank all the members of the Patel lab, past and present, for their contributions to this work. We apologize to all of our colleagues whose works may not have referenced by us, as time and space conspired against us.
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