Intestinal Apo B48 Secretion: A Novel Surrogate Marker of Pancreatic Exocrine Function

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Destruction of the exocrine pancreas as a result of chronic pancreatitis is an

insidious and generally relentless process that is eventually associated with

maldigestion and malabsorption of both macro- and micronutrients (1). The

substantial functional reserve of this organ accounts for the observation that

steatorrhea, one of the hallmarks of fat malabsorption associated with

pancreatic insufficiency, appears inconsistently and usually late in the course

of chronic pancreatitis (2). A direct corollary to this physiological threshold

effect—the absence of clinically apparent fat malabsorption until <10% of the

exocrine pancreas remains functional (3)—is the challenge of finding useful

prognostic indicators and benchmarks against which to estimate disease

progression. Accordingly, there is a recognized need for sensitive, readily

available, and technically robust approaches to evaluate pancreatic exocrine

function, particularly in the early stages of chronic pancreatitis (4).

A number of indirect approaches have been evaluated over the years, many founded

upon the anticipated defects in lipid digestion that accompany diminished

secretion of pancreatic lipase. The gold standard for this approach—chemical

determination of fecal triglyceride content as a fractional coefficient of

dietary intake—is cumbersome, time consuming, and esthetically unpleasant.

Additionally, for reasons alluded to above, it is an insensitive marker of early

disease. Variations on the determination of fecal triglyceride excretion, both

quantitative (5) and qualitative (6), have been proposed, but each appears to

have significant limitations, the most crucial of which is the marginal

discriminatory capacity between defective intraluminal lipolysis—the presumptive

defect in pancreatic insufficiency—and malabsorption due to small intestinal

disease. A more promising approach is suggested by finding that the absorption

of cholesterol and other sterols may be impaired early in the course of

pancreatic insufficiency and at a time when triglyceride digestion is only

marginally affected (7). This approach has the additional appeal that

gene-targeted, pancreatic cholesterol esterase knockout, mice have defective

cholesteryl ester absorption (8), thus providing an experimentally testable link

using a genetically defined animal model. Along these lines, a new 13C-labeled

cholesteryl octanoate breath test was recently shown to be of value in

diagnosing lipid maldigestion in patients with pancreatic exocrine insufficiency

(9); but this and other stable isotope-based approaches (10) are not widely

available, are expensive to perform, and require standardized conditions for

interpretation.

A novel approach to the diagnosis of lipid malabsorption is proposed in this

issue of The American Journal of Gastroenterology by Saviana et al. (11). It is

based on the " final common pathway " principle. Lipid absorption reflects a

multistep process involving intraluminal lipolysis, micellar solubilization,

delivery, and uptake across the microvillus membrane, intracellular reassembly

of complex lipids, apolipoprotein synthesis, and formation of lipoprotein

particles (principally chylomicrons), followed by their vectorial delivery to,

and ultimately disgorgement from, the basolateral surface of the small

intestinal enterocyte (12). The complex neutral lipid populations (triglycerides

and cholesteryl esters) are distributed largely within the core of these

particles, whereas the surface components—principally apolipoprotein A-I (apo

A-I), apo A-IV, apo B48, and phospholipid—form the exterior scaffold of the

chylomicron. The apolipoproteins are incorporated into the nascent particle as

it is formed in the endoplasmic reticulum of the small intestinal enterocyte

(12). Accordingly, the " final common pathway " for dietary lipid absorption

involves intestinal chylomicron assembly and secretion. In this process, the

presence of certain intestine-specific apolipoproteins offers a diagnostic

fingerprint to the proximate origin of these lipoprotein particles.

The proposal by Saviana et al. to examine apo B48 as a surrogate marker of

intestinal chylomicron secretion and, thus, the final common pathway of dietary

lipid absorption, has much appeal. A sample of blood is taken from a subject in

the fasting state and a second sample obtained 4 h after a standard meal. The

plasma samples are subjected to ultracentrifugation to float the least dense,

lipid rich chylomicron particles and the protein components analyzed by

SDS-PAGE. From a practical perspective, measuring plasma apo B48 levels is

relatively straightforward, and examining the incremental change in plasma apo

B48 levels (apo B48) after a lipid challenge offers an attractive approach to

longitudinal screening at repeated intervals. It is also less cumbersome than

determining retinyl ester clearance as a marker of the chylomicron core (13).

From a theoretical standpoint, the test makes sense. Apo B48 is the

intestine-specific product of a single APOB gene that is expressed at high

levels in both the liver and gut (14). Apo B48 represents the amino-terminal

half of the full-length (apo B100) protein and arises as a result of

posttranscriptional C to U RNA editing that produces premature termination after

48% of the mRNA has been translated (15, 16). In the adult human small

intestine, 85-90% of the apo B mRNA undergoes C to U editing (17). The remaining

10-15% of the intestinal apo B mRNA is unedited and encodes apo B100 (18).

Because a single molecule of apo B48 is permanently associated with each

chylomicron particle (19), the incremental change in apo B48 concentration

allows an accurate prediction, not only of lipoprotein particle number but also

unequivocal identification of the tissue source of these particles. Saviana et

al. demonstrate that changes in apo B48 levels >4.2 µg/ml have a discriminatory

sensitivity of 89% in subjects with chronic pancreatitis. More importantly, none

of the subjects with steatorrhea demonstrated an incremental change in apo B48

levels. Having established the feasibility of this approach, the authors clearly

recognize the importance of examining the postprandial change in apo B48 levels

after enzyme supplementation in their patients. The presumption is that

chylomicron secretion should be restored, and this will be an important

observation to establish directly.

Like all new approaches, the findings raise other issues. The group of chronic

pancreatitis patients examined demonstrated a wide range of steatorrhea

(5.2-31.8 g/day), yet there was no correlation between the degree of steatorrhea

and the apo B48 determination. This suggests that the apo B48 value may be a

highly sensitive indicator of the presence of lipid malabsorption, but not its

severity. Again, the physiological threshold effect noted earlier for fecal

triglyceride excretion may also apply to the apo B48 value. Another issue

concerns the range of particles secreted by the intestine in response to dietary

lipid. Although Saviana et al. examined apo B48 in the largest, most buoyant

chylomicion particles (11), there are good reasons to suspect that the intestine

secretes a broad range of particles with differing degrees of lipidation (20).

Accordingly, some measure of total apo B48—rather than just chylomicron apo

B48—values may be valuable. An ELISA for apo B48 has been reported and may be of

value once the reagents become more widely available (21, 22). It is reassuring

that the values determined by ELISA (21) for fasting apo B48 in normal subjects

(~0.5 µg/ml) are within the range reported by Saviana et al. (11), indicating

that two independent methods give similar values. Finally, there is an open

question as to whether the quantities of apo B100 produced by the small

intestine are of physiological importance (18). It is known from studies in

gene-targeted mice that intestinal lipoprotein secretion is indistinguishable in

apo B100-only animals versus apo B48-only animals, suggesting that the different

forms of apo B function interchangeably with respect to intestinal lipid

secretion (23). Intestinal apo B100 secretion would not be detected with the

approach proposed by Saviana et al., but its importance in the economy of lipid

absorption is currently undefined.

These issues notwithstanding, the approach proposed by Saviana et al. is likely

to give both clinicians and clinical investigators a new tool with which to

probe questions of pancreatic exocrine function with respect to lipid

absorption. It is a welcome addition to the armamentarium.

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Mark E. Armstrong

www.top5plus5.com

Oregon State Chapter Rep

Pancreatitis Association, International