Lipoprotein synthesis and secretion

INTRODUCTION

L.5.7. Apolipoprotein G

1.6. Lipoprotein synthesis and secretion

1.6.1. Triglyceride-rich

lipoproteins

The

intestine

and the liver of all

species, including human,

aÍe

the

major sites of

apolipoprotein synthesis

and

secretion

of

triglyceride-rich lipoproteins

(TRL)

(Jackson

et rl,

^{1976; Bisgaier}

and

Glickman, 1983). The

synthesis

of TRL

occurs

within the

intracellular membrane compartment of intestinal enterocytes and

liver

hepatocytes (Havel

et

â1, 1980).

1.6.1.1.

Chylomicrons

The fatty acids and 2-monoglycerol precursors of

chylomicron triglyceride are taken

up by the

enterocyte after being transported

to the

cells

by bile salt micelles

(Westergaard

and Dietschy,

1976). Subsequently ^the monoglycerides

are

re-esterified

to triglyceride in the

smooth endoplasmic

reticulum ^(SER).

The

synthesis

of

triglyceride appears

to

occur independently

of the

glyceraldehyde 3-phosphate pathway,

the

predominant pathway for synthesis

of

triglyceride

in the liver (Bell et al,

1981).

As a

consequence of

little or no de novo

synthesis

of

triglyceride

in the

enterocyte chylomicron triglycerides closely resemble those

of dietary fat

(Zilversmit, 1965). Unlike

triglyceride the source of

phospholipids

for

chylomicrons

originates

ⁱⁿ existing pools

of

mucosal phospholipid (Ardivson and Nilsson, ^{1972). Dietary} cholesterol

is also

incorporated

into

chylomicrons,

mainly as

cholesteryl esters synthesized

by

ACAT within the absorptive cells (Norum

et al,

^1983).

The non-polar esters

of

long-chain

fatty

acids are synthesized as are the

phospholipids

in the SER. From here the lipids are

transported

into

^the

cisternal space (Higgins and Hutson, 1984).

At this

stage there

is ^no

association

berween

lipid and

apolipoprotein (Christiansen

et rl,

^1983). Apolipoproteins

are

synthesized

on

attached ribosomes

of the rough

endoplasmic ^reticulum (RER). Intestinal cells synthesize apo B4g, ^apo

41,

apo

fu

^and

^C

apoproteins and

perhaps apo

E but only in

minor quantities (Alexander

et

â1, 1972; Marsh and

Sparks, 1979; Windmueller and Wu, 1981; Imaizumi

et al,

^1978).

Proteins and

lipid

come together a¡"

the point

where

the

RER and SER

come

into

contact (Alexander

et al,

^1976; Claude, 1970). The nascent particle is

then formed as the lipid and protein are

transported towards

the

^Golgi

apparatus

(Bell-Quint and Forte, 1981). Upon

reaching

the Golgi,

^more

phospholipid (Janero

et

â1, 1984) and cholesterol may

be

added and the nascent

particles are

concentrated

within

secretory

vesicles (Higgins and

^Hutson,

1984). These vesicles then appear

to

bud

off

from the distal end

of

the Golgi ^and fuse

with the

basolateral membrane

of the cell, from

where

they are

^secreted

into

the lymph (Bisgaier and Glickman, 1983;

Miller

and Small, ^1987).

The

secretion

of

chylomicron particles

is

however dependent upon the synthesis and secretion

of

apolipoproteins, apo

B in

particular (Glickman

et

^â1,

1986).

The

administration

of

cyclohexamide,

an inhibitor of

^{protein synthesis,}

blocks the secretion of nascent particles. In the genetic disorder

of abetalipoproteinemia, \ryere

there is a failure of both liver and

^{intestine to}

incorporate apo B into lipoproteins, the result is an

accumulation of triglyceride droplets

in

hepatocytes

and

enterocytes (Bisgaier

and

^Glickman,

1e83).

1.6.1.2. Very low density

lipoproteins

The

synthesis and secretion

of VLDL is

similar

to

that

of

chylomicrons,

however there are some

differences.

VLDL triglyceride, unlike that

^of

chylomicrons,

is

synthesised

via the

glyceraldehyde 3-phosphate ^pathway.

The fatty

acids

which are

incorporated

into VLDL lipids are

^{derived from} multiple sources, namely

de

novo synthesis

from

acetyl-CoA units ^{produced by} carbohydrate utilization, plasma

free fatty

acids, and

from

hydrolysis

of

^lipids

transported

to the liver by

^remnant lipoprotein particles (Hamilton

et

^{à1, 1967;}

Wilcox

et al,

1975). Cholesterol which

is

incorporated

into

^nascent

VLDL

maybe

derived from either ^plasma

or

de novo synthesis. Due

to

the

low

levels

of

ACAT

activity in

hepatic

cells

most cholesterol

in

nascent

VLDL is not

^esterified

(Erickson cholesterol

1982).

and Cooper,

1980),

feeding

and

increased

however

the

ACAT

activity

proportion

increases

(Dolphin, 1981; Guo

upon

et

^al,

The progression

of lipid

^and apoproteins towards

the Golgi

apparatus in hepatic

cells is the

same as

that in the

enterocyte. From

the Golgi

^apparatus secretory vesicles

bud off,

migrate

to,

and

fuse with the

membranes

at

the

sinusoidal

front, from

where they have access

to

the ^plasma

via the

sieve-plate fenestrae

of the

sinusoidal membrane (Alexander

et al,

^1972).

Apo B is

essential

not only for

chylomicron synthesis

but

also

for

the assembly

of VLDL

which are secreted from hepatic cells. Unlike intestinal cells

which are able

to

^produce both apo B4g and 8100, human hepatic cells ^produce

only

apo B1OO (Edge

et al,

1985).

In

the

rat

and mouse however, hepatic cells

produce

comparable amounts

of both apo

^B¿

g

and apo ^B _t o

o ^(Wu

^and

Windmueller, 1979; Sparks and Marsh, 1981).

Subcellular fractionation studies have demonstrated

a

^marked

^size

^and

compositional heterogeneity

of the Golgi lipoprotein population

^(Dolphin, 1981;

Howell and

Palade, 1982).

It has

been assumed

that

these particles

indicate immature precursors

of VLDL that

have

yet to

acquire

all of

^their

constituents (Howell and Palade, 1982) however

this

may

help to

^{explain the}

heterogeneous nature

of the VLDL

population present

in

^plasma.

1.6.2. Low density

lipoproteins

Low

density lipoprotein

is

normally formed as

a

^result

of

the catabolism

of

VLDL

to IDL

and subsequently

to LDL

^(Reardon

et al,

1978; Sigurdsson

et

^al,

1975; Ginsberg

et al,

1981).

In

addition,

in

normal subjects most

of

the

LDL

^apo

B is

derived from

the

catabolism

of VLDL

and

IDL.

However

in

the

rat

^(Fidge and Poulis, 1978), and

pig (Huff et al,

1985) much

of

^the

^LDL

production may

not be

derived from

VLDL or ^IDL,

^{and may therefore}

be

^{secreted directly into}

the

plasma.

This

may

be

supported

by

findings

of

both preformed and newly secreted

LDL-like

particles

in the

perfused

rat

(Fainaru

et

â1, 1977) and ^pig (Nakaya

et al,

^{1977) livers.}

An

alternate explanation

to the

direct synthesis

of LDL is that

apo B

could pass through

a

rapidly turning over

VLDL

^{pool (Beltz}

et al,

^{1985), which}

is not

traced

by the use of

^{exogenous labels,}

and be rapidly

converted to

particles

of

both

LDL

size and composition.

This

theory may help explain the

high LDL

apo

B

production rates

which are

generally greater than those of

VLDL in

subjects

with

severe and inherited hypercholesterolemia (Janus

et

al, 1980; Soutar et al, 1977).

1.6.3. High density

lipoproteins

High

density lipoproteins appear

to be

derived

from liver

and intestine,

and some may

form from the

redundant surface material

of

chylomicrons ^and

VLDL. Unlike TRL,

nascent

HDL

have

not

been successfully identified within subcellular compartments

or

isolated

from Golgi-rich

^fractions

(Gotto et

^â1,

1986), although

it

has been ^proposed

that HDL

may

be

^formed

by a

^process

similar

to

that

of VLDL

^(Melin

et al,

1984). Evidence

to

support this has come

from

hepatic perfusates where nascent

HDL

^particles

have been

observed

(Marsh, 1974; Hamilton, 1984).

HDL

secreted

by

cultured

rat

hepatocytes ^are discoidal

in

^shape

and

are composed

of a bilayer of

phospholipids, mainly

lecithin (Hamilton

et al,

^1976). When LCAT

is

active nascent HDL particles ^are spherical, and resemble those

in

plasma (Eisenberg, 1984).

High

density lipoproteins

may

also

be

formed

within

plasma

from

the

surface

material of

chylomicrons

and VLDL after lipolysis of

triglycerides

(Nicoll et â1, 1980). In vitro

demonstrations

have

shown

that

^following

lipolysis

of TRL

spherical complexes

of

^apo

A or

apo

C

and phospholipid can be

isolated. These particles resemble discoidal

HDL

particles and incubation with

LCAT results in the

formation

of

spherical

HDL-like

particles (Eisenberg, 1984).