released from cell membrane phospholipids
(prostaglandin E 3 , thromboxane A 3 and
leukotriene B 5 ) have anti- or weakly
inflammatory effects and only weak
aggregation-promoting effects. Because
linoleic and linolenic acid compete for
the same pathways of elongation and
desaturation, an increased supply of one
will decrease the elongation–desaturation
products of the other that are incorporated
into membrane phospholipids (Sardesai,
1992). In this way, the fatty acid composi-
tion of cell membranes can be altered by
the type of dietary fat. Interest has grown
in whether sources of -3 fatty acids such
as fish oil could be used to confer
advantages to animals by increasing
eicosapentaenoic acid and decreasing
arachidonic acid in cell membranes,
thereby decreasing the influence of the
series 2 eicosanoid products. Numerous
questions remain about the effectiveness
of such approaches, and to date few con-
clusive data are available.
Several fundamental questions remain
about the metabolism of essential fatty
acids in farm animal species. One of these
is how these fatty acids are transferred in
utero to the fetus, given the complex
nature of the placenta in ruminants and
pigs. A second issue is how ruminants are
able to obtain sufficient essential fatty
acids in the face of extensive rumen
microbial hydrogenation of dietary
unsaturated fatty acids (Noble, 1984). A
small, and evidently at least marginally
sufficient, amount of the essential fatty
acids escapes hydrogenation in the rumen
and is absorbed. Nearly all of the linoleic
(and presumably linolenic) acid that
reaches the small intestine is incorporated
into phospholipids (through reacylation of
lysophosphatidic acid) and cholesterol
esters (via the ACAT reaction). These
lipids have a very slow turnover in the
body, so that the essential fatty acids are
retained for their critical functions (Noble,
1984). Very few essential fatty acids
normally are incorporated into the triacyl-
glycerol fraction, which has a very rapid
turnover in the body. However, if pro-
tected unsaturated lipids are fed or the
rumen is by-passed experimentally to
allow absorption of large amounts of
unsaturated fatty acids, essential fatty
acids can be incorporated into lymph
triacylglycerols, which will then be trans-
ferred to tissues and milk (LaCount et al.,
1994). An additional factor in conserva-
tion of essential fatty acids by ruminants is
that they oxidize linoleic acid less
efficiently than other more abundant fatty
acids such as oleic, and less efficiently
than do non-ruminants (Reid and
Husbands, 1985).Role of Lipids in Cell Signalling and
Signal Transduction
Cells within and among tissues of animals
must communicate with one another to
ensure coordinated growth, differentiation,
metabolism and apoptosis (regulated cell
death). A variety of endocrine (hormonal),
paracrine and autocrine factors communi-
cate such information to the surface of
neighbouring or distant cells. During the
last two decades, research has exploded on
the ways in which receptor-borne messages
are translated into intracellular function.
These processes, which are referred to as
signal transduction mechanisms, have
profound effects on both normal growth
and carcinogenesis (Eyster, 1998). One of
the most exciting current areas of research
in lipid metabolism relates to the role of
lipids as signalling compounds.
Recent evidence has shown that
various polyunsaturated fatty acids can
serve directly as second messengers or
modulators of enzymes. Polyunsaturated
fatty acids play a key role in regulating
expression of genes for lipid-metabolizing
or lipogenic enzymes in both lipogenic
and non-lipogenic tissues (Sessler and
Ntambi, 1998). Other lipids, such as
platelet-activating factor and the eico-
sanoids, have regulatory effects on the
inflammatory response. Platelet-activating
factor is a type of ether-linked lipid called
a plasmalogen.
The first discovery of the involvement
of phospholipids was of the agonist-Lipid Metabolism 115