Marine lipids
Discussion
Alkyl-Gro are glycerol-derived lipids with potentially therapeutic properties (Brohult, 1963; Brohult et al., 1977). They resemble the monoglycerides closely in molecular size and shape, and also in their physical
properties but contain an alkyl group on sn-1 position of glycerol
instead of an acyl group of comparable chain lengths. The sn-3 position can be esterified by a choline group or an ethanolamine group and
alkyl-Gro from external or dietary sources can be used as precursors of
membrane PL (Snyder, 1972; Hichami et al., 1997). Actually, their
structure provides them the possibility of incorporation into cellular
membrane PL, so they could interfere with the release and/or with the
structure of lipidic second messengers such as DA(; (Robinson et al.,
1991 ). In our study, an incorporation of alkyl-Gro into PAEC membrane
PL was shown, mainly into PC.
Several studies have already described the major role of PKC in controlling
vascular permeability (Lum & Malik, 1994). PKC activation leads to
specific phosphorylation of linking proteins at the cell-cell and cell-matrix
junctions. PKC-dependent phosphorylations of these latter proteins
induce modifications in cellular shape which are strongly associated with
the increase of the endothelial permeability (Murray et al., 1991; Lum &
Malik, 1994). Our results confirmed that PKC activation was involved
in the modulation of this vascular function since PMA (2.10 - 7 M), a PKC
activator, induced a significant raise in albumin clearance rate. Moreover,
PKC activation is dependent on the bioactive lipidic molecules produced
by calcium-dependent phospholipase activation during cell triggering
(Exton, 1991). Some PKC subtypes need also calcium for activation
(Hug & Sarre, 1993; Nishizuka, 1995).
We observed that PMA-induced raise in endothelial permeability was
totally inhibited by an alkyl-Gro incorporation into membrane PL. We
also obtained an interesting result when endothelial monolayers which
had incorporated alkyl-Gro were stimulated by the calcium ionophore
A231S7: endothelial permeability was decreased below the basal level.
These inhibitory effects of alkyl-Gro on endothelial permeability were
possibly due to a PKC inhibition. Several authors showed that alkyl-Gro
could inhibit the activity of purified PKC (Warne et al., 1995). Robinson
et al. (1995) reported that alkyl-Gro were a potent and specific inhibitor
of PMA-stimulated arachidonic acid release from Madin-Darby canine
kidney cells, suggesting a potential bioregulatory role for alkyl-Gro as
an inhibitor of PKC.
To further examine the hypothesis of PKC inhibition, we investigated
the production of alkyl-acyl-Gro, an analogue of DAG inhibiting PKC
(Daniel et al., 1988). Since PC has been observed to be the major source
of both DAG and alkyl-acyl-Gro in response to IL-3 in murine mast cells
(Robinson et al., 1991) and since phorbol esters have been shown to stimulate accumulation of diacylglycerols in human neutrophils (Rider
et al., 1988), we measured {
3 H]-alkyl-acyl-Gro produced by PAEC
152
Discussion
Alkyl-Gro are glycerol-derived lipids with potentially therapeutic properties (Brohult, 1963; Brohult et al., 1977). They resemble the monoglycerides closely in molecular size and shape, and also in their physical
properties but contain an alkyl group on sn-1 position of glycerol
instead of an acyl group of comparable chain lengths. The sn-3 position can be esterified by a choline group or an ethanolamine group and
alkyl-Gro from external or dietary sources can be used as precursors of
membrane PL (Snyder, 1972; Hichami et al., 1997). Actually, their
structure provides them the possibility of incorporation into cellular
membrane PL, so they could interfere with the release and/or with the
structure of lipidic second messengers such as DA(; (Robinson et al.,
1991 ). In our study, an incorporation of alkyl-Gro into PAEC membrane
PL was shown, mainly into PC.
Several studies have already described the major role of PKC in controlling
vascular permeability (Lum & Malik, 1994). PKC activation leads to
specific phosphorylation of linking proteins at the cell-cell and cell-matrix
junctions. PKC-dependent phosphorylations of these latter proteins
induce modifications in cellular shape which are strongly associated with
the increase of the endothelial permeability (Murray et al., 1991; Lum &
Malik, 1994). Our results confirmed that PKC activation was involved
in the modulation of this vascular function since PMA (2.10 - 7 M), a PKC
activator, induced a significant raise in albumin clearance rate. Moreover,
PKC activation is dependent on the bioactive lipidic molecules produced
by calcium-dependent phospholipase activation during cell triggering
(Exton, 1991). Some PKC subtypes need also calcium for activation
(Hug & Sarre, 1993; Nishizuka, 1995).
We observed that PMA-induced raise in endothelial permeability was
totally inhibited by an alkyl-Gro incorporation into membrane PL. We
also obtained an interesting result when endothelial monolayers which
had incorporated alkyl-Gro were stimulated by the calcium ionophore
A231S7: endothelial permeability was decreased below the basal level.
These inhibitory effects of alkyl-Gro on endothelial permeability were
possibly due to a PKC inhibition. Several authors showed that alkyl-Gro
could inhibit the activity of purified PKC (Warne et al., 1995). Robinson
et al. (1995) reported that alkyl-Gro were a potent and specific inhibitor
of PMA-stimulated arachidonic acid release from Madin-Darby canine
kidney cells, suggesting a potential bioregulatory role for alkyl-Gro as
an inhibitor of PKC.
To further examine the hypothesis of PKC inhibition, we investigated
the production of alkyl-acyl-Gro, an analogue of DAG inhibiting PKC
(Daniel et al., 1988). Since PC has been observed to be the major source
of both DAG and alkyl-acyl-Gro in response to IL-3 in murine mast cells
(Robinson et al., 1991) and since phorbol esters have been shown to stimulate accumulation of diacylglycerols in human neutrophils (Rider
et al., 1988), we measured {
3 H]-alkyl-acyl-Gro produced by PAEC
152