Biochemistry, Volume 25: Ascorbic Acid: Biochemistry and
Biomedical Cell Biology
Edited by J. Robin Harris. Plenum Press, New York, 1996.
Reprinted with permission, ©1996.
Ivonne Pasqualli Ronchetti, D. Quaglino,
Jr., and G. Bergamini
1. SCURVY AND VITAMIN C
Observations of deficient wound healing in sailors suffering
from scurvy have been reported by explorers and physicians
since the sixteenth century, together with the observation
that citrus could have curative properties. Thereafter,
Wolbach and Howe (1926) found that in scorbutic guinea
pigs there was a deficient production of intercellular
matrix which could be reversed by administration of citrus.
The discovery, isolation, and chemical characterization
of vitamin C was performed in the early thirties. Since
then, several studies have been carried out with the aim
of characterizing the cellular and matrix defects in scurvy
and the effect of vitamin C on the healing process in
species unable to synthesize ascorbic acid, such as guinea
pigs and humans. Several models have been proposed, including
animals made scorbutic during fetal development (Rivers
et al., 1970) and postnatal growth (Barnes et al., 1970),
and cultured organs and cells grown on chemically defined
media in the absence (Jeffrey and Martin, 1966) or in
the presence of various concentrations of ascorbic acid
(Russell and Manske, 1991).
The great majority of these studies, however, were performed
on vitamin C-deficient guinea pigs and pointed to the
delay and to the histological, biochemical, and mechanical
features of healing of skin wounds or after laparatomy.
It was observed that vitamin C-deficient animals exhibited
persistent hemorrhaging; impaired production of granulation
tissue, such as reduced vessel formation and collagen
production; and slowed gain in wound strength (Lanman
and Ingalls, 1937; Hunt, 1940; Hartlett et al. 1942: Bourne,
1944). These early data were confirmed in a recent study
on pregnant sows with a hereditary defect in synthesizing
ascorbic acid, in which severe pathological alterations
were observed in the uterus and in the placenta as well
as in the fetuses on administration of a diet depleted
of vitamin C. These changes consisted of hemorrhages,
hematomas, and general edema in both placenta and fetuses
and in impaired ossification of the fetus skeleton (Wegger
and Palludan, 1994).
The distribution of ascorbic acid in wounded and intact
skin of guinea pigs was investigated in an attempt to
elucidate requirements of vitamin C during tissue repair,
and the authors reached the conclusion that in the early
stage of tissue regeneration there is a gradient distribution
of ascorbic acid in areas surrounding the wound depending
on the local physiological requirement (Kim et al., 1994).
Experimental vitamin C deficiency in humans was assayed
in the early forties by Crandon (1940), who underwent
a skin incision after six months on a diet essentially
free of ascorbate. Failure in the reparative process,
together with deficient formation of inter-cellular matrix
and vascular elements, were histologically observed in
a biopsy of the wound taken ten days after a skin incision.
Similar results were later confirmed by Wolfer and coworkers
Since then, numerous investigations have demonstrated
that in vitamin C deficiency the principal failure of
wound healing was impaired synthesis and secretion of
collagen (Robertson and Schwartz, 1953).
2. COLLAGEN AND VITAMIN C
The great majority of studies on the effects of vitamin
C have been performed in vitro; on cultured cells of different
origin and have pointed to the role of ascorbic acid in
collagen synthesis, maturation, and secretion.
In the early sixties, with the development of electron
microscopy, Ross and Benditt (1962, 1965) observed in
experimental scurvy a defective progression of labeled
proline through the altered cysternae of the rough endoplasmic
reticulum to the Golgi and the matrix. These results might
be interpreted as impaired processing of collagen and
storage of underhydroxylated molecules within the endoplasmic
reticulum of fibroblasts. which were stimulated to synthesize
collagen during wound healing in the absence of vitamin
C. An enormous number of studies performed in the sixties
demonstrated that the principal failure in wound healing
during vitamin C deficiency is the scarce synthesis and
secretion of collagen due to impaired hydroxylation of
proline residues in collagen types I and III (Gould and
Woessner, 1957; Gould, 1958).
Synthesis and maturation of interstitial collagens up
to their final cross-linking into insoluble cross-banded
fibrils is not the subject of this presentation, but it
is necessary to mention some of the steps involved in
the maturation of collagen molecules in order to understand
the effects of vitamin C. Like the great majority of secreted
proteins, procollagen molecules are synthesized in the
rough endoplasmic reticulum and require posttranslational
modifications before being extruded from the cell. However,
collagen is a rather peculiar protein. First of all, the
molecule is formed by three polypeptide chains that assume
a specific helical conformation due to the high content
of glycine which occupies every third position along most
of the length of the three polypeptide chains. Moreover,
collagen is characterized by the presence of hydroxyproline
and hydroxylysine formed by specific hydrolases during
the molecule assembly; furthermore, some of the hydroxylysine
residues undergo O-galactosyl and O-galactosyl-b-glycosyl
substitution. All these post-translational modifications
are necessary in order for collagen to be secreted from
the cells as procollagen.
In the extracellular space, procollagen is further modified
by enzymes which cut the C- and N-terminal portions of
the molecule and make tropocollagen suitable for self-assembly
into banded fibrils. The last enzymatic modification of
the collagen molecule is by lysyl oxidase, which initiates
a series of reactions leading to the formation of stable
intermolecular cross-links. As far as lysyl oxidase is
concerned, in vivo, this enzyme activity in rat skin does
not seem to be significantly affected by excess of vitamin
C in the diet; on the contrary, lysyl oxidase was inhibited
in a concentration-dependent manner in an in vitro assay
in which lysyl oxidase activity of chick embryo bones
was measured in the presence of increasing concentrations
of ascorbic acid (Quaglino et al., 1991). A similar reduction
was also observed by Faris et al. (1984) in cultures of
rabbit aortic smooth muscle cells.
With time, collagen undergoes other modifications such
as glycosylation and additional intermolecular cross-linking,
that do not suggest a role for vitamin C, but may be particularly
relevant in pathological processes.
2.1. Collagen Hydroxylation
As already mentioned, collagen contains the unique amino
acids hydroxyproline and hydroxylysine, which are necessary
for the stability of the molecule and for its complete
maturation. The synthesis of these amino acids occurs
posttranslationally during the assembly of the polypeptidic
chain (Uitto and Prockop, 1974), is independent of the
age (Brinckmann et al., 1994), and is catalyzed by prolyl
and lysyl hydroxylases in the presence of oxygen, a-ketoglutarate,
ferrous ions, and ascorbic acid (Hutton et al., 1967;
Kivirikko and Prockop, 1967). Ascorbic acid has been found
to be specifically required for the decarboxylation of
a-ketoglutarate in the prolyl-4-hydroxylase reaction,
where it may act as a com pound necessary for the reduction
of enzyme-bound ferric iron formed during proline hydroxylation
(Yu et al., 1988). In fact, ascorbate is not stoichiometrically
consumed during prolyl hydroxyletion (Tuderman et al.,
1977), and the reaction may continue for several cycles
in the absence of ascorbate, but then the reaction ceases
and vitamin C is required as a quite specific compound
to reactivate the enzyme (Myllyla et al., 1978). Hydroxylation
of a number of proline and lysine residues, at specific
sites of the nascent collagen molecule (Uitto and Prockop,
1974), is necessary for the polypeptidic chain to undergo
peculiar conformation and glycosylation of some of the
hydroxylated lysyl residues; all these steps are necessary
for the secretion of the procollagen molecule.
Hydroxylation of prolyl and lysyl residues may also occur
in vitro. Protocollagen, the unhydroxylaled form of collagen,
was isolated from cells cultured in the presence of the
iron chelator a,a-dipyridyl; moreover, the in vitro hydroxylation
of prolyl residues by prolyl hydroxylase was shown to
be dependent on the structure of prolyl-containing substrate
(Berg and Prockop, 1973a). Therefore, during the formation
of procollagen, prolyl and lysyl hydroxylases serve to
prepare the molecule to assume the correct conformation
necessary for its thermal stability and secretion (Uitto
and Prockop, 1974). In fact, underhydroxylated and underglycosylated
collagen has been shown to be retained within the cell
and to accumulate into large cyloplasmic vacuoles (Koss
and Benditt, 1965; Olsen and Prockop, 1974: Harwood et
al., 1975). The persistence within the endoplasmic reticulum
could be explained, at least partially, by the fact that
the underhydroxylated chains undergo a delay in the triple
helix formation and may stably associate with protein
disulfide isomerase, a multifactoral endoplasmic reticulum
resident enzyme, which is the b-subunit of prolyl-4-hydroxylase
(Bassuk and Berg, 1989). Therefore, prolyl-4-hydroxylase
specifically binds the non-triple helical procollagen
chain, playing a role in its intracellular retention (Olsen
et al., 1973). Similar retention has been observed in
a strain of fibroblasts harboring a deletion of 180 amino
acids in the pro-a2(I) chain, which causes a delay in
the molecule folding into the triple helix and in collagen
maturation (Chessler and Byers, 1992).
In the absence of vitamin C, underhydroxylated procollagen
molecules are not only retained within cells (Dehm and
Prockop, 1971), but are less stable and more temperature-sensitive(Berg
and Prockop, 1973b). Procollagens with different hydroxyproline
content were shown to be sensitive to pepsin digestion
at temperatures lower than physiological, and the phenomenon
was directly related to the extent of hydroxylation. Therefore,
at 37EC, hydroxyproline-deficient molecules might not
be in triple helical conformation within cells and could
be most sensitive to proteases (Rosenbloom et al., 1973).
This may imply that, in vitamin C deficiency, the impaired
collagen production is partly due to its cellular retention
and partly to its denaturation and destruction by unspecific
proteases within the cell.
Cell strains isolated from patients suffering osteogenesis
imperfecta, a connective tissue disorder caused by mutations
in the genes encoding type I collagen and affecting procollagen
chain association, were shown to increase procollagen
synthesis upon addition of ascorbic acid to the culture
medium, as well as the synthesis of BiP, an hsp70-related
stress protein, which was found to bind pro-a1(I) chains
stably (Chessler and Byers, 1993). This could be a mechanism
for retaining genetically altered and abnormally hydroxylated
(and/or glycosylated) collagen molecules inside cells.
As BiP synthesis is stimulated by ascorbic acid, this
could be an additional mechanism for preventing secretion
of abnormally configured collagen molecules.
2.2. Collagen Gene Expression
Human dermal fibroblasts and rabbit articular chondrocytes
were shown to produce higher amounts of collagen in the
presence of ascorbic acid in vitro (Hata and Senoo, 1989;
Hering et al., 1994); moreover, ascorbic acid was shown
to overcome the reduced proliferative capacity and to
ameliorate the reduced collagen synthesis of elderly human
fibroblasts (Phillips et al., 1994).
A stimulatory effect of ascorbate has been largely demonstrated
on the synthesis of collagen types I and III and recently
shown for collagen types II and X in chondrocytes (Leboy
et al., 1989; Hering et al., 1994) and for collagen type
IV in cultured rat skin epidermal cells (Ohkura et al.,
1990). On the contrary, a negative effect of vitamin C
has been described for collagen types V and VI in cultured
bovine aortic smooth muscle cells (Leushner and Haust,
These data might be explained by the fact that ascorbic
acid seems to play a role in collagen synthesis also at
the level of gene expression and/or mRNA stability in
cultured fibroblasts and chondrocytes (Lyons and Schwartz,
1984; Geesin et al., 1988; Sandell and Daniel, 1988; Quaglino
et al., 1989; Kurata and Hata, 1991; Kurata et al., 1993;
Phillips et al., 1994), as well as in vivo (Quaglino et
al., 1991). However, the mechanisms involved are still
not completely known. It is worthwhile mentioning that
the majority of studies of the effect of vitamin C on
collagen gene expression have been made in the presence
of connective tissue-modulating growth factors, such as
epidermal growth factor (EGF), transforming growth factor
(TGF-b), and fibroblast growth factor (FGF) (Hata et al.,
1988; Appling et al., 1989; Kurata and Hata, 1991; Phillips
et al., 1992; Geesin et al., 1993).
During the first weeks of vitamin C deprivation, scorbutic
animals exhibit reduced food intake, which correlates
with the rate of weight loss and with the decrease of
collagen and proteoglycan synthesis (Chojkier et al.,
1983). Later it was shown, however, that reduced collagen
mRNA expression and synthesis can be observed in several
tissues of vitamin C-deficient guinea pigs, which may
not be simply related to the reduced food intake or to
the role of vitamin C in the hydroxylation of proline
residues in collagen; therefore, other more complex mechanisms
and interactions among different cell products seem to
be involved (Gosiewska et al., 1994). In vitamin C-deficient
animals, elevated levels of IGF (insulin growth factor)
mRNA and of IGFBP (insulin growth factor binding protein)
mRNA have been identified, which seem to be responsible
for the inhibition of the IGF-I-dependent functions. Removal
of these inhibitors by specific antibodies restores collagen
gene expression. Therefore, vitamin C deficiency seems
to induce the synthesis of inhibitors of IGF dependent
functions, such as collagen gene expression (Goldstein
et al., 1989, Peterkofsky et al.,. 1994; Gosiewska et
Rather interestingly, vitamin C has been shown to stimulate
transcription of the gene and accumulation of mRNA for
the pro-a1(I) chain but has failed to stimulate transcription
and increase of the mRNA for the pro-a2(I) chain in fibroblasts
from a patient with a2(I)-chain defective Elhers Danlos
syndrome (Hata and Senoo, 1992; Kurata et al., 1993).
This may indicate the presence of different regulatory
elements responsible for transcriptional activation by
vitamin C in pro-a1 (I) and pro-a2 (I) genes in normal
fibroblasts (see also Chapter 3).
In a study we performed on the effects of excess of vitamin
C on collagen of growing rats, an increase or collagen
deposition as well as mRNA expression was observed in
the aorta after more than 10 days of treatment (Quaglino
et al., 1991).
According to some authors, ascorbic acid might stimulate
collagen gene expression through lipid peroxidation; vitamin
C, in fact, may induce lipid peroxidation with the formation
of aldehydic compounds, and some of these, such as malondialdehyde
have been shown to stimlulate collagen production and
raise procollagen a1 (I) mRNA levels in vitro (Brenner
and Chojkier, 1987; Chojkier et al., 1989). Moreover,
both lipid peroxidation and collagen production induced
by ascorbic acid have been shown to be inhibited by a-tocopherol,
a lipophilic antioxidant, or by iron chelators, suggesting
that the two processes are correlated and that an appropriate
redox state might be an important mechanism in controlling
collagen gene expression in vivo (Geesin et al., 1991).
More recent data, however, seem to point to a different
interpretation of the role of the ascorbate-induced lipid
peroxidation on collagen gene expression (Darr et al.,
1993). It has been suggested that lipid peroxidation and
collagen synthesis are coincidental but dissociated, as
metal chelators used to abolish the iron-ascorbic acid-induced
lipid peroxidation may also inhibit prolyl hydroxylase
and, as a consequence, collagen production. Moreover,
cell-impermeable iron chelators have been found to be
good inhibitors of ascorbate-mediated lipid peroxidation
but ineffective in inhibiting collagen synthesis (Darr
et al., 1993).
In any case, almost all studies on the promoting effect
of lipid peroxidation by vitamin C were performed in vitro;
the effect of vitamin C-induced lipid peroxidation might
not be so relevant in vivo. Under physiological conditions,
most of the iron is bound to proteins, and vitamin C might
prevent lipid peroxidation instead of stimulating it (Mukhopadhyay
et al., 1997). Moreover, the role of ascorbic acid in
vivo is generally considered to be one of cellular defense
against oxygen toxicity and lipid peroxidation (Chakraborty
et al., 1994) through a mechanism of free radical scavenging
followed by its conversion to dehydroascorbic acid. Collagen
indeed is susceptible to fragmentation by superoxide anion
with liberation of small hydroxyproline-containing peptides
(Monboisse and Borel, 1992), and, in vivo, vitamin C could
protect collagen from degradation by hydroxyl radicals
in the presence of oxygen.
Therefore, collagen synthesis, maturation, and secretion
as well as collagen degradation are tightly bound processes,
and vitamin C seems to be involved at different levels
of the whole process.
3. ELASTIN AND VITAMIN C
From the early studies on the effect of ascorbic acid
on collagen production, it was observed that vitamin C
deficiency did not affect elastin synthesis and secretion,
although it greatly influenced the degree of its proline
hydroxylation. In the presence of vitamin C, hydroxyproline
in elastin accounts for about one third of the total amino
acid content (Uitto et al., 1976), and proline/hydroxyproline
ratio in elastin is approximately 1:1; on the contrary,
in both skin and aortas of vitamin C-deprived guinea pigs,
the proline/hydroxyproline ratio in elastin was 20:1 (Barnes
et al., 1970). Though underhydroxylated collagen cannot
be excreted from cells, underhydroxylated elastin is secreted
at a normal rate (Rosenbloom and Cywinski, 1976) and,
similar to collagen, in a colchicine-sensitive way (Uitto
et al., 1976).
An influence of vitamin C on elastin synthesis was described
by Scott-Burden and coworkers (1979), who found that heart
smooth muscle cells in vitro incorporated radioactive
valine, an amino acid prevalent in elastin, to a greater
quantity in the absence of than in the presence of ascorbic
acid. Moreover, the elastin produced and secreted in the
absence of vitamin C underwent the cross-linking process
in the extracellular space more rapidly than that produced
in the presence of ascorbic acid (Scott-Burden et al.,
1979). Similar findings were published by DeClerck and
Jones (1980), who found that the amount of insoluble elastin
in the extracellular space was inversely proportional
to the ascorbic acid concentration in the medium. Therefore,
proline hydroxylation in elastin is not necessary for
secretion or for molecule assembly and cross-linking;
on the contrary, underhydroxylation in vitamin C deficiency
seems to favor elastin assembly and its stabilization
into the polymer. Moreover, hyperhydroxylated elastin,
produced in vitro in the presence of ascorbic acid, was
shown to contain more free lysine residues and to turn
over more rapidly (Dunn and Franzblau, 1982). At physiological
temperatures, both in vivo and in vitro, tropoelastin
molecules undergo a process of self-assembly into fibrillar
structures called coacervates (Cox et al., 1974; Volpin
and Pasquali-Ronchetti, 1977; Bressan et al., 1983, 1986).
This phenomenon also seems to happen in vivo and to be
a prerequisite for enzymatic cross-linking of tropoelastin
molecules, through the oxidative deamination of e-amino
groups of lysine residues on adjacent molecules by the
enzyme lysyl oxidase (Narayanan et al., 1978). The inhibition
of the molecular assembly by hyperhydroxylation of proline
would lead to a less cross-linked stable polymer (Urry
et al., 1979).
In order to investigate whether an excess of ascorbic
acid could modify in vivo the assembly of the elastic
fibers, we treated growing chicks and rats with excess
of vitamin C in their diet and drinking water, respectively
(Quaglino et al., 1991). Animals were killed after various
treatment times and the aorta examined by electron microscopy,
in situ hybridization, and biochemical methods. After
30 days treatment, the ultrastructural organization of
the aortic elastic fibers appeared to be significantly
affected by vitamin C treatment compared to control animals
(Fig. 1); in situ hybridization revealed a decreased expression
of elastin mRNA on slices from the aorta of vitamin C-treated
rats (Fig. 2); moreover, stereological measurements on
electron micrographs showed a significant increase in
collagen and decrease in the elastin content in the aortic
wall of treated animals (Fig. 3).
FIGURE 1.Electron microscopy of 30-day chick
aorta. Animals were fed a standard diet (a) or a
diet supplemented with 0.2% ascorbic acid from hatching
(b). Chick aorta is composed of layers of smooth
muscle cells (SMC) among which there are several
elastic fibers (E) and a few collagen bundles (C).
Vitamin C seems to cause an increase in collagen
bundle deposition and a decrease in elastic fiber
assembly. Bar: 1 Fm.
FIGURE 2. In situ hybridization
of rat aorta in 30-day old animals grown, at weaning,
on a standard diet (a) or on a standard diet plus
10% ascorbic acid added to the water (b). Sections
have been hybridized with a 1.0 kb cDNA fragment (cHE-4)
corresponding to exons of 18 to 36 of human elastin
and exposed for autoradiography. Animals treated with
an excess of vitamin C show a decreased expression
of elastin mRNA compared to control animals. Bar:
FIGURE 3. Morphometric
analysis of the rat aorta in 50-day old animals grown,
at weaning, on a standard diet plus 10% ascorbic acid
added to the water. Treatment causes an increased
deposition of collagen bundles and reduced amounts
of elastic fibers, whereas no significant changes
were observed in the cellular component or in the
remaining extracellular matrix.
4. MATRIX GLYCOPROTEINS AND VITAMIN C
Relatively few studies are available on the effects of
vitamin C on synthesis and secrction of matrix molecules
apart from collagen and elastin; however, in a number
of studies occasional mention is found of fibronectin,
proteoglycans, bone matrix glycoproteins, and elastin-associated
fibrillin (Schwartz et al., 1982; Kielty and Shuttleworth,
Ascorbic acid has been shown to stimulate in vitro differentiation
and production of matrix molecules by adipocytcs (Taylor
and Jones, 1979), fat-storing cells (Senoo and Hata, 1994),
chondrocytes (Leboy et al., 1989; Aulthouse, 1994), myoblasts
(Nandan et al., 1990; Mitsumoto et al., 1994), and osteoblasts
(Franceschi and Iyer, 1992). In this latter case, vitamin
C has been shown to be taken up by the cell through a
specific transport system (Wilson and Dixon, 1989) and
to influence osteoblast differentiation in a rather unusual
way. The expression of the osteoblast phenotype is regulated
by a series of factors, including growth factors, glucocorticoids,
parathyroid hormone, and 1,25-dihydroxyvitamin D3; however,
differentiation and mineralization seem to require the
presence of an extracellular collagen matrix. Vitamin
C has been shown to be necessary both for the production
of the collagen matrix and for the expression of osteoblast
markers, such as alkaline phosphatase and osteocalcin,
whereas it seems to have no effect on the level of osteopontin
mRNA (Franceschi and Iyer, 1992). In recent years, a number
of papers have pointed to the effect of ascorbic acid
on the differentiation of bone cells. Ascorbic acid has
also been shown to enhance the effect of retinoic acid
on mRNA expression of pro-a1(I) collagen and of alkaline
phosphatase in an immortalized strain of rat osteoblasts
in culture (Choong et al., 1993). Ascorbic acid was found
to stimulate cell proliferation, together with collagen,
non-collagenous protein, and alkaline phosphatase synthesis,
in pig bone cells in culture when added after cell confluence,
suggesting that it may interfere with cell differentiation
(Denis et al., 1994).
Ascorbic acid, in association with b-glycerophosphate,
was found to stimulate matrix mineralization by inducing
an increase of neutral metalloproteinase in matrix vesicles,
which may be able to degrade proteoglycans favoring mineral
precipitation (Brooks et al., 1994). Once again, vitamin
C, b-glycerophosphate, and dexamethasone induced an increases
of the mRNA level for collagen type 1, osteocalcin, bone
sialoprotein, and alkaline phosphatase in association
with the development of bone nodules in an in vitro system
(Malaval et al., 1994). In all these studies, ascorbic
acid seemed to act as a promoter for collagen synthesis
and secretion, whereas subsequent cell-matrix interactions
seemed to influence cell shape, metabolism, and differentiation
(Aulthouse, 1994). In fact, bone protein synthesis was
blocked by inhibitors of collagen triple helix formation
(Franceschi and Iyer, 1992), but these inhibitors were
ineffective if added after a certain amount of normally
hydroxylated collagen had been produced (Franceschi et
al., 1994). This could mean that bone cell differentiation
depends on vitamin C for the synthesis and secretion of
the first collagen matrix and that it may continue also
in the absence of ascorbic acid due to the already established
Addition of ascorbate to cultured calf aortic smooth
muscle cells was shown to increase collagen secretion
together with fibronectin and proteoglycans and the phenomenon
was associated both with changes in cell morphology, from
elongated to polygonal, and with an increase of the cell
growth rate (Schwartz et al., 1982). These findings could
be the result of the first deposition of a matrix collagen,
stimulated by ascorbic acid, followed by changes in cell
metabolism that are regulated by cell-collagen interactions.
The effect of ascorbic acid on the production of proteoglycans
is rather controversial. Edward and Oliver (1983) found
that both hyaluronate and sulfated glycosaminoglycan synthesis
by human skin fibroblasts was affected by vitamin C. Kao
and coworkers (1990) found that ascorbic acid stimulates
the production of glycosaminoglycans in cultured fibroblasts,
whereas Pacifici (1990) observed that in chick chondrocyte
cultures the secretion of keratin sulfate and chondroitin
sulfate-containing proteoglycans was not affected by ascorbic
acid in the culture medium.
In a recent study, vitamin C was shown to negatively
influence the synthesis of aggrecan and to abolish the
lag phase for decorin synthesis in cultured rabbit articular
chondrocytes (Hering et al., 1994). The production of
laminin and fibronectin was also shown to be increased
by vitamin C added to the cultured bovine trabecular meshwork
cells (Yue et al., 1990), suggesting a possible influence
on cellular adhesion molecules.
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