Pokaż cały numer - FPN - Farmaceutyczny PrzeglÄ d Naukowy

copyright © 2009 Grupa dr. A. R. Kwiecińskiego ISSN 1425-5073
ficult to explain why glucose deprivation results in protection
of the newly synthesised collagen against intracellular
degradation.
The endoplasmic reticulum is an organelle where secretory
or membrane proteins are synthesised. Approximately
one-third of all cellular proteins are translocated into the lumen
of the ER with its unique oxidising and Ca2+-rich environment,
where post-translational modification, folding and
oligomerisation of nascent proteins occur before they are
translocated to their final destination. Correct folding is an
important factor which allows translocation of protein molecules
to specific subcellular compartments, extracellular
matrix or to biological fluids [20-22]. The folding process is
regulated by a group of proteins, referred to as chaperones
[23]. ER molecular chaperones and folding enzymes associate
with the newly synthesised, unfolded and/or incompletely
glycosylated proteins to prevent their aggregation and
help them to fold and assemble correctly [21,22]. In some
instances, they function even in the refolding of denatured
proteins [19].
It is known that collagen folding depends on the hydroxylation
of prolyl residues to achieve sufficient amounts of
hydroxyprolyl residues to form a stable triple helical structure.
Properly folded collagen is secreted extracellularly and
serves as a substrate in the process of fibrogenesis. Underhydroxylated
collagen is unable to form a stable triple helical
structure and cannot be secreted from the synthesising cell.
It is degraded by the intracellular proteolytic system [24].
Many stress conditions, such as glucose deprivation, reduces
the folding process which results in the accumulation
of unfolded/misfolded proteins within the cell [25,26]. The
accumulation of unfolded proteins activates the so called
“unfolded protein response”, which enhances cell survival
by limiting the accumulation of unfolded or misfolded proteins
in the ER [25,26]. Molecular chaperones are induced in
these conditions, bind to unfolded/misfolded proteins, and help
them to be folded or refolded correctly [27]. An integral component
of the cellular response to environmental stress is the
expression, usually by de novo protein synthesis, of stress-associated
polypeptides termed oxygen-regulated protein [27,28] .
Several authors have reported that glucose deprivation
results in stimulation of protein degradation [1,6] and that
oxygen or glucose deprivation increases free radicals, which
in turn, oxidise proteins that are recognized and actively degraded
by proteasomes [1]. It is apparent from our results
that collagen (at least in our experimental conditions) is protected
against proteolysis, induced by glucose-deprivation.
This phenomenon is accompanied by an increase in the expression
of ORP150 – a chaperon, which protects intracellular
proteins against degradation. The appearance of ORP
150 in glucose deprived cultures coexisted with an increase
of gelatinolytic activity. Despite glucose shortage reduces
collagen synthesis, the increased expression of ORP150
may reduce the degradation of newly synthesised protein
and protect the cell culture against a massive loss of collagen.
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