coordinate lipogenic programmes [3]. Presence of glycogen has been
described also in various cancer cells and tumours. The level of
glycogen accumulation varies greatly across tumours. The levels of
glycogen were demonstrated to be particularly high in breast,
kidney, uterus, bladder, ovary, skin and brain cancer cell lines.
Glycogen content in these cells and in human colorectal cancer
tissues was inversely correlated with proliferation rate, suggesting
that glycogen is consumed to sustain cancer cell growth [4,5]. In
particular, ‘clear cell carcinomas’ represent a subset of tumours,
which are characterised by a prominent cellular enrichment in
glycogen. Their name derives from the clear, vacuolated appearance
of cellular cytoplasm caused by extraction of glycogen during
histology processing.
Both glycogen synthesis and degradation involve the activity of
several enzymes and regulatory proteins. Synthesis is performed in
the cytosol from extracellular glucose transported into the cells
through glucose transporters (direct pathway) or from gluconeogenic
substrates, such as lactate and amino acids (indirect
pathway). The indirect pathway occurs mainly in the liver,
following either the intrahepatic or extrahepatic conversion of
gluconeogenic precursors into glucose [6].
The first step of glycogen synthesis consists on the autoglucosylation
of the core protein, glycogenin (GYG). This provides
an oligosaccharide primer for glycogen synthase enzyme (GYS),
which elongates the glucose chain by attaching (activated) uridine
diphospho-glucose (UDP-glucose) units through a-1,4 glycosidic
bonds. Once the elongating chain consists of approximately
11 units, the glycogen branching enzyme (GBE) transfers a chain
of 7 units to an adjacent chain through a a-1,6 glycosidic bond [7]
(Fig. 1). The coordinated function of these two enzymes proceeds
to form spherical granules named b-particles, which are 20–50 nm
in diameter and contain up to 55,000 glucose units. The b-particles
can aggregate to form larger a-rosettes, of 200 nm in diameter.
coordinate lipogenic programmes [3]. Presence of glycogen has beendescribed also in various cancer cells and tumours. The level ofglycogen accumulation varies greatly across tumours. The levels ofglycogen were demonstrated to be particularly high in breast,kidney, uterus, bladder, ovary, skin and brain cancer cell lines.Glycogen content in these cells and in human colorectal cancertissues was inversely correlated with proliferation rate, suggestingthat glycogen is consumed to sustain cancer cell growth [4,5]. Inparticular, ‘clear cell carcinomas’ represent a subset of tumours,which are characterised by a prominent cellular enrichment inglycogen. Their name derives from the clear, vacuolated appearanceof cellular cytoplasm caused by extraction of glycogen duringhistology processing.Both glycogen synthesis and degradation involve the activity ofseveral enzymes and regulatory proteins. Synthesis is performed inthe cytosol from extracellular glucose transported into the cellsthrough glucose transporters (direct pathway) or from gluconeogenicsubstrates, such as lactate and amino acids (indirectpathway). The indirect pathway occurs mainly in the liver,following either the intrahepatic or extrahepatic conversion ofgluconeogenic precursors into glucose [6].The first step of glycogen synthesis consists on the autoglucosylationof the core protein, glycogenin (GYG). This providesan oligosaccharide primer for glycogen synthase enzyme (GYS),which elongates the glucose chain by attaching (activated) uridinediphospho-glucose (UDP-glucose) units through a-1,4 glycosidicbonds. Once the elongating chain consists of approximately11 units, the glycogen branching enzyme (GBE) transfers a chainof 7 units to an adjacent chain through a a-1,6 glycosidic bond [7](Fig. 1). The coordinated function of these two enzymes proceedsto form spherical granules named b-particles, which are 20–50 nmin diameter and contain up to 55,000 glucose units. The b-particlescan aggregate to form larger a-rosettes, of 200 nm in diameter.
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