AGGRESSIVE tumours generate their own vascular networks that nourish them and
help them spread to other parts of the body, say researchers in the US. This new
finding could help explain why drugs designed to cut off a tumour鈥檚 external
blood supply are only partially effective.
Andy Maniotis of the University of Iowa in Iowa City and his colleagues made
the discovery while studying skin melanoma and an unusual tumour called uveal
melanoma, which affects the inside of the eye. Using electron microscopy and
other techniques, they found that the most aggressive melanomas form their own
vascular channels that are physically distinct from normal blood vessels.
The newly discovered vascular networks lack the normal lining of endothelial
cells and are arranged in an organised pattern of loops around clusters of
tumour cells. Normal blood vessels are much more randomly organised.
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The team also detected red blood cells in the loops, which suggests that they
are involved in some form of circulation (The American Journal of
Pathology, vol 155, p 739). These vascular networks would explain how cells
manage to survive even at the centre of a large tumour, Maniotis says.
The pattern was the same when these cancers spread to other parts of a
patient鈥檚 body and formed secondary tumours. 鈥淲herever the tumour goes it makes
this very peculiar patterned circulation,鈥� says Bob Folberg, one of the
researchers on Maniotis鈥檚 team.
But the pattern did not show up in less invasive tumours that failed to
spread quickly. So searching for the loops in tumours may allow doctors to give
more accurate prognoses to people with these cancers, Folberg says.
The finding could apply to other types of cancer as well, and possibly to all
of them. 鈥淚t鈥檚 too early to tell if it is a trademark of all aggressive
tumours,鈥� says Maniotis. 鈥淏ut we have preliminary data that it could be true for
five different cancers.鈥�
The discovery could also explain why certain cancer treatments currently on
trial do not work as well as doctors hoped. These drugs are designed to block
angiogenesis鈥攖he process whereby a tumour recruits healthy tissue to form
blood vessels to feed it.
The new finding suggests that even if angiogenesis is halted, tumours can
rely on their own vascular networks to survive. However, the hope is that it
might eventually be possible to find new drugs that target the tumours鈥� vascular
system as well, Folberg adds.
Maniotis believes these strange vascular systems have not come to light
before because so much cancer research has been done on animals that have been
implanted with foreign, cancerous cells. This would encourage angiogenesis to
dominate, because the vessel-growing process is also triggered at sites where
the immune system detects infection by foreign bodies.
If so, says Maniotis, scientists will have to think again about how much such
animal studies tell us about cancer in people. 鈥淧eople are very complacent with
their animal models,鈥� he says. 鈥淏ut this begs the question of whether there
exists a good model of cancer.鈥�