IOWA CITY, IowaA surprising new study shows that some melanoma
cells can form themselves into vascular channels and provide a
tumors blood supply without the need for angiogenesis (the
growth of new blood vessels from existing blood vessels). The
resulting channels, which are lined by melanoma cells and basement
membrane (see Figure 1), function
as a vascular system for the tumor without the endo-thelium-lined
blood vessels produced through angiogenesis.
University of Iowa cancer biologist Mary J. C. Hendrix, PhD, and her
colleagues at Iowa, Hadassah University Hospital in Jerusalem, and
the National Human Genome Research Institute, discovered this
phenomenon of vasculo-genic mimicry in studies of human
intraocular (uveal) melanomas. Their work is reported in the
September 1999 issue of the American Journal of Pathology
The study suggests that aggressive melanomas have a special ability
to form their own tumor supply apart from the usual mechanisms of
angiogenesis, and that these vascular networks (which can be detected
with angiography) may provide a noninvasive method for detecting
The researchers found that:
Patterned vascular channels found in aggressive primary and
metastatic melanomas are different from angiogenic vessels derived
Highly invasive melanoma cells, but not poorly invasive ones, can
make similar patterned vascular channels in vitro in the absence of endothelium.
These aggressive melanoma cells are deregulated and express genes
associated with embryonic cells, including genes associated with
The fact that these deregulated aggressive melanoma cells can build
vascular channels suggests a means by which they provide tumor
microcirculation and promote metastasis.
Our collaborative team addressed an important question that Dr.
Robert Folberg (the pathologist on the study) had been focusing on
for years: What is the biological basis for the appearance of
microcirculatory networks and patterns in the tissues of patients
with poor-prognosis uveal melanoma? Dr. Hendrix said in an
interview. The study revealed that the aggressive tumor cells
(isolated from patients with a poor prognosis) had the ability to
form their own microcirculation in the absence of endothelial cells.
The combined biomechanical expertise of Dr. Andrew J. Maniotis
(who developed the 3-dimensional assays to prove that aggressive
tumor cells could form their own microvasculature) and my cancer
biology background, along with Dr. Folbergs pathology
expertise, made this study a once in a lifetime discovery, Dr.
Hendrix commented. The collaboration also included Paul S. Meltzer,
MD, of the National Human Genome Research Institute, who performed
the genetic analysis of the tumor cells.
The investigators conducted both in vivo and in vitro studies of
human uveal melanomas. Their goal was to determine why melanomas that
produce fine web-like patterns of vascular channels are associated
with poor prognosis. They approached this by comparing the
tube-forming ability of cells from aggressive and nonaggressive
melanomas when cultured with endothelial cells.
They expected to find a difference in the ability of cells from these
two tumor groups to induce endothelial cells to form blood vessels,
but they actually found something much more surprising: Cells from
aggressive melanomas could form vascular channels even without the
help of endothelial cells. Furthermore, these channels strongly
resembled the microcirculatory structures observed in tissue from
patients with aggressive uveal melanomas but not in tissue from
patients with less aggressive disease.
Possible Use in Tumor Staging
The researchers also found similar channels in metastatic cutaneous
melanoma tissue and in liver metastases from uveal melanomas. Dr.
Hendrix said that they have made similar observations in other
aggressive tumor models and have received independent confirmation
from other laboratories that their findings have been repeated and validated.
This is particularly important because channels of this type can be
visualized noninvasively in vivo using indocyanine green angiography (Figure
2, left photo), and histopathologic staining shows that these
channels are periodic acid-Schiff (PAS)-positive looping patterns and
networks (Figure 2, right photo).
The association between the density of tumor-generated vascular
channels and prognosis may mean that examination of these networks
could be used as a noninvasive method for tumor staging.
There is good evidence from our report and also from previous
studies by Dr. Folberg and others to suggest that imaging may provide
important information regarding the metastatic potential of the
tumors under examination, Dr. Hendrix said.
The basic structure of these vascular channels is an inner basement
membrane covered by an outer layer of melanoma cells. Red blood cells
were frequently observed within these channels. The investigators are
currently characterizing the constituents of the basement membrane.
The 3D culture method developed by Dr. Maniotis was key to this
project. These cultures are produced by first establishing a
polymerized Matrigel or Type I collagen matrix on a coverslip, then
seeding tumor cells on top of the gels and watching to see what
happens. When cells from aggressive melanomas were seeded onto the
gels, they readily formed vascular loops and networks that looked
very much like those seen with angiography and in PAS-stained tissue
samples. The researchers also found that these channels could hold
injected dye and distribute it through the generated network, even in
culture. Cells from less invasive melanomas did not form these networks.
Furthermore, this process was not affected by conditioned medium from
invasive cells or by a variety of growth factors. It was not
inhibited by conditioned medium from poorly invasive cells or by
blocking antibodies. These two findings suggest that vascular channel
formation is a process intrinsic to the melanoma cells themselves
rather than a response to some external signal.
Cultures from aggressive melanomas also had a strong ability to
contract and alter the shape of floating collagen gels. Dr. Hendrix
said that this is a good test of the ability of cells to remodel and
contact extracellular matricesan important mechanism in the
formation of blood vessels and microvascular channels.
These behavior differences in cells from the aggressive melanomas led
the investigators to ask whether they were expressing different genes
than cells from poorly invasive melanomas. Hybridization to cDNA
microarrays was used to analyze the expression of 5,000 genes. More
than 200 genes were differentially expressed in these aggressive vs
less aggressive tumors, and 15 of those have been associated with
phenotypes characteristic of endothelial or vascular cells.
This suggests that the more aggressive melanoma cells have moved back
toward a more pluripotent, embryonic-like genotype. The
aggressive (but not the nonaggressive) melanoma tumor cells somewhat
resemble embryonic cells in their ability to express myriad markers
and characteristics, Dr. Hendrix said. They also express
certain genes similar to those expressed by endothelial cells.
However, endothelial cells produce blood vessels, and the aggressive
tumor cells in our study produced vascular channels that are not
lined by endothelial cells.
The researchers noted that formation of a microcirculation by cells
other than endothelial cells has been reported in normal embryonic
tissues but not previously in the context of tumor progression.
The investigators concluded that their findings call for reappraisal
of the current assumption that endothelial-cell-mediated angiogenesis
is the only mechanism underlying tumor growth and metastasis. Dr.
Hendrix said, This work adds a new dimension to our thinking
about the microcirculation of aggressive tumors. It provides an
interesting vascular channel formation mechanism and new molecular
markers for diagnostic application and therapeutic intervention.
These studies suggest that angiogenesis inhibitors alone are not
going to be the magic bullet that kills all solid tumors, since the
aggressive tumors in this study were able to form their own
functional vascular supply system without angiogenesis or endothelial cells.
I feel that our work offers new opportunities and strategies to
target aggressive tumors and may, in fact, complement the ongoing
studies of Folkman and colleagues in a very important mannerby
directly targeting the tumor cells themselves within an aggressive
tumor, Dr. Hendrix said. We must begin testing the
efficacy of antiangiogenic agents in our 3D models, in addition to
developing specific agents that will target the molecular markers
revealed in our study.
This study also raises new questions about drug testing. For example,
in studies of angiogenesis inhibitors, will it be necessary to test
patients for the presence of endothelium-lined tumor vessels as, for
example, breast cancer patients are tested for HER2 before starting
treatment with Herceptin (trastuzumab)?