Laminin is famous for being shaped like a cross but it should be valued for its role in preventing cancer development. Credit: National Institutes of Health |
Laminin,
long thought to be only a structural support protein in the microenvironment of
breast and other epithelial tissue, is “famous” for its cross-like shape.
However, laminin is far more than just a support player with a “pretty face.”
Two studies have shown how laminin plays a central role in the development of
breast cancer.
In one study it was shown how laminin influences the genetic
information inside a cell’s nucleus. In the other study it was shown how
destruction of laminin can play a detrimental role in the early stages of tumor
development.
Mina
Bissell, a “Distinguished Scientist” with the Lawrence Berkeley National Laboratory, is
famous for having discovered the critical role in breast cancer development
played by the extracellular matrix (ECM), the network of fibrous and globular
proteins surrounding a breast cell. Her “dynamic reciprocity” theory holds that
the fate of cells hinges on the chemical signals exchanged between the ECM and
a cell’s nucleus. In these latest studies, Bissell and her collaborators
focused on laminin and its connections with two other proteins—actin, a
cytoplasmic protein that has been linked to nuclear activities; and MMP9, an
enzyme that is secreted outside the cells and is known to break down ECM
constituents.
Laminin
and cell quiescence
“Quiescence” is the process by which a biological cell stops growing or dividing.
This is the opposite of a cancerous state, in which cell growth and division is
often unchecked. Signals from laminin-111, an ECM protein that helps the cell
and its ECM stick together, have been linked to cell quiescence but the
mechanism was unknown. Bissell and postdoctoral fellow, Virginia Spencer, in
Berkeley Lab’s Life Sciences Division, have now shown that the addition of
laminin-111 leads to quiescence in breast epithelial cells through changes in
nuclear actin.
“We
found that high levels of laminin-111 depleted nuclear actin and this in turn
induced cell quiescence,” Bissell says. “Furthermore, this process can be
prevented if a form of actin that can not exit the nucleus is introduced. Under
these conditions the cells do not stop growing even in the presence of
laminin.”
In
their study, Bissell and Spencer and their colleagues used a three-dimensional
cell culture assay developed by Bissell’s research group, and worked with mouse
and human mammary epithelial cells. Through the addition of laminin-111, they
were able to decrease nuclear actin levels in the cultured cells, which reduced
DNA synthesis and transcription. When nuclear actin levels were deliberately
over-expressed, the effects were reversed and cells were prevented from
becoming quiescent even in the presence of laminin-111. Furthermore, the high
levels of nuclear actin were concentrated in regions of the breast cells where
there was little or no laminin-111. Taken together, the results implicate
laminin-111 as the regulator of nuclear actin and nuclear actin as a key
mediator of epithelial cell quiescence.
“In
collaboration with Ole Petersen’s laboratory, we had found previously that the
ECM surrounding tissues from breast cancers has a dramatic reduction in
laminin-111 in comparison to the ECM surrounding a normal breast cell, which is
rich in laminin-111,” Bissell says. “However, just giving laminin back to
cancer cells was not enough to make them normal so other factors are clearly
also involved and one such factor we now know is how laminin-111 and nuclear
actin talk to each other!”
Says
Spencer, “Ours is the first study to actually identify laminin-111 as the
physiological regulator of nuclear actin and to implicate the loss of nuclear
actin as a key step in reaching quiescence and homeostasis in the mammary gland
in vivo and in culture.”
Spencer
believes that the interaction between laminin-111 and nuclear actin could
provide a new target for diagnostic therapeutic efforts, but this will require
further study.
“While
it remains to be determined whether dysregulation of the levels or organization
of nuclear actin is responsible for the inability of malignant cells to respond
to growth-inhibitory signals from laminin-111, our preliminary results point in
this direction,” she says. “In addition, the findings that laminin-111
expression is lost in myoepithelial cells isolated from human tumors should
place the interaction of laminin-111 and breast tumor cells at the forefront of
future investigations.”
A
paper detailing the results of this study appears in the Journal of Cell
Science.
Laminin,
MMP9 and tumor growth
In the second study, which was related to the role of laminin-111 in cell
quiescence, Bissell and another group of collaborators examined laminin-111 in
the context of matrix metalloproteinase-9 (MMP9), a zinc-dependent enzyme that
plays a huge role in tissue function by virtue of its ability to cleave or
degrade many of the ECM constituent proteins, including laminin-111.
A 3D cell culture assay developed by Mina Bissell and her research group enables breast cells to reproduce actual structural units, an advantage that was essential for understanding the role of laminin in breast cancer development. Credit: Bissell group |
“Organization
into polarized three-dimensional tissue structures defines whether epithelial
cells are normal or malignant,” Bissell says. “We have shown that when MMP9
degrades laminin-111 in the ECM, the tissue architecture of breast cells
becomes lost and cell proliferation is initiated. This is the first
demonstration of how the degradation of laminin-111 by MMP9 in a physiological
context contributes to tumor progression.”
A
paper detailing the results of this study has appeared in the journal Genes
and Development.
Using
a model of human breast cancer where breast epithelial cells were grown in
three-dimensional cultures of basement membrane, a thin layer of ECM material
that envelops breast and other glandular tissue, Bissell and her co-authors
found that not only did excessive MMP9 activity disrupt tissue architecture,
but that silencing MMP9 restored tissue architecture and decreased the ability
of human beast cancer cells to form tumors in mice.
“We
found that in all conditions where tumor cells could be reverted to a normal
phenotype in our 3D assays, a novel signaling loop through a pathway of
Raf/MEK/ERK proteins was responsible for MMP9 activity in the breast tumor
cells,” says co-author Joni Mott, a researcher with Bissell’s group in Berkeley
Lab’s Life Sciences Division. “Once MMP9 was activated, the proteinase targeted
the destruction of laminin-111 within the basement membrane.”
Laminin-111
in the basement membrane, Mott explains, allows mammary epithelial cells to
establish a normal polarized structural unit called an “acinus,” which is
responsible for storing milk and making it available for babies when they
suckle.
In
the Genes and Development paper, the team reported that when the
integrity of the tissue architecture was compromised by laminin proteolysis,
the basement membrane no longer provided the appropriate signals to restrain
epithelial cell proliferation. The result was a sustained signaling of the
Raf/MEK/ERK pathway that leads to continued MMP9 production and further
disruption of tissue architecture and loss of cell growth control.
“This
work is particularly poignant because it provides potential new therapeutic
targets for controlling breast cancer and revitalizes the possibility of
targeting MMPs in cancer therapy,” Bissell says. “New information on how MMP9
and other MMPs truly function may provide highly targeted and effective
therapeutic strategies to control MMP activity in cancer, and may soon lead to
the development of novel cancer treatments.”
Both
studies were funded in part by grants from the U.S. Department of Energy’s
Office of Science, the National Cancer Institute, and the U.S. Department of
Defense.