Scientists have identified a single gene that appears to be central to the maintenance of non-small cell lung cancer (NSCLC) cancer stem cells, as well as the initiation of tumor formation, growth and metastasis in vivo. A Mayo Clinic College of Medicine team found that cancer stem cells
(CSCs) in oncosphere cultures demonstrated high expression levels of
matrix metalloproteinase-10 (Mmp-10; or stromelysin 2), and that
knocking down Mmp10 expression in these cells resulted in the loss of
stem cell markers and the inhibition of oncosphere growth, clonal
expansion, and tumor development. Mmp10-deficient oncospheres were in
addition far less able to initiate tumor development than unmodified
oncospheres when injected into experimental mice.
Reporting their findings in PLoS One, Alan P. Fields, M.D., and colleagues say subsequent analysis of gene expression
data from human cancers highlighted a strong correlation between tumor
Mmp10 expression and metastasis in a range of tumor types. “Our data
demonstrate for the first time that Mmp10 is a critical lung cancer stem
cell gene and novel therapeutic target for lung cancer stem cells,”
they conclude. The investigators’ results are published in a paper
titled “Matrix Metalloproteinase-10 Is Required for Lung Cancer Stem
Cell Maintenance, Tumor Initiation and Metastatic Potential.”
MMPs have previously been implicated in lung tumor proliferation,
invasion, and metastasis. The Mayo team’s own prior work suggested that
Mmp10 is needed for transformed growth and invasion of human NSCLC cells
in vitro, and mediates tumor initiation through the control of
tumor-initiating bronchio-alveolar stem cell expansion.
Building on these findings the investigators have now looked more
closely at the role of Mmp10 in the maintenance and tumorigenic
potential of fully transformed mouse lung CSCs, a cell population
characterized by stem-like properties including increased
anchorage-independent growth in vitro, and enhanced tumor initiation,
growth, and metastatic spread as orthotopic tumors in syngeneic mice.
The CSCs in these oncosphere cultures were found to express high
levels of mRNAs for numerous genes associated with stem cell phenotype,
and also high levels of Mmp10, but not other MMPs previously linked with
lung cancer. The cultures in addition secreted much higher levels of
Mmp10 protein into the medium in comparison with parental or
redifferentiated cultures. As expected, oncosphere cultures demonstrated
a loss of stem cell markers and Mmp10 expression when allowed to
redifferentiate in adherent culture.
Notably, using an Mmp10-targeting RNAi to knock down Mmp10 mRNA
expression in the oncosphere cultures led to an inhibition of
transformed growth, loss of stem cell markers, and inhibition of clonal
expansion, without impacting on cell viability. These effects could be
restored by the addition of exogenous Mmp10.
Initial tests in vivo showed that while transplanting cells from
oncosphere cultures into the lungs of syngeneic mice routinely led to
the development of large tumors, cells from Mmp10-knockdown cultures
generated fewer and small tumors, and also fewer metastases.
Importantly, the role of Mmp10 in tumorigenesis related to the gene’s
expression by CSCs specifically, and not by other cells in the tumor
environment. When the researchers injected unmodified oncospheres into
Mmp10 knockout mice, tumor growth, size, and metastasis were equivalent
to those resulting from the injection of unmodified oncospheres into
wild-type syngeneic mice. This indicates that “Mmp10 expressed by
oncospheres, but not from Mmp10 produced by other tumor-associated
cells, is critical for tumor formation,” they write.
In a final set of analyses, the team analyzed publicly available
gene-expression datasets of human tumors and found a strong positive
correlation between Mmp10 expression and metastatic potential not only
in human NSCLC, but also in colorectal cancer, melanoma, breast cancer,
renal cell carcinoma, and prostate cancer.
The observation that Mmp10 appears to play a dual role in cancer,
both in terms of maintaining the cancer stem cell population and
facilitating metastasis, was an unexpected finding, the investigators
note. Most other MMPs are expressed in the tumor microenvironment and
cells and tissues surrounding the tumor, where they act to modify the
tumor environment and facilitate cancer cell spread, Dr. Field explains.
Conversely, “Mmp10 acts to keep these cancer stem cells healthy and
self-renewing, which also helps explain why these cells escape
conventional chemotherapy that might destroy the rest of the tumor.”
The team is now trying to define the mechanism by which Mmp10
stimulates the growth of cancer stem cells, and identify the design of
inhibitors that block its activity. “Given the dual role in cancer stem
cells and metastasis, targeting Mmp10 may be especially effective in
treating these tumors,” he concludes.
