Isaiah J. Fidler, DVM, PhD
To produce clinically relevant metastasis, tumor
cells must complete a series of sequential and selective steps that include
growth, neovascularization (angiogenesis), detachment (motility), invasion,
survival in the circulation, adhesion to vessel walls (cells), extravasation
into the organ parenchyma, and growth coupled with neovascularization.
In 1889, Paget asked, "What is it that decides what
organs shall suffer in a case of disseminated cancer?" Paget’s study was
motivated by the discrepancy between consideration of blood flow and the
frequency of metastases in different organs. After examining the autopsy
records of 735 women who died of breast cancer and many other patients
with different neoplasms, he concluded that metastases occurred only when
certain favored tumor cells (the "seed") had a special affinity for the
growth milieu provided by certain specific organs (the "soil"). The formation
of metastasis required the interaction of the right cells with the compatible
organ environment.
A current definition of the "seed and soil" hypothesis
consists of three principles. First, neoplasms are biologically heterogeneous
and contain subpopulations of cells with different angiogenic, invasive,
and metastatic properties. Second, although some of the steps in this process
contain stochastic elements, metastasis as a whole favors the survival
and growth of a few subpopulations of cells that pre-exist within the parent
neoplasm. Thus, metastases can have a clonal origin, and different metastases
can originate from the proliferation of different single cells. Third,
the outcome of metastasis depends on multiple interactions ("cross talk")
of metastatic cells with homeostatic mechanisms, which the tumor cells
can usurp.
The survival and growth of cells (normal and tumor)
in the body are dependent on an adequate blood supply. Tumors smaller than
1 to 2 mm in diameter can receive nutrients by diffusion, but further growth
of the lesions (like any other tissue) must be preceded by the formation
of new blood vessels, ie, angiogenesis. The prevascular stage of a tumor
is associated with local, noninvasive, benign tumors, whereas the vascular
stage is associated with aggressive, invasive, and metastatic tumors. Indeed,
the extent of neovascularization in different malignancies, such as melanoma
and breast and prostate cancers, is associated with their potential for
invasion and metastasis. Angiogenesis is a multistep process emanating
from microvascular endothelial cells. To generate capillary sprouts, endothelial
cells must proliferate, migrate, and invade host stroma, the direction
of migration generally pointing toward the source of angiogenic molecules.
The capillary sprout subsequently expands and undergoes morphogenesis to
yield a capillary.
The induction of angiogenesis is mediated by multiple
molecules that are released by some tumor cells and host cells. Among these
molecules are members of the fibroblast growth factor (FGF) family: vascular
endothelial cell growth factor/vascular permeability factor (VEGF/VPF)
and interleukin 8 (IL-8). The extent of angiogenesis is determined by the
balance between factors that stimulate and those that inhibit the process.
In quiescent normal tissues, the inhibitory factors predominate. Both stimulating
and inhibiting molecules of angiogenesis can also be produced by leukocytes,
and the organ microenvironment can directly contribute to the induction
and maintenance of the angiogenic factors basic FGF, VEGF, and IL-8.
Several factors that down-regulate or inhibit angiogenesis
have already been incorporated into clinical trials, the most widely studied
being interferon alfa (IFN-a). We tested the ability of IFNs to down-regulate
bFGF mRNA expression and protein production in human carcinoma cell lines.
We found that IFN-a or IFN-b (but not IFN-g) down-regulated the steady-state
mRNA expression and protein production of bFGF and collagenase type IV
in human renal, bladder, colon, and prostate cancer cells by mechanisms
independent of their antiproliferative effects. The inhibition of bFGF
mRNA and protein production required long-term exposure (more than four
days) of cells to IFNs. Moreover, once IFN was withdrawn, cells resumed
production of bFGF. These observations were consistent with the clinical
experience that IFN-a must be given for many months to bring about involution
of hemangiomas. We next analyzed the expression of angiogenic molecules
in surgical specimens of human hemangiomas resected during the proliferative,
involuting, and involuted phases. Proliferation was associated with production
of bFGF and VEGF and absence of IFN-b, whereas involuted lesions and normal
skin expressed high levels of IFN-b and low levels of bFGF and VEGF. Further
immunohistochemical analysis revealed that IFN-b is produced by differentiated
epithelial cells of the skin, aerodigestive tract, gastrointestinal tract,
and genitourinary tract and not by normal dividing cells or by carcinomas.
These data suggest that cell division associated with production of positive
angiogenic molecules is inversely correlated with production of IFN-b,
a negative regulator of angiogenesis.
The mouse and human IFN-b genes have been cloned.
Transduction with viral vectors containing the IFN-b cDNA produced down-regulation
of collagenase type IV and bFGF in tumor cells and activation of inducible
nitric oxide in infiltrating macrophages. The inhibition of angiogenesis
coupled with activation of tumoricidal properties in macrophages produced
regression of human colon, prostate, ovarian, and bladder carcinomas implanted
into orthotopic sites in nude mice. Collectively, these data indicate that
cytokines produced by leukocytes and different organ environments can regulate
the process of angiogenesis that is an integral part of the metastatic
cascade.
These data suggest that therapy against metastasis
can be targeted not only against the tumor cells, but also against the
homeostatic factors that are favorable to tumor metastasis, growth, and
survival.
General References
Fidler IJ. Critical factors in the biology of human cancer metastasis:
Twenty-Eighth G.H.A. Clowes memorial award lecture. Cancer Res.
1990;50:6130-6138.
Fidler IJ. Modulation of the organ microenvironment for the treatment
of cancer metastasis. J Natl Cancer Inst. 1995;87:1588-1592.
Ellis LM, Fidler IJ. Angiogenesis and metastasis. Eur J Cancer. 1996;32A:2451-2460.