Metastasis: A Therapeutic Target for Cancer

Patricia S Steeg; Dan Theodorescu

Disclosures

Nat Clin Pract Oncol. 2008;5(4):206-219. 

In This Article

Metastatic Colonization

It is important to understand that, in the metastatic process, not all the component processes may be of comparable therapeutic benefit. We propose that the final steps in the metastatic process, outgrowth at a distant site (herein termed 'metastatic colonization'), may hold the most therapeutic promise. Molecular assays will be used to reliably segregate patients with cancer into three categories within the foreseeable future: those at very low risk of metastasis, for whom no further treatment can be considered; those at high risk, in whom distant metastases have not been detected; and those with known metastatic disease. The high-risk group might benefit most from therapies that interrupt metastatic colonization. National Cancer Institute Surveillance Epidemiology and End Result (SEER) data indicate that more than 20% of patients with breast, kidney and pancreas cancers; more than 30% of patients with colon, cervix, lung and stomach cancers; and more than 40% of patients with oral cancers comprise this category at the time of initial diagnosis and surgery.[1] At this stage of disease, tumor samples will show evidence of invasion, meaning it is too late to stop this aspect of metastasis from occurring. It is the growth of distant micrometastases to clinically detectable, large, life-threatening metastases (metastatic colonization) that remains incomplete in this setting and, thus, vulnerable to therapeutic intervention.

Metastatic colonization involves reciprocal interactions between tumor cells and a foreign microenvironment. Microenvironments consist of extracellular matrix and normal cells such as fibroblasts, endothelial cells and infiltrating inflammatory cells. Products of these resident and transient cells include growth factors, chemokines, cytokines and proteases. Microenvironments that contain tumor cells are distinct from normal tissues. Differences include the presence of hypoxia that drives angiogenesis and invasion, low pH, low glucose concentrations, alterations in extracellular matrix proteins and liberation of previously bound growth factors.[20,21]

Figure 2 summarizes steps involved in metastatic colonization. Some colonization can occur in the vasculature of the distant organ.[22] Most cells, however, extravasate and then begin colonization within the organ parenchyma. Hematopoietic precursor cells can migrate from the bone marrow to potential sites of metastasis, which they then condition as a 'premetastatic niche' upon which tumor cells then metastasize.[23] Tumor cell growth in a foreign microenvironment may be distinct from that at the primary tumor site, selecting for either autonomous growth or survival and proliferation in response to local or circulatory signals. An excellent example is the context-specific activation of the metastasis suppressors MKK4 and MKK7 in prostate tumor cells disseminated to the lung, but not in the primary tumor.[24] Angiogenesis or the use (co-option) of existing vessels is required for metastatic colonization beyond the limits of oxygen diffusion and also to provide growth factors, metabolites and nutrients. Metastatic colonization might not be a continuous process. Metastatic tumor cells can enter periods of dormancy at any stage of colonization. Both general and tissue-specific pathways influence metastatic colonization.

Metastatic colonization. Metastatic colonization represents a prime window of opportunity to interrupt the metastatic process. Growth in a distant site has similarities and differences to that in the primary tumor site. Steps in metastatic colonization are listed on the left, with potential breaks for dormancy shown. Potential therapeutic strategies are listed to the right.

The relationship between vascularity and metastatic colonization is complex. Hypoxic conditions in tumors stimulate angiogenesis. Angiogenesis is regulated by the balance of angiogenic and antiangiogenic factors. The most widely studied angiogenic factor is VEGF, which induces the proliferation, migration and organization of endothelial cells to form new capillaries. VEGF has several isoforms and is a multifunctional protein. In addition to its role in angiogenesis, VEGF regulates vascular permeability, the proliferation of some tumor cells and the formation of a premetastatic niche.[1] Overexpression of VEGF induced greater numbers of tumor microvessels and increased metastasis.[25] Both the angiogenic and vascular permeability functions can contribute to VEGF regulation of metastatic colonization.[19] Bevacizumab, a humanized monoclonal antibody to VEGF, has clinical activity in combination with cytotoxics in multiple cancer histologies.[26,27]

A gene-expression analysis of primary breast tumors and metastasis samples identified complex differential expression patterns of multiple angiogenic and antiangiogenic factors between tumor sites and between patients.[28] The sprouting of capillaries is not necessarily the ultimate end point of angiogenesis, and signaling involved in pericyte coverage and barrier function may be therapeutically important. In a prostate cancer model system, low levels of the CXCL12 chemokine were found in the primary tumor, which permitted the CXCR4 chemokine receptor to stimulate tumor secretion of the glycolytic enzyme phosphoglycerate kinase 1 (PGK1). PGK1 then induced the secretion of the antiangiogenic factor angiostatin and inhibited production of VEGF and interleukins 6 and 8 (IL-6, IL-8). In metastatic sites such as the bone and liver, however, ample CXCL12 negated this pathway, resulting in less PGK1 and angiostatin and greater angiogenesis factor production, facilitating angiogenesis and colonization.[31] In preclinical models, combinations of antiangiogenesis tactics showed optimum efficacy.[29,30]

Experimental evidence supports several alternative scenarios to the traditional angiogenesis concept. First, both liver and brain metastatic tumor cells can co-opt the existing host vasculature rather than induce angiogenesis.[32,33] In the brain, co-option of the existing vasculature resulted in an intact blood-brain barrier, which could contribute to chemotherapy resistance even in large metastases. Second, recent reports correlated increased vascular density directly with poor tumor growth. This apparent contradiction was explained by the contribution of Dll4, which stimulated angiogenesis to a nonproductive level, resulting in poor perfusion, increased hypoxia and reduced tumor outgrowth.[34,35] Dll4 blockade was demonstrated to reduce the growth of VEGF-inhibitor-resistant experimental tumors.

Tumor dormancy is a well-known clinical phenomenon. Metastases from certain histological types of cancer can occur more than 10 years after successful treatment for the primary tumor. Fatal melanoma was noted in patients who underwent kidney transplant 16 years after surgical cure.[36] Dormant cells are plastic, highly regulated by their interaction with the metastatic microenvironment. Dormant tumor cells have been harvested from mouse tissues, expanded in tissue culture, and reinjected into animals to produce growing primary tumors that spawned dormant micrometastases.[37] Experimental data support two models of dormancy in the metastatic site: first, tumor cells that lack a vascular connection and balance apoptosis and proliferation;[38] and, second, tumor cells that enter a viable but nondividing phase.

Several molecular pathways operative in metastatic dormancy have been identified in model systems. The myc, integrin, Bcl-Xl antiapoptotic and p38 stress pathways have been shown to influence tumor dormancy. KISS1, the first secreted metastasis suppressor, induced and maintained the dormancy of melanoma cells.[15] Another postulated antidormancy strategy involves thrombin, which can impact proliferation through the protease-activated receptor.[39]

Metastatic colonization might affect drug responsiveness, and vice versa. Tumor cells that grow in the bone-marrow microenvironment stimulate bone marrow production of 'survival' factors such as IL-6 and insulin-like growth factors (IGFs), which protect the tumor cells from cytotoxic drugs.[40] Similarly, bone marrow cells produce osteoprotegerin (OPG) that protects bone metastatic breast-cancer cells from apoptosis induced by TRAIL.[41] Conversely, drugs can modify the interaction between tumor cell and microenvironment, which can impact metastatic colonization. Analysis of biopsy samples from a clinical trial of preoperative thalidomide in patients with prostate cancer revealed changes in protein expression in the metastatic microenvironment (including tumor cells, endothelial cells and stromal cells) that favored a less metastatic state.[42] Further mechanistic advances in this research area are vital to therapeutic advances.

This Review will focus on bone metastasis as an example of progress and challenges in site-specific metastatic colonization. The delineation of bone metastatic signaling pathways was fostered by the development of in vivo metastasis assays. These model systems have limitations and require validation to confirm that the model is actually representative of molecular events found in the human disease. In addition, a single model cannot represent the heterogeneity found in human disease.[1,43,44,45]

Normal bone remodeling is a dynamic balance of osteoblastic (bone forming) and osteolytic (bone degrading) activity. Breast and prostate cancers and multiple myeloma most frequently produce bone metastases. A pathologic analysis of resected breast cancer osteoclastic lesions showed that 72% of the lesions were a mixture of osteoclastic and osteoblastic, while the remaining 28% were purely osteolytic. Osteolytic metastases can evolve through a series of tumor-microenvironment interactions known as the 'vicious cycle' (Figure 3).[46] Tumor cells secrete factors that stimulate osteoblast activation of osteoclasts. Activated osteoclasts degrade the bone matrix and release factors that stimulate tumor cells. Many other proteins also participate in osteolytic bone metastasis.[1,46] Overexpression of the chemokine CXCR4 increased bone metastasis in a model system and this activity was enhanced by coordinate expression of IL-11, osteopontin and/or connective tissue growth factor.[47]

The bone metastasis 'vicious' cycle with recent updates. Metastatic tumor cells interact with the bone microenvironment to facilitate osteolytic colonization. Tumor cells secrete PTHrP, which stimulates osteoblasts (bone-forming cells) to produce both a membrane-bound RANKL and OPG, a soluble decoy receptor for RANKL. It is the ratio of RANKL to OPG that determines osteoclast (bone-degrading cell) activation, through its receptor for RANKL. Activated osteoclasts degrade the bone matrix, releasing into the local microenvironment embedded growth factors including TGF-ß. TGF-ß stimulates tumor-cell PTHrP production, renewing the cycle. Recently, RANKL was reported to stimulate tumor cells as well as osteoclasts, inducing motility that could spread bone colonization.[92] Tumor cells also produce GM-CSF, a hematopoietic growth factor used in cancer therapy. GM-CSF, in turn, stimulated bone marrow cells to produce more osteoclasts, amplifying the cycle.[93] Abbreviations: GM-CSF, granulocyte macrophage-colony stimulating factor; OBL, osteoblast-like cells; OCL, osteoclast-like cells; OPG, osteoprotegerin; PTHrP, parathyroid-hormone-related protein; RANKL, receptor activator of NF-κB ligand; TGF-ß, transforming growth factor-ß.

An autopsy study of bone metastases from androgen-independent prostate cancer found tumor cells at an average of 14 sites within the marrow. Heterogeneity was observed between metastases from a single patient in the fraction of tumor cells that were positive for prostate-specific antigen (PSA) or chromogranin A (CGA).[48] No single vicious cycle has been mapped out for osteoblastic lesions. Debate continues on the relative and temporal role of osteolysis. Osteoblastic activity may be increased by PSA cleavage of parathyroid-hormone-related protein, breaking the osteolytic vicious cycle.[46] In addition, growth factors such as transforming growth factor-ß (TGF-ß), fibroblast growth factor (FGF), IGF and endothelin-1 (ET-1) activate the RUNX2 transcription factor in osteoblasts, which results in osteoblastic gene expression.[49]

Numerous compounds have been tested in model systems for potential activity against bone metastases (Supplementary Table 2). These include compounds that are active against confirmed bone metastasis targets (OPG, TGF-ß, and colony-stimulating factor), in addition to others that target tumor cells more generally. Most model systems involve injection of tumor cells into the circulation; alternatively, tumor cells can be directly implanted into bone, which eliminates the extravasation and initial micrometastatic colonization events. Few studies permit a metastasis to grow before asking whether the compound will have an antitumor effect, comparable to early clinical trials conducted in the metastatic setting. Even fewer studies report pharmacokinetic data. One reason why so many mice, but not humans, are cured may be that mice are given compounds at levels unachievable in humans.

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