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The extracellular matrix is a crucial component in determining cell fate. Fibrillar collagen in its native form inhibits cell proliferation, whereas in its monomeric form it stimulates proliferation. The observation of elevated levels of p27(KIP1) in cells plated in the presence of fibrillar collagen has led to the assumption that this kinase inhibitor was responsible for cell cycle arrest on fibrillar collagen. Here we provide evidence that p15(INK4b), rather than p27(KIP1), is the cyclin-dependent kinase inhibitor responsible for G0/G1 arrest of human melanoma cells grown on fibrillar collagen. Additionally, we demonstrate that fibrillar collagen can also arrest cells at the G2 phase, which is mediated in part by p21(CIP1). Our data, in addition to identifying cyclin-dependent kinase inhibitors important in cell cycle arrest mediated by fibrillar collagen, demonstrate the complexity of cell cycle regulation and indicate that modulating a single cyclin-dependent kinase inhibitor does not disrupt cell proliferation in the presence of fibrillar collagen.

Neuroblastoma is the second most common solid tumor in children that is metastatic in 70% of patients at the time of diagnosis. The ability of neuroblastoma cells to colonize distant organs like the bone marrow and the bone is the result of close interactions between tumor cells and the microenvironment. Significant progress has been recently made in our understanding of the mechanisms that promote the colonization and invasion of the bone by neuroblastoma cells and these mechanisms are reviewed in this article. How this understanding is now allowing us to test new therapeutic agents specifically targeted at interfering with neuroblastoma metastasis is then discussed.

In the 1980's, as the importance of matrix metalloproteinases (MMPs) in cancer progression was discovered, it was recognized that in most tumors these proteases were abundantly and sometimes exclusively expressed not by tumor cells, but by normal host-derived cells like fibroblasts, vascular endothelial cells, myofibroblasts, pericytes or inflammatory cells that contribute to the tumor microenvironment. Later experiments in mice deficient in specific MMPs revealed that host-derived MMPs play a critical role not only in tumor cell invasion, but also in carcinogenesis, angiogenesis, vasculogenesis and metastasis. Tumor cells secrete many factors, cytokines and chemokines that directly or indirectly increase the expression of these MMPs in the tumor microenvironment where they exert extracellular matrix (ECM) degrading and sheddase activities. The knowledge of the complex role that stromal-derived MMPs play in the interaction between tumor cells and stromal cells should allow us to consider specific windows in cancer treatment when MMP inhibition could have a valuable therapeutic effect.

Bone is the second most common site of metastasis in neuroblastoma. Over the last several years, our understanding of the mechanism of bone metastasis in neuroblastoma has significantly improved. Like breast cancer and myeloma, neuroblastoma cells activate osteoclasts to form osteolytic lesions. Activation occurs via the receptor activator of NFkappaB ligand (RANKL) or in the absence of RANKL via activation of bone marrow mesenchymal stem cells and stimulation by these cells of the expression of IL-6, a potent osteoclast activating factor. Several targets for therapeutic intervention can now be identified. Inhibition of osteoclast activation by bisphosphonates has already shown to be effective in preclinical models of neuroblastoma bone metastasis and should now be tested in phase I clinical studies. Inhibition of RANKL and IL-6 are other potential targets that require preclinical studies before being tested in patients. This article provides a review of our current understanding of the mechanisms involved in bone metastasis in neuroblastoma and discusses how this knowledge is leading to the identification of new targets for therapeutic intervention.

Rodent models provide an important means of assessing antitumor activity vs toxicity for new cancer therapies. Tumors are often grown subcutaneously on the flank or back of animals, allowing accurate serial determination of tumor volume with calipers by measuring the tumors in three dimensions. The advantages of assessing tumor volume in subcutaneous tumors must be balanced against the potential artifacts induced by growth of tumor cells in subcutaneous tissue. Various orthotopic models have been developed. However, they are more labor-intensive and generally do not allow accurate assessment of tumor growth and/or response unless investigators have access to small animal cross-sectional imaging. Use of small-animal magnetic resonance imaging (MRI) allows one to assess the growth and response of intracavitary tumors, but the cost and labor-intensive nature of MRI limits its use in drug testing. Another approach to intracavitary solid tumor models is the intravenous injection of tumor cells, which can produce lung, liver, or bone metastases (depending on the cell line used), whereas direct injection of tumor cells into the femur or tibia of mice can cause local growth in bone. Progression of both lung metastases and bone lesions can be assessed by small-animal analog X-ray techniques that are more easily available and less labor-intensive to use, and are proving useful for selected therapeutic and biological studies.

Neuroblastoma is the second most common solid tumor in childhood and frequently metastasizes to the bone marrow and the bone matrix. The mechanism involved in bone metastasis and destruction in neuroblastoma is poorly understood. Using a model of bone invasion in immunodeficient mice, we demonstrated that neuroblastoma cells recruited osteoclasts to generate osteolytic lesions and invade the bone matrix. In further support of a contributory role for osteoclasts in neuroblastoma bone invasion, we demonstrated that treatment with the bisphosphonate compound, ibandronate, significantly delayed the progression of osteolytic lesions. The data suggest that bisphosphonates may be clinically effective in the treatment of bone metastases in neuroblastoma.