Current Cancer Drug Targets - Volume 6, Issue 2, 2006

Anchorage-independent growth, anoikis resistance, and most steps of metastasis formation are integrin-mediated or -dependent processes, which are characteristics of malignant tumor cells. Acting as oncogenes or tumor suppressor genes, integrins may be involved in the oncogenic transformation of normal cells and their growth into a primary tumor node. During tumorigenesis, a switch of integrin expression can be observed, inasmuch as growth-promoting and growth-attenuating integrins are up- and down-regulated, respectively. ECM-ligand binding to an integrin initiates signals, which eradiating from the integrins are transmitted via different yet interconnecting pathways and elicit various cell functions, such as morphological changes, adhesion, migration and gene activation. Any of these functions takes part in the metastatic cascade of tumor progression, such as epithelial-to-mesenchymal transition of carcinoma cells, tumor cell contact with the basement membrane, invasion into neighboring tissues as well as production and activation of ECM-degrading MMPs. Besides their direct involvement in tumor progression as cell surface molecules on tumor cells, integrins in normal cells surrounding a tumor, e.g. endothelial cells, can also determine various cancer characteristics, such as tumor-induced neoangiogenesis and immune resistance. Hence, integrins are relevant pharmacological targets in tumor biology. Spurred by the recent success to generate pharmaceutical mimetics of RGD-dependent integrins and by the integrin's easy accessibility on the cell surface, the hope is rising that also RGD-independent integrins, such as the collagen- and laminin-binding integrins, can be pharmacologically manipulated to fight integrin-dependent functions of cancer cells, which are necessary and at least partially specific for their proliferation and progression

A family of latent cytoplasmic transcription factors, signal transducers and activators of transcription (STATs), mediates the responsiveness of cells to several cytokines and growth factors. Although mutations of STATs have not been described in human tumors, the activity of several members of the family, such as STAT1, STAT3 and STAT5, is deregulated in a variety of human tumors. STAT3 and STAT5 acquire oncogenic potential through constitutive phosphorylation on tyrosine, and their activity has been shown to be required to sustain a transformed phenotype. Disruption of STAT3 and STAT5 signaling in transformed cells therefore represents an excellent opportunity for targeted cancer therapy. In contrast to STAT3 and STAT5, STAT1 negatively regulates cell proliferation and angiogenesis and thereby inhibits tumor formation. Consistent with its tumor suppressive properties, STAT1 and its downstream targets have been shown to be reduced in a variety of human tumors and STAT1 deficient mice are highly susceptible to tumor formation. In recent years we have gained mechanistic understanding of the pathways whereby STATs convey signals from the cytoplasm to the nucleus. In addition, several endogenous regulators of the JAK/STAT pathway have been described - and their mechanism of action revealed - that profoundly affect signaling by STATs. Both should greatly facilitate the design of drugs with potential to modulate STAT signaling and to restore the homeostasis in tissues where STATs have gone awry.

About one third of osteosarcoma patients develop lung metastasis refractory to chemotherapy. Recent studies indicate that biological response modifiers activating the patient's immune system may help controlling minimal residual disease via pathways distinct from those used by cytotoxic drugs, and therefore prove effective against tumor resistance. Muramyl tripeptide phosphatidylethanolamine (MTP-PE) is a synthetic lipophilic glycopeptide capable of activating monocytes and macrophages to a tumoricidal state. When intercalated in multilamellar liposomes (L-MTP-PE) and injected intravenously, it targets lung, liver, and spleen macrophages. Therapeutic activity of L-MTP-PE was demonstrated in several preclinical models of experimental lung metastasis and in clinical trials in dogs with osteosarcoma. Although macrophage activation was shown to be directly involved in the in vivo anti-metastatic activity of this molecule, cytokine and chemokine secretion by activated macrophages could induce recruitment and stimulation of other immune cells, which may in turn indirectly contribute to the anti-tumor effect. L-MTP-PE has undergone clinical development in humans. In early trials, most side effects of L-MTP-PE were minimal. L-MTP-PE showed signs of efficacy in treatment of patients with recurrent osteosarcoma and the encouraging results from phase II studies led to a phase III trial conducted by the Children's Oncology Group in patients with newly diagnosed high-grade osteosarcoma. Patients were treated with or without L-MTP-PE in combination with multi-drug chemotherapy in adjuvant setting; significantly higher overall survival and disease-free survival were observed in the group receiving L-MTP-PE.

Epidemiological studies have revealed an inverse correlation between the intake of cruciferous vegetables and the risk of certain types of cancer. In animal studies, results suggest that the anti-cancerous effect of cruciferous vegetables is due to isothiocyanates that exist as thioglucoside conjugates in a variety of edible plants, including broccoli cabbage for example. Among isothiocyanates (ITC), Sulforaphane (SF) has received a great deal of interest due to its potent anti-tumoral properties in carcinogen-treated animals. The molecular pathways mediating the effects of SF have not been fully elucidated. However, many studies have shown that SF (as well as other ITCs) can induce phase II drug metabolizing enzymes in vitro as well as in animals. This commonly occurs via the activation of a basic leucine zipper transcription factor, Nrf2. In addition, accumulating evidence now indicates that SF can inhibit the proliferation of cancer cells in culture through the induction of cell cycle arrest via the regulation of cell cycle protein levels and/or cyclindependent kinase activity, tubulin polymerization and histone acetylation. Furthermore, ITCs have been shown to induce apoptotic cell death via a P53 dependent or independent pathway. Here, it is proposed to review the different intracellular targets involved in the in vitro effects of SF in various cancer cell lines. The relationship will then be discussed that exists between the various cell signaling pathways involved in this effect, and finally, the important aspects will be identified that must be addressed to fully understand the exact mechanism of action of SF.

Telomeres are located at the ends of eukaryotic chromosomes. Human telomerase, a cellular reverse transcriptase, is a ribonucleoprotein enzyme that catalyzes the synthesis and extension of telomeric DNA. It is composed of at least, a template RNA component (hTR; human Telomerase RNA) and a catalytic subunit, the telomerase reverse transcriptase (hTERT). The absence of telomerase is associated with telomere shortening and aging of somatic cells, while high telomerase activity is observed in over 85% of human cancer cells, strongly indicating its key role during tumorigenesis. Several details regarding telomere structure and telomerase regulation have already been elucidated, providing new targets for therapeutic exploitation. Further support for anti-telomerase approaches comes from recent studies indicating that telomerase is endowed of additional functions in the control of growth and survival of tumor cells that do not depend only on the ability of this enzyme to maintain telomere length. This observation suggests that inhibiting telomerase or its synthesis may have additional anti-proliferative and apoptosis inducing effect, independently of the reduction of telomere length during cell divisions. This article reviews the basic information about the biology of telomeres and telomerase and attempts to present various approaches that are currently under investigation to inhibit its expression and its activity. We summarize herein distinct anti-telomerase approaches like antisense strategies, reverse transcriptase inhibitors, and G-quadruplex interacting agents, and also review molecules targeting hTERT expression, such as retinoids and evaluate them for their therapeutic potential. "They conceive a certain theory, and everything has to fit into that theory. If one little fact will not fit it, they throw it aside. But it is always the facts that will not fit in that are significant". "Death on the Nile". Agatha Christie.