Tumor dormancy: potential therapeutic target in tumor recurrence and metastasis prevention.

In past decades, cancer patient survival has been improved with earlier detection and advancements in therapy. However, many patients who exhibit no clinical symptoms after frontline therapy subsequently suffer, often many years later, aggressive tumor recurrence. Cancer recurrence represents a critical clinical challenge in effectively treating malignancies and for patients' quality of life. Tumor cell dormancy may help to explain treatment resistance and recurrence or metastatic reactivation. Understanding the dormant stage of tumor cells may help in discovering ways to maintain the dormant state or permanently eliminate dormant residual disseminated tumor cells. Over the past decade, numerous studies indicate that various mechanisms of tumor dormancy exist, including cellular dormancy (quiescence), angiogenic dormancy, and immunologic dormancy. In this short review, we summarize recent experimental and clinical evidence for these three mechanisms and other possible tumor microenvironment mechanisms that may influence tumor dormancy.

Caspase inhibition blocks cell death and enhances mitophagy but fails to promote T-cell lymphoma.

Caspase-9 is a component of the apoptosome that mediates cell death following release of cytochrome c from mitochondria. Inhibition of Caspase-9 with a dominant negative construct (Casp9DN) blocks apoptosome function, promotes viability and has been implicated in carcinogenesis. Inhibition of the apoptosome in vitro impairs mitochondrial function and promotes mitophagy. To examine whether inhibition of the apoptosome would enhance mitophagy and promote oncogenesis in vivo, transgenic mice were generated that express Casp9DN in the T cell lineage. The effects of Casp9DN on thymocyte viability, mitophagy and thymic tumor formation were examined. In primary thymocytes, Casp9DN delayed dexamethasone (Dex)-induced cell death, altered mitochondrial structure, and decreased oxidant production. Transmission electron microscopy (TEM) revealed that inhibition of the apoptosome resulted in structurally abnormal mitochondria that in some cases were engulfed by double-membrane structures resembling autophagosomes. Consistent with mitochondria being engulfed by autophagosomes (mitophagy), confocal microscopy showed colocalization of LC3-GFP and mitochondria. However, Casp9DN did not significantly accelerate T-cell lymphoma alone, or in combination with Lck-Bax38/1, or with Beclin 1+/- mice, two tumor-prone strains in which altered mitochondrial function has been implicated in promoting tumor development. In addition, heterozygous disruption of Beclin 1 had no effect on T-cell lymphoma formation in Lck-Bax38/1 mice. Further studies showed that Beclin 1 levels had no effect on Casp9DN-induced loss of mitochondrial function. These results demonstrate that neither inhibition of apoptosome function nor Beclin 1 haploinsufficiency accelerate T-cell lymphoma development in mice.

Mechanisms of ascorbate-induced cytotoxicity in pancreatic cancer.

PURPOSE: Pharmacologic concentrations of ascorbate may be effective in cancer therapeutics. We hypothesized that ascorbate concentrations achievable with i.v. dosing would be cytotoxic in pancreatic cancer for which the 5-year survival is <3%. EXPERIMENTAL DESIGN: Pancreatic cancer cell lines were treated with ascorbate (0, 5, or 10 mmol/L) for 1 hour, then viability and clonogenic survival were determined. Pancreatic tumor cells were delivered s.c. into the flank region of nude mice and allowed to grow at which time they were randomized to receive either ascorbate (4 g/kg) or osmotically equivalent saline (1 mol/L) i.p. for 2 weeks. RESULTS: There was a time- and dose-dependent increase in measured H(2)O(2) production with increased concentrations of ascorbate. Ascorbate decreased viability in all pancreatic cancer cell lines but had no effect on an immortalized pancreatic ductal epithelial cell line. Ascorbate decreased clonogenic survival of the pancreatic cancer cell lines, which was reversed by treatment of cells with scavengers of H(2)O(2). Treatment with ascorbate induced a caspase-independent cell death that was associated with autophagy. In vivo, treatment with ascorbate inhibited tumor growth and prolonged survival. CONCLUSIONS: These results show that pharmacologic doses of ascorbate, easily achievable in humans, may have potential for therapy in pancreatic cancer.