Cutting edge: systemic inhibition of angiogenesis underlies resistance to tumors during acute toxoplasmosis.

The ability of various infections to suppress neoplastic growth has been well documented. This phenomenon has been traditionally attributed to infection-induced concomitant, cell-mediated antitumor immunity. We found that infection with Toxoplasma gondii effectively blocked neoplastic growth of a nonimmunogenic B16.F10 melanoma. Moreover, this effect was independent of cytotoxic T or NK cells, production of NO by macrophages, or the function of the cytokines IL-12 and TNF-alpha. These findings suggested that antitumor cytotoxicity was not the primary mechanism of resistance. However, infection was accompanied by strong, systemic suppression of angiogenesis, both in a model system and inside the nascent tumor. This suppression resulted in severe hypoxia and avascular necrosis that are incompatible with progressive neoplastic growth. Our results identify the suppression of tumor neovascularization as a novel mechanism critical for infection-induced resistance to tumors.

Activation of the myc oncoprotein leads to increased turnover of thrombospondin-1 mRNA.

The Myc oncoprotein is implicated in transcriptional regulation of a variety of genes pertaining to cell cycle and neoplastic transformation. Examples of both positive and negative regulation have been reported that involve E-box and initiator (Inr) promoter elements, respectively. In both cases, Myc is thought to induce changes in transcription initiation. We have previously shown that overexpression of Myc causes down-regulation of the thrombospondin-1 (tsp-1) gene, an important negative modulator of tumor angiogenesis. In this study, we demonstrate that Myc in combination with Max can bind, albeit with low affinity, to an E-box-like element in the tsp-1 promoter. However, the 2.7 kb DNA segment containing both this non-canonical E-box and an Inr-like sequence does not constitute a Myc-responsive element in a transient expression system. Furthermore, Myc does not significantly affect the rate of initiation or elongation of the tsp-1 mRNA. Thus, in this instance Myc does not act as a canonical transcription factor. Instead, as demonstrated by blocking de novo RNA synthesis, down-regulation of the tsp-1 gene by Myc occurs through increased mRNA turnover. To our knowledge, this is the first example of gene regulation by Myc that involves mRNA destabilization.

An in vivo function for the transforming Myc protein: elicitation of the angiogenic phenotype.

The ability of neoplastic cells to recruit blood vasculature is crucial to their survival in the host organism. However, the evidence linking dominant oncogenes to the angiogenic switch remains incomplete. We demonstrate here that Myc, an oncoprotein implicated in many human malignancies, stimulates neovascularization. As an experimental model, we used Rat-1A fibroblasts that form vascular tumors upon transformation by Myc in immunocompromised mice. Our previous work and the use of neutralizing antibodies reveal that in these cells, the angiogenic switch is achieved via down-modulation of thrombospondin-1, a secreted inhibitor of angiogenesis, whereas the levels of vascular endothelial growth factor, a major activator of angiogenesis, remain high and unaffected by Myc. Consistent with this finding, overexpression of Myc confers upon the conditioned media the ability to promote migration of adjacent endothelial cells in vitro and corneal neovascularization in vivo. Furthermore, mobilization of estrogen-dependent Myc in vivo with the appropriate steroid provokes neovascularization of cell implants embedded in Matrigel. These data suggest that Myc is fully competent to trigger the angiogenic switch in vivo and that secondary events may not be required for neovascularization of Myc-induced tumors.