TGF-β antagonists: Why suppress a tumor suppressor? (original) (raw)

The articles in this issue of the JCI (7, 8) have pushed the story two steps further, firstly by applying soluble Fc:TβRII as an injectable drug to prove efficacy in suppression of breast tumor metastasis in vivo (7), and secondly by screening for any adverse effects on the mice after lifetime exposure to high-level circulating Fc:TβRII (8). Muraoka et al. (7), using the MMTV-PyV mT transgenic model of mammary tumorigenesis, show that twice-weekly intraperitoneal injection of Fc:TβRII reduces lung metastasis tenfold. Fc:TβRII treatment also inhibits metastasis of two metastatic mammary cell lines. In all three cases, Fc:TβRII has no effect on proliferative rate of the primary tumor cells. Yang et al. (8) take a different approach, focusing on possible adverse effects in transgenic mice that stably express soluble Fc:TβRII. Circulating Fc:TβRII, which is found at about 1 mg/ml in the blood, not only reduces metastasis formation of melanoma cells injected into the tail vein of the mice but also reduces metastasis to the lung from endogenous mammary tumors that arise when the mice are crossed onto the MMTV-Neu transgenic model of mammary carcinogenesis. Both groups find that Fc:TβRII leads to no changes in tumor latency, yield, or size.

Taken together, the two papers (7, 8) show that soluble Fc:TβRII is efficacious in reducing tumor metastasis, whether delivered genetically from within the neoplastic cell or administered as an injectable circulating drug. Both groups also addressed the mechanisms of action of Fc:TβRII in attenuating metastatic spread. In the MMTV-PyV mT model, Muraoka et al. (7) specifically exclude an effect on TGF-β–induced angiogenesis. In their model, Fc:TβRII appears to decrease tumor cell intravasation and/or decrease survival of tumor cells in the circulation, since the number of circulating tumor cells is lower in the Fc:TβRII-treated mice than in controls (7). In support of this mechanism, Smad2 activation has recently been shown to drive tumor cell extravasation in a skin tumor model (6). Consistent with an effect on tumor intravasation, the Fc:TβRII-treated mammary tumor cells are altered toward a more differentiated, less motile/migratory phenotype than is seen in untreated tumor cells. Production of active matrix metalloproteinase 2 (MMP2) and MMP9, proteases thought to be important in tumor invasion, migration, and intravasation, is diminished and apoptosis is elevated in response to Fc:TβRII (7).

In the injectable melanoma model, Yang et al. (8) argue, the effect of Fc:TβRII on metastasis is likely indirect, possibly including decreased angiogenesis and/or elevated immunosuppression. Although me-tastasis is diminished in the Fc:TβRII transgenic mice following tail vein injections of melanoma cells, the initial appearance of micro-metastases is no different from that seen in wild-type mice. Since TGF-β has multiple actions that can drive tumor metastasis, the exact mechanisms involved are probably context-specific, de-pending on the tumor type, genetic constitution, and the exact stage of carcinogenesis of the tumor. However, the exciting take-home message is that soluble TGF-β antagonists can significantly decrease metastasis in models of breast cancer and melanoma, as previously suggested for thymoma (13), glioma (14), and pancreatic (15) and mammary carcinoma (16).

The complete absence of TGF-β1 in mice leads to death resulting from uncontrolled systemic inflammation, and even a T cell–specific deficit in TGF-β causes lethal immune system defects (discussed in ref. 8). Therefore, the apparent absence of side effects, even after lifetime exposure to approximately 1 mg/ml circulating Fc:TβRII (8), is particularly encouraging. No immune phenotype was seen in the study of Yang et al. (8), apart from a minimal increase in memory T cells, and a nonsignificant increase in lymphocytic infiltration into organs of aged mice. The authors employed several techniques to demonstrate that Fc:TβRII at this level does not completely block all TGF-β1 bioactivity in vivo (8). Indeed, although circulating TGF-β levels are reduced in the Fc:TβRII transgenics to probably ≤10% of the wild-type level, these animals still thrive in the laboratory. It would be interesting to examine how the mice fare when challenged with other environmental hazards such as foreign antigens and pathogens.

Also heartening is the finding by both groups that soluble Fc:TβRII had no tumor-promoting action in vivo. Conversely, mice in which TGF-β activity is diminished globally, as a result of hemizygosity for Tgfb1, are tumor-prone (10, 11), as are animals in which this factor is ablated in a tissue-specific manner using dominant negative (DN) TβRII. The basis of this discrepancy is uncertain, but it may be that Fc:TβRII preferentially targets circulating TGF-β1 because it is too bulky to gain access to the more functionally important TGF-β tightly bound to the cell surface or ECM. In addition, different thresholds of TGF-β activity required for the growth-suppressing and the metastasis-promoting effects of TGF-β could help account for the tumor inci-dence seen in DN-TβRII transgenic strains. Growth inhibition, for example, is more sensitive than other TGF-β responses to decreases in the level of TβRII (3, 17). Moreover, recent studies in a skin carcinogene-sis model do indeed show that a high threshold of Smad2 activation must be surpassed in order to drive metastatic spread (6).

Nevertheless, since Tgfb1+/– animals have an increased incidence of chemically induced tumorigenesis (10), one might expect a similar phenotype in Fc:TβRII mice (8), but this is not the case. The explanation probably lies in the different models used. The tumors and cell lines studied by Muraoka et al. (7) have already lost growth sensitivity to TGF-β, as assessed by BrdU incorporation, so tumor-suppressive effects of TGF-β would not be expected. The TGF-β growth sensitivity of MMTV-Neu tumor cells has not been studied, but transfection of normal differentiated thyroid cells with ErbB2 (Neu) attenuates the growth-inhibitory response to TGF-β, suggesting that Neu does indeed attenuate growth sensitivity to TGF-β (18). In this context, chemical carcinogenesis studies on Fc:TβRII transgenic mice are warranted to uncover any tumor-promoting effects of Fc:TβRII, especially in view of the fact that a soluble TβRII transfected into a hepatoma cell line has been shown to promote tumor development (19).

Despite these reservations, Fc:TRII clearly is highly efficacious in reducing metastasis and is of exceptionally low toxicity in mice. Indeed, many drugs for treatment of both malignant and nonmalignant conditions, such as cyclosporin, have tumor-promoting activity (12), and most cancer drugs show general cytotoxicity levels orders of magnitude higher than does this soluble TβRII receptor.