Human breast tumors override the antiangiogenic effect of stromal thrombospondin-1 in vivo.

The antiangiogenic extracellular matrix protein thrombospondin-1 (TSP-1) inhibits tumor growth and metastasis in animals. However, the clinical relevance of such findings are equivocal as increased stromal TSP-1 expression has been associated with either good or poor prognosis. In an effort to obtain a more integrated understanding of the role of TSP-1 in breast cancer, we first used a breast tumorigenesis model in which tumor-associated stromal fibroblasts were engineered to produce high levels of TSP-1. We demonstrate here that stromal TSP-1 delayed human MDA-MB-231/B02 breast tumor growth. However, this delay in MDA-MB-231/B02 tumor growth upon exposure to TSP-1 was associated with an increased vascular endothelial growth factor (VEGF) expression in tumor cells themselves, leading to a tumor growth rate comparable to that of tumors whose fibroblasts did not overproduce TSP-1. Clinical evidence also suggested that primary breast carcinomas have adapted to escape the effects of stromal TSP-1. TSP-1 was found to be expressed in the stroma of human breast carcinomas where, although its level correlated with decreased vascularization, it was unexpectedly associated with a reduction of relapse-free survival. In metastatic axillary lymph nodes, tumor cells expressed high levels of VEGF and TSP-1 expression were no longer associated with a decreased vascularization. Overall, these results suggest that a resistance may develop early in human breast cancers as a result of high in situ exposure to stromal TSP-1, leading to disease progression.

In vitro and in vivo antitumor effects of bisphosphonates.

Bisphosphonates are powerful inhibitors of osteoclast-mediated bone resorption. They are currently used in the palliative treatment of bone metastases. However, bisphosphonates do not only act on osteoclasts. There is now extensive in vitro preclinical evidence that bisphosphonates can act on tumor cells: they inhibit tumor cell adhesion to mineralized bone as well as tumor cell invasion and proliferation. Bisphosphonates induce also tumor cell apoptosis and stimulate gammadelta T cell cytotoxicity against tumor cells. In vivo, bisphosphonates inhibit bone metastasis formation and reduce skeletal tumor burden. This may reflect direct antitumor effects and indirect effects via inhibition of bone resorption. In addition, bisphosphonates inhibit experimental angiogenesis in vitro and in vivo. Understanding the molecular mechanisms through which bisphosphonates act on tumor and endothelial cells will be undoubtedly an important task in the future. It will allow the design of clinical trials to investigate whether the antitumor activity of bisphosphonates can be realized in the clinical setting.

Bisphosphonates inhibit angiogenesis in vitro and testosterone-stimulated vascular regrowth in the ventral prostate in castrated rats.

Bisphosphonates (BPs) are used currently in the treatment of patients with bone metastases because these compounds inhibit bone resorption. We examined here the effects of BPs on inhibition of endothelial cell functions in vitro and in vivo. Treatment of endothelial cells with BPs (clodronate, risedronate, ibandronate, and zoledronic acid) reduced proliferation, induced apoptosis, and decreased capillary-like tube formation in vitro. Quantification of blood vessels in bone biopsy specimens from patients with Paget's disease before and after clodronate treatment showed a 40% reduction of the vascularization after BP treatment. However, such a decreased vascularity could be secondary to a reduction of bone resorption. Therefore, the tissue distribution of [14C]BPs in male rats was examined to develop an angiogenesis model in a noncalcified tissue where BPs could accumulate. [14C]BPs (zoledronic acid, ibandronate, and clodronate) not only accumulated in bone but also transiently accumulated in the prostate. The effects of BPs on testosterone-induced revascularization of the prostate gland in castrated rats were then studied. Testosterone in combination with ibandronate or zoledronic acid induced a 17-35% reduction of the prostate weight compared with castrated rats treated with testosterone alone. Blood vessel immunostaining on prostate tissue sections revealed that both ibandronate and zoledronic acid induced a 50% reduction of the revascularization of the prostate gland. Moreover, zoledronic acid did not alter testosterone-induced activity of a luciferase gene reporter construct transfected in androgen-dependent prostatic cells, indicating that this BP did not directly interfere with testosterone. In conclusion, BPs have in vivo antiangiogenic properties, which could be of relevance to improve therapy and prevention of bone metastasis. In addition, our results extend the potential clinical use of BPs to patients with early prostate cancer.