Abstract
Hypoxia is a common feature of solid tumors and is associated with their malignant phenotype. The transcription factor hypoxia-inducible factor-1 (HIF-1) is a major regulator of adaptation to hypoxia and is implicated in the malignant progression of cancers. Here, we studied whether hypoxia and HIF-1 expression contribute to the development of bone metastases using a well-characterized animal model of bone metastasis in MDA-MB-231 human breast cancer cells. To study the role of hypoxia in bone metastases, we tested the effects of the fusion protein (TOP3), the oxygen-dependent degradation domain of HIF-1alpha fused with HIV-TAT, and procaspase-3. TOP3 selectively induced apoptosis in hypoxic tumor cells in vitro and significantly reduced bone metastases in vivo. We next examined the role of HIF-1 in bone metastases by establishing MDA-MB-231 cells overexpressing constitutively active or dominant-negative HIF-1alpha (MDA/CA-HIF or MDA/DN-HIF, respectively). Bone metastases of MDA/CA-HIF were significantly increased with elevated number of CD31-positive blood vessels. In contrast, bone metastases were significantly reduced in MDA/DN-HIF. Because the progression of osteolytic bone metastases is due in part to the imbalance between bone formation and bone resorption, we examined the effects of hypoxia and HIF-1 on the differentiation of osteoblasts and osteoclasts. Hypoxia and CA-HIF overexpression markedly inhibited osteoblastic differentiation, whereas hypoxia increased osteoclast-like cell formation. In conclusion, these results suggest that tumor-associated hypoxia and HIF-1 expression promote the progression of bone metastases in breast cancer. Our results also suggest that hypoxia and HIF-1 lead to the development of osteolytic bone metastases by suppressing osteoblast differentiation and promoting osteoclastogenesis.
Highlights
Hypoxia is a common feature of solid tumors, which is caused by reduced or inadequate oxygen supply [1,2,3]
Because pimonidazole accumulates in the cells that have oxygen concentration
Consistent with the previous report [12], our in vitro study using TAT-protein transduction domain (PTD)-oxygen-dependent degradation domain (ODD)-h-Gal showed that h-Gal activity was markedly increased in MDA-MB-231 cells cultured under the hypoxic condition compared with the cells cultured under normoxia, verifying the www.aacrjournals.org oxygen-dependent stability of TAT-PTD-ODD fusion proteins in MDAMB-231 cells (Supplementary Fig. S1)
Summary
Hypoxia is a common feature of solid tumors, which is caused by reduced or inadequate oxygen supply [1,2,3]. Doi:10.1158/0008-5472.CAN-06-2355 tumor hypoxia is associated with malignant biological phenotype, including enhanced invasiveness, angiogenesis, and distant metastasis [1,2,3]. More than 60 putative direct HIF-1 target genes, including glycolytic enzymes, glucose transporter, angiogenic factors, growth factors, enzymes, and proteins involved in tumor invasiveness and metastasis, and apoptosis resistance–related proteins, have been identified [2, 5]. HIF-1a possesses a unique oxygen-dependent degradation domain (ODD) that controls protein stability [2, 5, 6]. Clinicopathologic studies have shown that the expression level of HIF-1a in breast cancer increases as the pathologic stage elevates [7]. Increased levels of HIF1a are associated with increased proliferation, metastasis, and poor prognosis in breast cancer patients [8,9,10]
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