TGF-β signaling blockade inhibits PTHrP secretion by breast cancer cells and bone metastases development

Abstract

Breast cancer frequently metastasizes to the skeleton, and the associated bone destruction is mediated by the osteoclast. Growth factors, including transforming growth factor-β (TGF-β), released from bone matrix by the action of osteoclasts, may foster metastatic growth. Because TGF-β inhibits growth of epithelial cells, and carcinoma cells are often defective in TGF-β responses, any role of TGF-β in metastasis is likely to be mediated by effects on the surrounding normal tissue. However, we present evidence that TGF-β promotes breast cancer metastasis by acting directly on the tumor cells. Expression of a dominant-negative mutant (TβRII∆cyt) of the TGF-β type II receptor rendered the human breast cancer cell line MDA-MB-231 unresponsive to TGF-β. In a murine model of bone metastases, expression of TβRI-I∆cyt by MDA-MB-231 resulted in less bone destruction, less tumor with fewer associated osteoclasts, and prolonged survival compared with controls. Reversal of the dominant-negative signaling blockade by expression of a constitutively active TGF-β type I receptor in the breast cancer cells increased tumor production of parathyroid hormone-related protein (PTHrP), enhanced osteolytic bone metastasis, and decreased survival. Transfection of MDA-MB-231 cells that expressed the dominant-negative TβRII∆− cyt with the cDNA for PTHrP resulted in constitutive tumor PTHrP production and accelerated bone metastases. These data demonstrate an important role for TGF-β in the development of breast cancer metastasis to bone, via the TGF-β receptor-mediated signaling pathway in tumor cells, and suggest that the bone destruction is mediated by PTHrP.

Transforming growth factor-β1 is the predominant isoform required for breast cancer cell outgrowth in bone

“…Expression of the ligand trap significantly inhibited the ability of breast cancer cells to grow effectively in the bone and suppressed TGF-b-induced expression of PTHrP and IL-11, two osteoclastic factors that are important for the formation of breast cancer bone metastases (Manolagas, 1995;Yin et al, 1999;Sotiriou et al, 2001;Kakonen et al, 2002;Kang et al, 2003Kang et al, , 2005Deckers et al, 2006;Liao and McCauley, 2006). These results are consistent with the observation that a dominant negative TbRII or a TbRI inhibitor impaired the ability of breast cancer cells to metastasize to bone (Yin et al, 1999 that an increase in the local concentration of the ligand trap would be achieved as Fc:TbRII(ECD)-producing tumors progressively grow in the bone. These increasing amounts of the secreted ligand trap would neutralize TGF-b1 and TGF-b3 that are (1) produced by resident bone cells, (2) released from the matrix during the induction of an osteolytic response or (3) secreted by the mammary tumor cells themselves.…”

“…Transforming growth factor-b is known to induce the expression of factors important for osteoclast differentiation and function, such as PTHrP and IL-11 (Yin et al, 1999;Kang et al, 2003Kang et al, , 2005Deckers et al, 2006). Thus, we investigated whether expression of the neutralizing ligand trap could blunt TGF-b-induced expression of these two factors.…”

“…Expression of a dominant negative TbRII significantly reduces the ability of MDA-MB-231 breast cancer cells to form osteolytic bone metastases, which can be rescued by expression of constitutively active TbRI (Yin et al, 1999). In vivo selection approaches to isolate highly aggressive bone metastatic MDA-MB-231 breast cancer cells have identified TGF-b targets, such as parathyroid hormone-related protein (PTHrP), interleukin-11 (IL-11) and connective tissue growth factor, which play important roles in breast cancer metastasis to bone (Yoneda et al, 2001;Kang et al, 2003).…”

New insights into the actions of bisphosphonate zoledronic acid in breast cancer cells by dual RhoA-dependent and -independent effects

“…Finally, in bone, breast-cancer cells interact mainly with boneresorbing osteoclasts, supplying them with stimulatory factors, which lead to disruption of bone structure and release of stromabound factors that in turn stimulate the growth of cancer cells (Yin et al, 1999). Invasive breast cancers, which constitutively express inducible Cox-2, enhance osteoclast formation through the production of high levels of PGE2 and subsequently an increase of osteolysis (Ono et al, 2002).…”

Mechanisms of cancer metastasis to the bone

“…Thus, tumor cells, tumor-produced PTHrP, osteoclasts and bone derived TGFβ form a vicious cycle to promote the development and progression of bone metastasis. Disruption of any step in the cycle may significantly decrease bone metastasis [7,25].…”