BDNF/TrkB signaling as an anti-tumor target

Several examples of molecularly targeted anticancer therapies successfully translated from the laboratory bench to the clinical setting, including imatinib, gefitinib, trastuzumab, erlotinib and cetuximab, illustrate the potential therapeutic usefulness of targeting growth factors or their cell surface receptors. Among growth factor signaling pathways, neurotrophins and their receptors represent emerging targets for the development of novel anticancer therapies. Brain-derived neurotrophic factor (BDNF), a prototypical member of the neurotrophin family of growth factors, was first identified and isolated from pig brains by Ivens Barde and colleagues in the early 1980s, and cloned by the same group in 1989 (for a recent review, see [1]). In 1991, BDNF was identified as a ligand for TrkB, a member of the tropomyosin receptor kinase (Trk) family of cell membrane receptors. Trks are receptor tyrosine kinases activated by neurotrophins and other growth factors. TrkA, TrkB and TrkC are the preferred receptors for the neurotrophins NGF, BDNF and neurotrophin-3 (NT-3), respectively [1,2]. Trk was first identified by Dionisio Martin-Zanca and colleagues as an oncogene, present in a human colon carcinoma and capable of mediating transformation of fibroblasts (reviewed in [3]), and several research laboratories have since focused on examining the role of Trks in cancer. Still, over the years, neurotrophins and Trks have been characterized primarily in the field of neurobiology as molecules involved in CNS development, neuronal survival and synaptic plasticity, while their role in cancer has remained relatively less explored.

To date, the function of neurotrophins and Trks in cancer has been largely characterized in neural tumors, particularly neuroblastoma. High levels of TrkA and TrkC expression in neuroblastoma are associated with better prognosis, whereas BDNF and TrkB are preferentially expressed in aggressive tumors, and increased BDNF/TrkB signaling in neuroblastoma cells might represent an autocrine system to promote tumor growth, invasion and metastasis. In addition, TrkB activation by BDNF promotes resistance to chemotherapy in neuroblastoma cells through a mechanism mediated by PI3K and Akt (see [3–5] for reviews). A recent study showed that, in neuroblastoma cells with high TrkB expression, treatment with BDNF protected from etoposide-induced cell death in vitro, and neuroblastomas with high TrkB expression were less sensitive to etoposide treatment in vivo than tumors with low TrkB expression in a xenograft mouse model [6]. In medulloblastoma, a neural tumor that is the most common brain cancer of childhood, TrkC expression correlates with a better response to therapy, while TrkB probably promotes cell survival (reviewed in [5]). However, we have recently found that, at least in some human medulloblastoma cell lines, human recombinant BDNF decreases cell viability, suggesting that stimulation of BDNF/TrkB signaling might also inhibit medulloblastoma growth [7].

Recent evidence for a role of BDNF/TrkB in cancer has also come from studies on non-neurogenic tumors [3–5]. For example, human lung adenocarcinomas have been shown to express TrkB, and treating A549 lung adenocarcinoma cells with BDNF stimulated the prosurvival Akt pathway, while the Trk inhibitor K252a inhibited cell growth and induced apoptotic cell death [8]. TrkB mutations have been identified in non-small-cell lung cancer; however, these mutations did not result in enhanced cell transformation and migration [9]. Both BDNF and TrkB are overexpressed in human bladder cancer specimens [10], and BDNF enhanced cell proliferation and survival [11], whereas a TrkB antibody induced cytotoxicity and suppressed migration and invasion, in transitional cell carcinoma cells [12]. Two recent studies have also suggested a role for BDNF/TrkB signaling in breast cancer. Expression of BDNF and TrkB was detected in breast cancer cells lines and tumor specimens. BDNF induced resistance to apoptosis in breast cancer cells, while injection of an anti-BDNF antibody reduced the growth of breast tumors xenografted in immunodeficient mice [13]. BDNF expression was shown to be higher in breast cancer samples compared with normal tissue, and higher levels of BDNF transcripts were significantly associated with unfavorable pathological parameters and adverse clinical outcomes including poor prognosis and death from breast cancer [14]. The expression of BDNF and TrkB mRNA was also higher in human cervical cancer cell lines and squamous cell carcinoma of the uterine cervix than in normal tissues, and was related to clinicopathological parameters related to early-stage squamous cell carcinoma [15]. Other solid tumors in which TrkB expression is elevated and might be associated with a poorer disease outcome include Wilm’s tumor, ovarian cancer and pancreatic ductal adenocarcinoma (reviewed in [4]).

Cancer types in which BDNF/TrkB signaling possibly plays a stimulatory role also include gastrointestinal and hepatic tumors. For example, high mRNA and protein levels of BDNF were associated with the development and recurrence of tumors in a rat model of hepatocellular carcinoma, and treatment with BDNF promoted hepatocellular carcinoma cell proliferation [16]. In human colorectal cancer, two different sites of the TrkB kinase domain (TRKB^T6951 and TRKB^D751N) have been found to be mutated [17], although the functional significance of these mutations remains unclear [18]. We have recently found that BDNF and TrkB were expressed in samples of sporadic colorectal adenocarcinoma, and BDNF protein levels were higher in colorectal tumor samples compared with non-neoplastic adjacent tissue. We went on to investigate the function of BDNF/TrkB in colorectal cancer cells. Treating human HT-29 cells with human recombinant BDNF prevented the antiproliferative effect of RC-3095, a synthetic peptide with anti-tumor activity that acts by blocking gastrin-releasing peptide receptors. Conversely, treatment with K252a resulted in a dose-dependent decrease in proliferation. Moreover, HT-29 cells showed an increase in both BDNF mRNA expression and BDNF protein secretion in response to treatment with RC-3095. This increase, which was likely mediated by a mechanism requiring EGF receptors, was associated with resistance to RC-3095-induced reduction of cell survival. These findings suggest that increased BDNF/TrkB may play a role in the progression of colorectal cancer and contribute to resistance to anti-tumor agents targeting other growth factor receptors [19]. This possibility should be further investigated.

At the cellular level, the pro-oncogenic actions of TrkB may involve a variety of mechanisms, including downstream activation of protein kinase pathways that are established targets in cancer, such as PI3K, MAPK and PKC [1–5]. TrkB has been shown to suppress anoikis (apoptosis resulting from loss of cell-matrix interactions) through a mechanism dependent on PI3K/Akt in nonmalignant epithelial cells, an effect associated with the formation of rapidly growing and invasive tumors in mice (reviewed in [4]). One possibility that should be further explored is that BDNF/TrkB signaling stimulates angiogenesis. Recent evidence supporting this possibility includes the findings by Lam et al. that endothelial cells overexpressing BDNF contributed to tumor angiogenesis and growth in a mouse model of liver cancer [20].

Taken together, the evidence suggests that small-molecule compounds that act as TrkB antagonists, or monoclonal antibodies against either BDNF or TrkB, could be developed as promising novel therapies for the treatment of some types of cancer. Phase I clinical trials of Trk inhibitors have already been carried out, and Phase II trials are currently underway [3,4]. However, further understanding of the role of BDNF/TrkB in cancer, as well as of the effects of pharmacological blockade of BDNF signaling, is clearly needed. Although Phase I trials of Trk inhibitors have suggested that they were well tolerated, given the crucial roles of BDNF in normal nervous system function, it remains to be established whether blocking this pathway in an individual would be clinically acceptable. On the biological aspect, it will be crucial to understand how TrkB cross-talks with other growth factor receptors and interacts with downstream protein kinase pathways in regulating cell proliferation and transformation. As BDNF/TrkB signaling is one among many examples of molecular links between cancer progression and neural development and plasticity, studies on BDNF biology in the normal nervous system will contribute to elucidate its role in cancer.

Financial & competing interests disclosure

The authors are supported by the National Council for Scientific and Technological Development (CNPq; grant number 303703/2009-1 to Rafael Roesler), the National Institute for Translational Medicine, the South American Office for Anticancer Drug Development and the Children’s Cancer Institute. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

No writing assistance was utilized in the production of this manuscript.