Proteins are the chief regulators of cancer cells and exist intracellular as well as extracellular. Receptor tyrosine kinases (RTKs), integrins, ligands, chemokine transmitters, and cytokine receptors are all kinds of membrane-bound proteins that characterize the cell. This cell signature determines where a cell is capable of. All the membrane-bound proteins are there for a reason; otherwise they would probably be degraded. Cells can regulate important cell functions through these membrane proteins, but they can also be isolated on the basis of this signature. Over time we isolated and characterized more and more different cell types on the basis of the proteins they express on their cell membrane.

Membrane proteins can be activated by secreted factors produced by themselves and by other cells. Membrane proteins can also be activated by cell–cell contact inducing interactions that phosphorylate the membrane protein. These activations might have different consequences for a cell. In most cases, activated membrane proteins will induce intrinsic downstream signaling through phosphorylation of the downstream kinases. The activated signaling pathways demonstrate much overlap in the activating similar kinases as demonstrated in Fig. 1.

Fig. 1
figure 1

Cancer cells express many membrane proteins that can all induce downstream signaling through activation of downstream kinases, appointing their overlap. This figure demonstrates how different membrane proteins can induce similar signaling pathways

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Membrane Proteins in Cancer Biology

Kinases—for example, Erk and Akt—are overactive as a result of induced membrane protein phosphorylation and have been found to be important throughout a whole range of cancers (Steelman et al. 2011). If we take a look into the top 5 most frequently occurring forms of cancer, following the World Health Organization, we can observe that increased phosphorylation of Erk and Akt is a common event in all these cancers, as is depicted in Table 1 (Fukuda et al. 2003; Gan et al. 2010; Sah et al. 2004; Serra et al. 2011; Shimizu et al. 2010; Zhang et al. 2011). Of course many translocations result from the production of fusion proteins that contribute to these overactive signaling pathways, but cells need the membrane proteins to the able to transmit the signals and/or to interact with other cells. The membrane receptors involved in Erk/Akt activation in the top 5 most frequent cancers in humans are demonstrated to be dominated by three major receptor signaling pathways. Vascular endothelial growth factor receptors (VEGFRs), epidermal growth factor receptor (EGFR, ErbB-1), and human epidermal growth factor receptor 2 (ErbB-2, i.e. HER2) seem to be the most important receptors; all three are capable of activating the same Akt and Erk signaling pathways in cancer. In breast cancer it has been shown that downstream inhibition of the PI3 kinase/Akt pathway induces the phosphorylation of Erk (Serra et al. 2011). This observation has been made for several cancers, and it suggests that targeting a single pathway may not result in the desired effect. Targeting multiple kinases might give a more promising result by inducing apoptosis in an additive way. Another thought is that it might be more effective to inhibit the upstream kinases: the membrane proteins/receptors. As we learn how different receptors in diverse cancers can activate similar downstream signaling pathways, it would be of great value to inhibit multiple receptors instead of a single receptor. Examples of small molecule inhibitors designed for specific targeting of VEGFR-2 (PTK787/ZK222584), VEGFR-2 and platelet-derived growth factor receptor-β (PDGFR-β) (sunitinib), and Raf kinase (sorafenib) have been demonstrated to be “dirty” inhibitors that target multiple membrane receptors (Sosman et al. 2007). Nevertheless, these inhibitors have been proven effective in the treatment of several cancers.

Table 1 Erk/Akt activation in the top 5 most frequent cancers
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Membrane Proteins Involved in Cancer Prognosis

Membrane-bound proteins have been connected to patient outcome in a variety of cancers. In breast cancer, HER2 expression is associated with poor prognosis, similar to EGFR expression (Noguchi et al. 1994; Toikkanen et al. 1992). Conventional therapy combined with an anti-HER2 antibody treatment has been demonstrated to be effective in HER2-positive breast cancers. This therapy has greatly improved the prognosis of these poor-outcome patients. HER2-negative and metastatic breast cancer tumors are currently being studied in a phase III clinical trial using an anti-VEGFR-2 antibody treatment (ramucirumab, IMC1121b) to sensitize these breast cancer cells (Mackey et al. 2009). In prostate cancer the active form of EGFR is associated with a poor prognosis and resistance to standard treatment (Schlomm et al. 2007). Tumors that express higher amounts of phosphorylated EGFR have been shown to be in a more advanced state of disease, as they are more aggressive tumors. In vitro studies have demonstrated that targeting EGFR shows therapeutic potential, but clinical trials have not yet been started.

Membrane Proteins in Cancer Therapy

Might membrane-bound proteins/receptors be the ideal target to treat cancer? Translocations in cancer cannot be changed; they are a given failure of the system. We can, however, target the cause of the translocation, as we currently do in patients with chronic myeloid leukemia who have the BCR–Abl fusion protein. Unfortunately, targeting the fusion protein cannot cure all chronic myeloid leukemia patients; some of them seem to be resistant to the therapy (Assouline and Lipton 2011). In many cancers we have not yet managed to find the proper therapy to overcome these failure problems. During the last 20 years we have observed an increase in new treatment options that include membrane protein inhibition, as shown in Table 2 (Bonner et al. 2010; Cook et al. 2010; Czuczman and Gregory 2010; Langenberg et al. 2010; Mackey et al. 2009; Metro et al. 2006; Okines et al. 2010; Roboz et al. 2006; Spratlin et al. 2010; Viani et al. 2007; Woyach et al. 2011; Yau et al. 2010; You and Chen 2011). Not all of them are as successful as would be desired. Rituximab is a well-studied and approved therapy for the treatment of B cell lymphomas (Czuczman and Gregory 2010). Rituximab is a human anti-CD20 monoclonal antibody that binds to its target cancer cell on the basis of membrane protein expression of CD20. These examples demonstrate that new treatment options are becoming increasingly specific. New treatment options are developed to increase overall survival rates, decrease recurrence rates, and reduce the risk of side effects. For many diseases we still need improved treatment options; thus the search for membrane-specific signatures remains an important topic. Targeting membrane-bound proteins is a specific way to target a cancer cell. However, a pitfall of the cancer cell phenotype is that the cancer cells arise from “normal” cells and demonstrate kind of the same signature. The gene expression profiles of the leukemic compartment in patients suffering from leukemia demonstrated that genes up-regulated in the leukemic stem cell population are shown to be high in the hematopoietic stem cell compartment from healthy control subjects as well (Gentles et al. 2010). This result demonstrates that the up-regulated genes are most likely involved in self-renewal, quiescence, and other stem cell-specific signatures. In other words, leukemic stem cells seem quite similar to the normal hematopoietic stem cells they arise from.

Table 2 The developed membrane receptor targeted therapy approaches in different cancers
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Nanocarrier Membrane Protein Targeted Therapy in Cancer

A new treatment strategy involving nanocarriers has been tested in a variety of cancers. This method demonstrates an increasing interest in membrane-bound protein overexpression for accurate drug delivery. The receptor-targeted therapy strategy has not been used in clinical trials so far, but it has produced a wide range of therapies already commercially available and studied in vitro and in vivo (Peer et al. 2007). In the case of hepatic cancer, doxorubicin-loaded nanoparticles that specifically target the folate receptor overexpressed on hepatocellular carcinoma cells have been used (Maeng et al. 2010). The folate receptor nanocarrier system has also been proven effective in gliomas, where doxorubicin-loaded polyethylene glycol liposomal nanocarriers were used to target the overexpressed folate receptor (McNeeley et al. 2009). Nanocarrier-targeted therapy in breast and ovarian cancer also introduced a receptor drug delivery system that uses an EGFR-targeting peptide (Milane et al. 2011). This system was shown to be effective in these cancers. The nanocarrier drug delivery system still needs much research before patients might benefit from this therapy approach.

Membrane Protein Fluorescent Imaging for Accurate Tumor Dissection in Surgery

Although cancer cell-specific signatures have been targeted to improve cancer treatment, they are also becoming more and more prominent for fluorescent imaging during surgeries. Breast cancer, ovarian cancer, prostate cancer, gliomas, and so on are cancers that have been monitored by fluorescent dyes to accomplish more precise tumor dissection (Chiun-Wei et al. 2011; Crane et al. 2011; Kantelhardt et al. 2010; Lee et al. 2011; Poptani 2010; Sampath et al. 2007; Smith-Jones et al. 2008; Themelis et al. 2011). In some cases they can use the receptors that are overexpressed on the tumor cell membrane to visualize the exact boundaries of tumors by fluorescent probes or anti-receptor antibodies to enhance the accuracy of dissecting the total tumor. Traztuzumab has demonstrated encouraging results in breast cancer xenograft near-infrared fluorescence imaging, but also showed some aspecific staining that makes tumor excision inaccurate (Sampath et al. 2007) (Table 2). In ovarian cancer much research has been performed over the last 10 years concerning fluorescence imaging, with innovative results (Crane et al. 2011). For intraoperative fluorescent imaging in ovarian cancer, the folate receptor overexpression on ovarian cancer cells may be used (Smith-Jones et al. 2008). Targeting folate receptor with a fluorescent probe has been demonstrated to clearly distinguish ovarian tumor tissue from normal tissue. Clinical trials are currently in process to evaluate whether this method could be included as a new standard surgical tool for tumor dissection.

Gene Expression Profiling: There is More to Discover

Gene expression profiling of cancers is an important topic in cancer research to evaluate cancer signature profiling and thereby predict patient outcomes. Gene expression profiles demonstrate the significance of membrane protein involvement. Gene expression profiling of tumorigenic breast cancer cells from patients with early breast cancer (n = 295) were compared with normal breast epithelium cells (Liu et al. 2007). The invasiveness gene signature (IGS) included 183 genes and was demonstrated to be associated with prognosis in breast cancer (n = 295), medulloblastoma (n = 60), lung cancer (n = 62), and prostate cancer (n = 21). Tumors with a correlation not similar to IGS (correlation < 0) were demonstrated to have a better outcome than tumors with a gene expression pattern similar to IGS (correlation > 0). Membrane-bound proteins included in the IGS are XPR1, CD59, LRP2, HSPC163, C5orf18, CDw92, TMC4, ZDHHC2, TICAM2, KDELR3, and ErbB-4. Most of these proteins are still relatively unknown, and the role and function of these membrane proteins remains to be investigated. CD59 has been shown to be involved in colorectal cancer, breast cancer, and prostate cancer (Koretz et al. 1993; Madjd et al. 2003; Xu et al. 2005). In all these cancers CD59 correlates with good patient prognosis. The fact that this gene expression profiling study included early breast cancer patients confirms that CD59 is correlated to good prognosis; however, CD59 is included in the “invasiveness” gene signature. Membrane protein ZDHHC2 is related to cancers as well. ZDHHC2 is found to be mutated in colorectal cancer and hepatocellular carcinomas (Oyama et al. 2000). Therefore, ZDHHC2 research might provide substantial innovative information concerning breast cancer pathogenesis. ErbB-4 is a family member of EGFR (ErbB-1). Alterations in ErbB family members 1 and 2 have been associated to breast cancer tumorigenesis, but less attention has been paid to the biological significance of ErbB-4 signaling in these tumors. The ErbB-4 gene seems to be hypermethylated in a subgroup of breast cancer patients (Das et al. 2010). This means that the ErbB-4 gene is not expressed in a subset of breast cancer tumors, which leads to preventing the tumor cells from going into apoptosis. Results suggest a significant role for ErbB-4 in breast cancer as a tumor suppressor gene. Analyzing gene expression profiling demonstrates that investigating the function of prognostic important membrane proteins might obtain revealing and promising results for future research.

According to the World Health Organization, cancer claims around 7.5 million lives a year, and its incidence is predicted to continue to rise to over 11 million in 2030. The search for new and better treatment options remains necessary to enhance the overall survival rates of cancer patients. Research has brought us a whole range of new treatment options based on the membrane proteins that cancer cells express on their cell membrane, some of them already clinically used and some of them under investigation. Yet many membrane-bound proteins still need to be discovered, identified, investigated, and characterized to develop more accurate therapies to cure cancer patients or to considerably prolong life while improving the patient’s quality of life.