Prostate cancer is the most commonly diagnosed malignancy and the second leading cause of cancer-related deaths in the Western male population[8
]. Its incidence in China has greatly increased from 1.71 per 100,000 men in 1993 to 7.9 per 100,000 men in 2005. PNI is one of the most important characteristic of prostate cancer, as well as the cause of frequent recurrence and poor prognosis. PNI usually occurs in critical local nerves, and influences patients' quality of life and causes the disease difficult to cure[9
]. Recurrence and metastasis are common when the disease is diagnosed in the later stages and mean a poor prognosis. Residual tumor tissue in nerve tissue is to blame for extracapsular spread and recurrence, which often results in treatment failure. Reports suggest that 79% of prostate cancer samples obtained during surgery display PNI [10
] and that PNI is associated with tumor pathological grade, clinical stage and Gleason score. Elucidation of the trigger and mechanism involved in prostate cancer PNI is critical for improving prognosis and promoting clinicians choose the most appropriate therapy.
Besides their well-established roles in the inflammatory reaction, chemokines participate in many physiological and pathological behaviors including lymphocyte homing, immunity, infection, autoimmune disease, angiogenesis and so on [11
]. Of late, increasing evidences suggested that chemokines were closely related to tumor growth, invasion and metastasis[12
]. Among the family of chemokines and their receptors that significantly contribute to the initiation, progression, invasion and metastasis of tumors, tumor cells expressing high CXCR4 are often found in tissue expressing CXCL12, perhaps due to organ-specific metastasis[13
]. One possible mechanism involves chemokines combining with their receptors to induce membrane wrinkling in tumor cells that forms pseudopodia. The tumor cells then adhere to and traverse through the extracellular matrix and basal membrane to enter the blood circulation and metastasize. However, there are not reports about chemokines and their recepters involving with PNI. Furthermore, The previous views suggested that tumor cells express high CXCR4 and the metastasis-targeted organs express high CXCL12, so the tumor cells were attracted to the ligand in these organs[14
]. In our study, tumor cells not only expressed CXCR4, but also CXCL12. CXCR4 and CXCL12 jointly promote prostate cancer PNI. This phenomenon indicates that there may be autocrine stimulation by CXCL12 in tumor tissue. Some research validated that tumor cells can also express CXCL12. Taichman[15
] showed that PC3 and DU145 prostate cancer cell lines and hormone-resistant LnCaP and C4-2B prostate cancer cell lines can express CXCL12. Prostate cancer cells with significant CXCR4 expression may escape from primary tumor foci, enter into the lymphatic and blood vessels and migrate toward cells expressing CXCL12. CXCL12 in normal tissue attracts the CXCR4 on the cancer cells, stimulating cell proliferation and inducing angiogenesis [16
The previous explanation for tumor PNI is that interstitial tension around nerves is generally weak, making the microenvironment surrounding the lamellar sheath suitable for tumor cell growth. However, this explanation cannot adequately detail the molecular mechanism involved in prostate cancer PNI. Our study concludes with the idea that chemokines and their receptors interacting with MMPs contributed to prostate cancer PNI. CXCL12 can induce tumor cell secretion of MMP-9 that then degrades the blood vessel basal membrane, remodels vessels and stimulates growth of new vessels. All of these features together make it possible for tumor cells to easily enter the circulation. At the same time, MMPs can similarly induce CXCR4 expression in tumor cells.
] reported that CXCL12 can markedly upregulate the expression and activity of MMP-9 in osteosarcoma. In the progression of prostate cancer PNI, CXCL12 and CXCR4 secreted by tumor cells and nerve tissue induce tumor cells to migrate toward nerves. At the same time, tumor cells secrete MMP-2 and MMP-9 that degrade the matrix around the tumor and the nerve tissue, promoting PNI. Our research demonstrated that the expression of CXCL12, CXCR4, MMP-2, and MMP-9 in prostate cancer was much higher than in hyperplastic prostate tissue, implying that these proteins interact with each other and together may regulate chemotactic ability and invasiveness of tumor cells.
Exogenous CXCL12 can enhance PC3 cells' ability to penetrate ECM gel. Initially in this process, chemokines combining with their receptors cause actin to concentrate and enhance cell mobility. Muller [20
] reported that CXCL12 could increase the concentration of F-actin in breast cancer cells in vivo, thus facilitate the breast cancer cells' migration and invasion in a specific direction. Secondly, CXCR4 combining with CXC12 can rapidly stimulate transmembrane transporting of Ca2+
] showed the specific antibody against human CXCR4 scFv could significantly inhibit the Ca2+
transport mediated by CXCL12 and CXCR4. Thirdly, CXCL12 promotes tumor cell adhesion to the basement membrane components.
AMD3100 is a small highly specific non-peptide antagonist of CXCR4[6
]. It can inhibit the signal transduction of CXCL12 mainly through a route that does not involve the ligand-receptor interaction. AMD3100 is capable of inhibiting infiltration by acute lymphoblastic leukemia and ovarian cancer cells[7
]. AMD3100 competitively inhibited the CXCR4 binding with CXCL12 and blocked the downstream pathway. In addition, AMD3100 can promote MMP secretion by blocking the downstream of signal transduction pathway about the CXCL12 and CXCR4 interaction.
Nerve growth factor (NGF) not only promotes nerve tissue growth, development and regeneration, but regulates the growth and metastasis of pancreatic cancer, prostate cancer, lung cancer and retinal glioblastoma cells[22
]. Results of Oelmann et al[23
]. indicated NGF promotes proliferation in three kinds of lung cancer cell lines, and the effect took on dose-dependent. This effect could be blocked by NGF antibody. Research from Zhu et al. [24
] showed that pancreatic cancer cells expressed NGF and lamellar sheath expressed TrkA, a combination of receptor and ligand that attracts tumor cells to nerves, leading to invasion and metastasis along nerves. In addition, NGF stimulated the expression of MMPs in tumor cells. Our results indicated that the expression of NGF in the CXCL12-treated group was much higher than that in the control group and the AMD3100-treated group. Of the three groups, the CXCL12-treated group had the highest average number of nerves around tumor. This study suggested that CXCL12 could increase NGF expression in PC3 cells. NGF could combine with its receptor expressed on the cell surface to activate the corresponding signal transduction pathway, enhancing the cells' invasiveness and mobility toward nerves. NGF secreted by tumor cells could also enhance the synthesis and release of some factors including MMPs that promoted cell invasion and metastasis. Furthermore, NGF diffuses around tumor tissue and stimulates nerve growth along this concentration gradient, enabling tumor cells to invade nerves and metastasize. Meanwhile, the increased number of nerves in tumor mass can secrete numerous factors to promote tumor cell growth.