Testicular Cancer is cancer that occurs in the testicles, an important part of the male reproductive system that produces sperm and the hormone testosterone. Typical symptoms in the early stage of the cancer are often a lump or swelling in the testes. Other symptoms including firmness or differences in the appearance of a testicle, dull ache, or sharp pain in the testicles or scrotum may arise as cancer progresses. Cryptorchidism is a major factor in the development of cancer, the testicular tumors tend to be larger combined with cryptorchidism. Inguinal hernias, Klinefelter syndrome, and mumps orchitis are also associated with an increased risk of testicular cancer. Over 95% of the testicular cancers are testicular germ cell tumors (TGCTs), which include two main subtypes, the seminomas, and the non-seminomas. While the left types of testicular cancer are generally Leydig cell tumors or Sertoli cell tumors. Testicular cancer is one of the most treatable and curable types of cancer, with 95% five-year survival rate in the US and 98% in England and Wales. Common treatment strategies for testicular cancer include surgery, radiation therapy, chemotherapy, and stem cell transplantation. The cure rate was over 80% even cancer has spread widely, which was better in the localized tumors.
Figure 1. Testicular Cancer Signaling Pathway
The Nodal signaling pathway belongs to the TGFβ superfamily, which participates in the regulation of cell differentiation, proliferation, and apoptosis. The pathway is activated through the binding of Nodal to Activin receptors (two serine/threonine receptors, the type I receptor Alk4/7 and type II receptor AlkRIIA/B) in the presence of the obligate co-receptor, Cripto (teratocarcinoma derived growth factor 1, TDGF-1). After binding of the extracellular domain of type II receptor to Nodal, it is conformational changed and dimerized with the type I receptor, which results in the activation caused by the phosphorylation of their intracellular domains. The complex in turn activates the downstream signal transducer Smad2/3 together with Smad4 and finally regulates the transcription of target genes. The pathway up-regulates the expression of Nodal, as well as TGFβ molecules, Lefty1 and Lefty2, both of which operate as dose-dependent feedback inhibitors of the pathway.
The insulin and insulin-like growth factor (IGF) signaling pathway is involved in various physiological and pathological cellular processes. The IGF family mainly consists of three ligands (insulin, IGF1, and IGF2), three receptors (insulin receptor (IR), IGF1R, and IGFR2), and six high-affinity ligand-binding proteins (IGFBP1-6). The IGF signaling pathway is complex. Due to the high homology between tyrosine kinase IR and IGF1R and the subunits IR-1 and IR-B produced by alternative splicing of the INSR gene, there are seven different receptors with different affinities for the IGF ligands. The signal could be transferred by any receptor containing at least one IGR1R subunit, some of which in turn activates the anti-apoptotic mechanisms and lead to increased cellular proliferation and growth in malignant tissues. A recent study has reported the phosphorylation of IGF1R in TGCT cell lines of non-seminoma type, and the insulin receptor that can heterodimerize with IGF1R was also found highly phosphorylated, indicating the activation of IGF pathway in TGCT. Additionally, the expression of IGF1R was found higher in non-seminoma and seminoma patients than in normal testis.
The SCF/c-KIT signaling pathway is activated by the specific binding of the stem cell factor (SCF) and the tyrosine kinase receptor c-KIT, a member of the type III receptor tyrosine kinase (RTK) family. SCF is a noncovalent homodimer cytokine composed of two protomers, each protomer tail has a hydrophobic crevice with a charged region that functions as the receptor binding site. Thus, the c-KIT receptors are conformational changed and dimerized on the binding of SCF, leading to its autophosphorylation and finally resulting in the initiation of multiple downstream signaling pathways such as the phosphatidylinositol 3-kinase (PI3K), the Src, the Janus kinase/signal transducers and activators of transcription (JAK/STAT), and the mitogen-activated protein kinase (MAPK). The interaction between SCF and c-KIT is playing an important role in the regulation of cell survival, proliferation, differentiation, and migration. Overactivation of c-KIT is reported to be found in cancer cases due to the presence of mutations or receptor overexpression, making it a concerned target for cancer treatment.
The Wnt/β-catenin signaling pathway is involved in the regulation of multiple biological processes such as cell proliferation and differentiation. It has been reported that the aberrant activation of Wnt was detected in TGCTs. The scaffold protein axis inhibition protein 1 (Axin1), a negative regulator of the Wnt signaling pathway, is found to inhibit PI3K/Akt/mTOR signaling pathway in TGCT cells. Research also showed the knockdown of Axin1 could increase the cell viability of the embryonal carcinoma cell line, which was decreased by overexpression of Axin1. Additionally, overexpression of Axin1 can also upregulate the expression of the pro-apoptotic Bax protein and downregulate the anti-apoptotic Bcl-2 protein levels in cell apoptosis.
Testicular cancer has the highest survival rates among patients with cancer due to advances in the treatment of TGCT, such as improved disease staging techniques, effective combined chemotherapy, and aggressive surgical treatment. Additionally, the development and widespread use of sensitive and reliable biomarkers contribute to the favorable outcomes in TGCT. Almost all testis tumors are TGCTs, which are divided into seminoma or nonseminoma germ cell tumor (NSGCT). NCGCTs mainly include four subtypes: choriocarcinoma, yolk sac, embryonal, and teratoma.
Biomarkers of TGCTs are playing crucial roles in providing clinical guidance for disease management. Although extensive evidence is proving the importance of markers in TGCT treatment, they are still underutilized. Studies have shown that the detection of testicular cancer markers is important in assigning the appropriate disease stage, determining patient prognosis, deciding appropriate therapy, as well as monitoring disease response. Furthermore, measurement of the biomarkers can help with the early identification of recurrence during post-therapy surveillance.
Protein markers are important prognostic factors in the diagnosis and staging of testicular cancer. The main classic tumor markers include alpha fetoprotein (AFP), human chorionic gonadotropin (HCG), and lactate dehydrogenase (LDH). AFP is a 70kDa single-chain glycoprotein synthesized in the yolk sac, liver, and intestine in the fetus. The level of AFP peaks during 12 to 14 weeks after pregnancy and steadily decline to 15 ng/mL a year after birth. As for TGCTs, a recent study on nearly 1500 patients with testicular cancer showed increased AFP levels in over 60% of the patients, which makes it a common tumor marker in TGCT. hCG is another glycoprotein produced during pregnancy. The molecular weight of hCG was 38kDa, which is composed of α and β subunits. The elevated serum level of hCG is found in all patients with choriocarcinoma and 40%-60% of embryonal cell cancer. Additionally, increased hCG level can also be seen in 10%-20% of patients with stage I seminoma and 30%-50% of disseminated seminoma though the concentration is generally below 500Miu/m. LDH is a 134 kDa cellular enzyme produced by muscle, liver, kidney, and brain. LDH-1 is the most commonly elevated isoenzyme among the five unique isoforms of LDH. In contrast to AFP and hCG, measurement of LDH shows the enzymatic activity instead of the actual quantity, the elevated index reflects the entities of other diseases. It needs to be taken in the context of other markers when making management decisions.
Cellular receptor pathways are targets of interest for the treatment of testicular cancer in clinic. The epidermal growth factor receptor (EGFR) pathway regulates the activation of intracellular signal transduction and leads to the transcription of target genes responsible for cellular proliferation, differentiation, angiogenesis, and survival. Thus, blocking of the pathways could inhibit tumor progression. Overexpression of EGFR is found in up to 65% of nonseminomatous germ cell tumors. Though erlotinib is proved to inhibit the proliferation of cisplatin-sensitive and cisplatin-resistant embryonal carcinoma cells in vitro, EGFR targeted therapy did not show a significant therapeutic effect in germ cell tumors. A Phase II study of gefitinib in patients with chemo-refractory germ cell tumor was conducted with no data published.
Table 1 Clinical trials of EGFR targeted therapies in TCGTs
| NCT ID | Status | Lead sponsor | Study first posted |
| NCT00772694 | Recruiting | Institute of Oncology | September, 2008 |
| NCT01743482 | Completed | Fondazione IRCCS Istituto Nazionale dei Tumori | May, 2013 |
| NCT01037790 | Completed | Abramson Cancer Center | December, 2009 |
| NCT01242631 | Completed | Hannover Medical School | November, 2010 |
| NCT01962896 | Terminated | University of Texas Southwestern Medical Center | October, 2013 |
| NCT01851200 | Completed | Fondazione IRCCS Istituto Nazionale dei Tumori | May, 2013 |
| NCT01461538 | Completed | Seagen Inc. | October, 2011 |
The proto-oncogene c-Kit is mainly expressed in seminomas. Kit signaling is of great importance for the development and survival of germ cells, and overactivation of c-Kit is found in testicular cases due to the presence of mutations or receptor overexpression. The c-Kit inhibitor imatinib can act on exon 11 mutation of c-Kit but is unresponsive to exon 17 mutation. However, multiple clinical trials using imatinib for the treatment of seminoma did not receive a unified therapeutic effect, which still needs further study before clinical use.
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