Biomarkers and Antibodies Development for Cancer (Part I)

Posted by beauty33 on August 9th, 2019

Tumor markers refer to a class of substances that abnormally change due to the expression of related genes of tumor cells or collective response to tumors during tumor cell growth, proliferation, metastasis or recurrence. Tumor markers have been in existence for more than 100 years since their discovery. Since the 1960s, they have been widely used clinically and have played an important role in the discovery and treatment of tumors. With the development of biotechnology, various new markers have been discovered, and the specificity and sensitivity have been continuously improved.

According to clinical evaluation criteria, tumor markers should have the following characteristics:

(1) Tumor markers must be produced by malignant tumor cells and can be detected in blood, tissue fluid, secretion or tumor tissue;

(2) Tumor markers are low in normal tissues or benign tumors;

(3) Tumor markers of a tumor can be detected in most patients with the tumor;

(4) Tumor markers can be detected before clinical diagnosis of tumors;

(5) The amount of tumor markers can reflect the size of the tumor;

(6) Tumor markers can help to estimate the therapeutic effect and predict tumor recurrence and metastasis to a certain extent.

    The expression of CK is maintained during the malignant transformation, which is a powerfully useful tool in oncology diagnostics. Since CK expression is often preserved in tumorous cells, the application of specific antibodies is valuable in determining the origin of metastatic carcinomas. 

1. CK18

    CK18 stands for cytokeratin 18, the gene of which is located on chromosome 12q13 and is 3791 base pairs long. CK18 encodes a type I intermediate filament protein that is primarily discovered in varieties of single-layered or “simple” epithelial tissues localizing in the cytoplasm and perinuclear region. It is reported that CK18 is able to modulate intracellular signaling and operate in conjunction with wide ranges of related proteins. The structural protein CK18 and its co-expressed complementary partner CK8 are persistently expressed in a variety of adult epithelial organs, including the lung, liver, kidney, pancreas, mammary gland, and gastrointestinal tract, as well as expressed in cancers that arise from these tissues. In the absence of CK8, the protein CK18 is degraded and the keratin intermediate filaments cannot be formed.

CK18 is a primary component of intermediate filaments of simple epithelial cells and epithelial-derived tumors and consists of approximately 5% of the total cell protein. This protein is cleaved into proteolytic fragments by caspase 3, caspase 7, and caspase 9 during apoptosis. The biological functions of CK18 increasingly draw more attention and known aspects are involved in structuring the cytoplasm by providing flexible intracellular scaffolding, resisting external stresses applied to cells, and maintaining normal mitochondrial structures. It is also essential for cellular processes such as apoptosis, mitosis, cell signaling, and cell cycle progression. Notably, CK18 has been recognized for years as an epithelial marker in diagnostic histopathology and has important effects on tumor cell behaviors.

    Apoptosis is a programmed death with many triggering factors, suggesting that there are different initiation mechanisms and pathways in the interior; apoptosis is a normal cell renewal and abnormal cell clearance in the body, but in pathology. Abnormal apoptosis in the state is closely related to the occurrence and development of multiple system diseases such as tumors. CK18 with low molecular weight is usually expressed in both monolayer glandular epithelium and pseudostratified epithelium, which is closely related to the type, proliferation and differentiation of epithelial cells. Tumors can be mediated through a variety of signal transduction pathways including PI3K/Akt, Wnt and extracellular signal-regulated kinase/mitogen-activated protein kinase signaling pathways, so they are diagnosed from monolayer glandular epithelium. Tumors that migrate to the epithelium are helpful. Squamous cell carcinoma, basal cell carcinoma, and myoepithelial tumor do not express CK18. Tumors derived from epithelial tissue are most common. Therefore, studies about CK18 and different systemic tumors have been reported at home and abroad, including tumors of the respiratory system, digestive system, urinary system and gynecology.

 1.1. CK18 and respiratory tumors

Cytokeratin can be released from tumor cells, thus providing us with a useful serum marker for evaluating the progression of patients with "epithelial malignancies". Tissue polypeptide antigen (TPA) was originally thought to be a tumor antigen present in soluble fragments of human tumor cells. Recent studies have shown that it is a polypeptide substance related to CK8, CK18, and CK19 marker, which is produced and released by proliferating cells and is increased in epithelial tumors. Scholars worldwide have reported that TPA combined with carcinoembryonic antigen and neuron-specific enolase detection have important value for early diagnosis, distant metastasis and therapeutic observation of lung cancer patients.

1.2. CK18 and digestive system tumors

Since the human digestive system is mostly covered by epithelial tissues, gastrointestinal tumors derived from epithelial tissues can also express markers such as CK8, CK18 and CK19. In patients with colorectal cancer who underwent surgery and chemotherapy, the results showed that CK18 was positively associated with tumor burden and risk of recurrence in patients with colorectal cancer. The expression levels of CK18M30 and CK18M65 in gastric cancer patients and healthy people were significantly higher in the gastric cancer group than in the normal human group, and the expression level was correlated with the progression of gastric cancer suggesting that CK18M30 and CK18M65 can be used as a prognosis independent predictor for gastric cancer. The expression level of serum CK18 is important for the diagnosis of gastric cancer. Monitoring the dynamic changes of CK18 is also important for the observation of the efficacy and prognosis of gastric cancer patients.

1.3. CK18 and urinary tumors

The transitional epithelium is distributed in the renal pelvis, ureter and bladder of the urinary system to adapt to changes in the volume of the lumen. Therefore, the expression of CK8, CK18 and CK19 can also be detected in tumors of the transitional epithelium of the urinary tract. International scholars have confirmed the correlation between CK18 expression levels and clinical efficacy by monitoring the levels of CK18 M30 and CK18 M65 fragments in the urine of patients with bladder cancer. Recently, it has been reported that RT-QPCR and immunohistochemistry quantify the expression and protein of CK18 mRNA in renal cell tumors, suggesting that CK18 expression is positively correlated with renal cell carcinoma progression.

1.4. CK18 and gynecological tumors

Apoptosis is a mechanism of cell clearance during follicular atresia and luteal regression. A prospective study of 49 patients with endometrial cancer who underwent surgical resection showed that CK8 and CK18-positive tumor tissues and cells were positively correlated with tumor stage. Some scholars used immune-histochemical techniques to detect tissue samples from 33 patients with endometrial cancer and found that 100% expressed CK18. Foreign scholars have studied that Annexin A1 protein (ANXA1), which is also a cell membrane framework, can mediate the transmission of related signals after complex complexes with CK18/CK8, and thus play an important role in the occurrence and development of breast cancer.

To be continued in Part II…

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