Biological Characteristics of Caspase-14 and Its Expression in Neoplastic Diseases in the View of Translational Medicine
LIU Kang-sheng1, LYU Juan2, LI Ping3, ZHONG Tian-ying4,*
1. Department of Medical Laboratory, Nanjing Maternity and Child Health Care Hospital, Nanjing, Jiangsu, 210029, China
2. Department of Gynecology and Obstetrics, Nanjing Maternity and Child Health Care Hospital, Nanjing, Jiangsu, 210029, China
3. Department of Pharmacy, Nanjing BenQ Medical Center, Nanjing, Jiangsu, 210019, China
4. Hospital Office, Nanjing Maternity and Child Health Care Hospital, Nanjing, Jiangsu, 210029, China
*Corresponding Author:ZHONG Tian-ying, E-mail: tianyingzhong@163.com
Abstract

Caspase-14, a member of caspase family, only exists in mammals. As the most divergent member in the family of mammalian caspases, caspase-14 displays a variety of unique characteristics. It is expressed in a limited number of tissues and has the shortest amino acid sequence within the caspase protein family. At present, it has been found that caspase-14 is functionally different from the inflammatory reaction group of typical caspase family members. It exerts a certain effect in the promotion of final differentiation of epidermal cells and hydration of stratum corneum so as to maintain the steady state of skin barrier. In recent years, caspase-14 expression has been discovered in neoplastic diseases. Translational medicine integrates experimental research results and clinical guidance into the optimal implementation criteria for promoting the prediction, prevention and treatment of diseases. Via human genomics and molecular biology, translational medicine offers a possibility of screening molecular markers so that it can be used to diagnose the neoplastic diseases. In this article, the biological characteristics and substrates of caspase-14 as well as its expression in embryonic period and neoplastic diseases were reviewed.

Key words: Caspase-14; Neoplastic diseases; Embryonic period; Biological characteristics
Introduction

With the development of economy, aggravation of environmental pollution and increase of aging population in our country, the incidence and mortality of the patients with neoplastic diseases will increase gradually. Translational medicine integrates experimental research results and clinical guidance into the optimal implementation criteria for promoting the prediction, prevention and treatment of diseases. Via human genomics and molecular biology, translational medicine offers a possibility of screening molecular markers so that it can be used to diagnose the neoplastic diseases [1]. Meanwhile, the polygenic tumor which greatly threatens the health of human beings has become a hotspot in the field of translational medicine [2]. In recent years, the studies on molecular biology develop fast, but the incidence of tumors still does not decrease substantially [3]. Caspase-14, a member of caspase family, only exists in mammals. At present, it has been found that caspase-14 is functionally different from the in flammatory reaction group of typical caspase family members. Caspase-14 exerts a certain effect in the promotion of final differentiation of epidermal cells and hydration of stratum corneum so as to maintain the steady state of skin barrier. Joehlin-Price et al.[4] found that the expression of caspase-14 frequently decreased in premalignant and malignant vulvar squamous lesions, but upregulated in vulvar squamous cell carcinoma (VSCC) cell culture by black raspberry extract (BRB-E). Therefore, study on the biological characteristics of caspase-14 and its expression in neoplastic diseases is of great importance for tumor diagnosis and treatment, and is also an important embodiment of translational medicine. In this article, the biological characteristics and substrates of caspase-14 as well as its expression in embryonic period and neoplastic diseases were reviewed.

Biological Characteristics and Substrates of Caspase-14
Biological characteristics of caspase-14

Different from classical caspase family proteins in function, caspase-14 is a new sort of protein with an extremely short source domain, whose catalytic domain is closely related to inflammation. Human caspase-14 gene includes 7 exons, and its total length of mRNA translation fragment is about 2.4 kb. cDNA encodes the protein with 257 amino acids, and its existence form is procapase-14 in vivo. Human caspase-14 is located at chromosome 19P13.1. Caspase-14 is mainly expressed in keratoderma and stratum granulosum, but it is unclear about its specific mechanisms. At present, it is believed that caspsase-14 has the following characteristics: (1) It cannot be activated by classical apoptotic signals, and the cell apoptosis cannot be induced by its over-expression. (2) It cannot be separated into small and big subunits when over-expressed. (3) It has the function of catalyzing classical amino acid residues, and has potential activated sites and conventional pentapeptide sequence of QACRG box, but lacks of N-terminal prodomain and one caspase recruitment domain. Therefore, caspase-14 is one of the down-stream caspases that unnecessarily rely on the activation of initial caspase.

Mature caspase-14 is cleaved into subunits P20 and P10 in keratosic epithelial cells by proteolytic enzyme. A protein sensing ring with amino acid sequences of caspase-14 in mammals is present between subunits P20 and P10, illustrating that the cleavage site of caspase-14 is N-terminal conventional hydrophobic spots[6]. Different from other hydrolytic caspases on aspartate residues, the hydrolytic site of caspase-14 is between Ile152 and LyS153 [7]. The study revealed that both procaspase-14 and activated caspase-14 were present in all epidermal extracts, but only activated caspase-14 existed in the corneum [8]. It was found that activation of caspase-14 and formation of corneum were conducted synchronously at the stage of embryonic development, suggesting that caspase-14 is mainly expressed in the corneum [9].

Substrates of caspase-14

Interestingly, the substrate preference of human caspase-14 is different from murine substrate preference. Human caspase-14 preferentially accommodates tryptophan or tyrosine in the S4 subsite, while murine caspase-14 is more tolerant, with almost equal preferences for β -branched and aromatic amino acids [10]. For instance, both human and murine caspase-14 can effectively cleave the fluorescent peptide substrate WEHD-amc, but only murine one can cleave IETD-amc as effectively[11]. Due to substrate preferences, human and murine caspase-14 is classified into inflammatory caspase and inflammatory and apoptotic initiator caspase, respectively. Nevertheless, human caspase-14 cannot proteolytically activate the inflammatory cytokines pro-interleukin-1 or -18, and there are no studies that support a direct role of caspase-14 in apoptosis. Whether the substrate preferences of human and murine caspase-14 are observed in vitro, it is still unclear about the occurrence of peptide substrates in vivo. It is noteworthy that profilaggrin, a major structural protein in the differentiating epidermis, has been shown to be a physiological substrate of caspase-14 [12]. Caspase-14 is expressed predominantly in cornifying epithelia and catalyzes the degradation of profilaggrin. Additionally, identification of additional substrates and determination of cleavage sites will provide more insight into the preferred recognition sequence of caspase-14 in the context of a protein.

Caspase-14 Expression in Embryonic Period

The expression pattern of caspase-14 is unique among caspases because it primarily exists in cornifying epithelia, including the epidermis, the Hassall’ s bodies of thymus and the forestomach of rodents [13]. Caspase-3 suffers from a classical apoptosis when the damage factors of keratinocytes, such as chemical substance and heavy metal, appear. Meanwhile, caspase-14 encounters programmed cell death in continuous cell differentiation, finally leading to formation of stratum corneum [14]. By detecting the expression level and activation status of caspase-3 and caspase-14 during development of fetal mouse epidermis, Fischer et al. [15] found that caspase-3 was not activated obviously, but caspase-14 increased dramatically from embryonic day 14.5 (E14.5) onwards and consistently localized to the suprabasal layers of fetal epidermis. The caspase-14 pro-enzyme was processed into its catalytic subunits, a step required for enzyme activity, on day E17.5, coinciding with the formation of stratum corneum. Hence, processing of procaspase-14 is not confined to air-exposed mature skin but also occurs during epidermal development in uteri. These research results suggest that caspase-14 plays a key role in the formation of skin barrier in embryonic period. Glial cells missing homolog 1G (GCM1), a transcription factor, is critical for placental development. Through immunohistochemistry, Chiang et al. [16]demonstrated that GCM1 and caspase-14 were over-expressed in placental trophoblastic cells on the 8th gestation. Interestingly, GCM1 was expressed significantly when placental oxygen tension elevated on the 10th gestation. With gestational progression, both GCM1 and caspase-14 were commonly expressed in full-term placental trophoblastic cells. Caspase-14 can suppress placental cell differentiation through down-regulation of GCM1 activity, which is of positive importance for placental homeostasis in the third trimester of pregnancy, but it is still unclear with regard to its regulatory mechanisms [17]. However, Western blot analysis revealed increased expression in the first trimester when compared with the third trimester of pregnancy; immunohistochemistry for caspase-14 showed diffuse expression in the trophoblast layer, not only in occasional cells, illustrating that caspase-14 is present in the human placenta, primarily in the trophoblast, but its function is not clear [18]. Since the expression of hCG, KLF4 and cytokeratin-18 proteins is normally increased with trophoblast differentiation, it is implied that caspase-14 inhibits trophoblast differentiation[19].

Caspase-14 Expression in Various Neoplastic diseases

Caspase-14 is expressed in various benign and malignant tumors, such as vulvar carcinoma, cervical carcinoma, lung squamous cell carcinoma and oral squamous cell carcinoma. However, its gene is rarely mutated in colorectal cancer, but not mutated in gastric, lung, breast and hepatocellular carcinoma, suggesting that caspase-14 mutation may not be a direct target of inactivation in tumorigenesis of common carcinomas [20].

Fang et al.[21]found that monoclonal antibodies were specific to caspase-14, and that caspase-14 was highly expressed in lung adenocarcinomas. Caspase-14 over-expression was correlated with tumor staging, cell differentiation and lymphovascular involvement, suggesting that caspase-14 was associated with tumor cell growth and metastatic potential. In addition, caspase-14 in vitro interacted with apoptosis-inducing factor, and silencing of caspase-14 expression could decrase cisplatin resistance. These research results all showed that caspase-14 is an anti-apoptotic protein targeting apoptosis-inducing factor and increases cisplatin resistance in lung adenocarcinoma cells. Caspase-14 expression was reduced in gastric and colon adenocarcinomas, whereas increased in breast and lung adenocarcinomas frequently. At present, caspase-14 expression in salivary gland adenocarcinomas has not been evaluated. Lemound et al. [22] believed that caspase-14 was not expressed in normal salivary glands, while in a subfraction of carcinomas (32%) an aberrant expression was found, suggesting that caspase-14 cannot be used as a biomarker for a specific carcinoma subtype of the salivary gland.

Caspase-14 expression in skin cancer

Caspase-14 is an important protease in the proper formation of a fully functional skin barrier. Hoste et al. [23] found that lack of caspase-14 expression is prone to develop parakeratosis and caspase-14 plays an important role in keratinocyte terminal differentiation and maintenance of normal stratum corneum, especially in conditions causing epidermal hyperproliferation. Jung et al. [24] analyzed the correlations of pyrrolidone carboxylic acid (PCA), caspase-14 and cytokines in corneocytes with clinical severity, barrier function and skin inflammation, and the results showed that in patients with atopic dermatitis (AD), the expression of caspase-14 decreased in inflammatory lesions than non-lesion; in the lesion of patients with AD, the amounts of PCA and caspase-14 were correlated with clinical severity as determined by eczema area, severity index score and skin barrier function. Caspase-14 was expressed in normal epidermis and skin lesions of psoriasis vulgaris, and its positive expression was only seen in cytoplasm of basal layer [25]. The upper keratinocytes were, the stronger the expression of caspase-14 was. By comparing the skin squamous cell carcinoma (SCCs) induced by ultraviolet (UV) irradiation with normal skin, Rundhaug et al. [26] found that compared with normal skin, more than 200 genes were differentially expressed in SCCs and the genes related to epidermal proliferation and differentiation were deregulated, in which caspase-14 reduced 8.5-fold in SCCs. Wang et al. [27]used reverse transcription PCR and Western blot assay to detect the caspase-14 expression in malignant melanoma cells, they found that caspase-14 was significantly expressed in melanoma cells, and caspase-14 mRNA expression could be detected in malignant melanoma tissues and dermal nevus. Besides, the positive rate of caspase-14 was extremely high in malignant melanoma cells, and it was higher in melanoma-associated antigen 1 recognized by T cells (MART-1) positive cells than in MART-1 negative cells. Hsu et al. [28] believed that expression of exogenous caspase-14 led to growth inhibition and reduced the tumorigenicity of A431 cells, illustrating that caspase-14 could be used as a novel approach to skin cancer therapy via gene delivery systems.

Caspase-14 expression in gynecolgical neoplastic diseases

As is known to all, caspase 14 proteins are reduced in cervical intraepithelial neoplasms and invasive cervical carcinomas during neoplastic dedifferentiation. Krajewska et al. [29] explored caspase-14 expression in several types of human epithelial malignancies, and found that caspase-14 expression went down significantly in cervical, ovarian, and colon cancers when compared with normal epithelium. Caspase-14 expression decreased gradually from cervical intraepithelial neoplasia I (CINI) to CINIII, and was almost negative in cervical squamous carcinoma. In addition, caspase-14 expression was not related to the age, clinical staging and lymph node metastasis in cervical cancer tissues, but was positively correlated with caspase-3 in CIN and cervical squamous carcinoma [30]. Joehlin-Price et al. [4] found that caspase-14 was frequently decreased in premalignant and malignant vulvar squamous lesions, and was upregulated in vulvar squamous cell carcinoma (VSCC) cell culture by black raspberry extract (BRB-E).

Conclusion

Oncological translational medicine embodies a current tendency of biomedical research mode revolution. By integrating the separated molecular biology, drug research and development as well as clinical research in traditional mode, it transforms clinical experimental results into clinical application timely so as to improve the cooperation of molecular biology with the clinic [32]. Nowadays, study on tumor molecular markers is the embodiment of translational medicine, and early prevention, diagnosis and treatment for tumors is also a major research direction for oncological translational medicine.

Early cloning research on caspase-14 plays an important role primarily in maintenance of epidermal stable equilibrium. The balance between apoptosis and proliferation can be broken when caspase-14 expression decreases, consequently leading to a serious of diseases, including tumorigenesis. Caspase-14 can be considered as a new biomarker in the early diagnosis of diseases, but its signaling pathway is uncertain in cell apoptosis, it still needs further experiments to verify.

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