NCBI Summary:
KRT18 encodes the type I intermediate filament chain keratin 18. Keratin 18, together with its filament partner keratin 8, are perhaps the most commonly found members of the intermediate filament gene family. They are expressed in single layer epithelial tissues of the body. Mutations in this gene have been linked to cryptogenic cirrhosis. Two transcript variants encoding the same protein have been found for this gene. [provided by RefSeq, Jul 2008]
General function
Cytoskeleton
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Cellular localization
Cytoskeleton
Comment
Apoptotic markers indicate nonalcoholic steatohepatitis in polycystic ovary syndrome. Tan S et al. (2010) Polycystic ovary syndrome (PCOS) characterized by chronic anovulation and hyperandrogenism is highly associated with obesity and insulin resistance (IR), two key features of nonalcoholic steatohepatitis (NASH). NASH often leads to cirrhosis, including portal hypertension, liver failure, and hepatocellular carcinoma as long-term complications. The caspase 3-cleaved fragment of cytokeratin 18 (CK18) emerging from ongoing cell death during apoptosis process has been established as a serum marker for NASH. This study was conducted to evaluate the prevalence of NASH in PCOS patients by caspase-cleaved CK18 measurement. In 192 PCOS patients [age, 29.0 +/- 6.7 yr; body mass index (BMI), 31.5 +/- 8.2 kg/m(2)] and 73 age-matched controls (age, 28.6 +/- 8.0 yr; BMI, 24.1 +/- 4.6 kg/m(2)), obesity and IR were determined by BMI and area under the curve of insulin response (AUCI), respectively. Apoptotic cell death was measured by M30 ELISA detecting caspase-cleaved CK18 only. M30 levels were significantly elevated in PCOS patients after correction for BMI (304.7 +/- 223.1 vs. 86.3 +/- 165.6 U/liter; P < 0.001). M30 correlated significantly with BMI, AUCI, glucose secretion, low-density lipoprotein, low high-density lipoprotein, and free androgen index. AUCI turned out to be the only independent M30-determining factor in the multiple regression analysis with an effect size of 7.9%. Fifty-one of 186 (27.4%) PCOS patients showed M30 levels of at least 395 U/liter, indicating NASH. These data demonstrate elevation of apoptotic cell death, its correlation with IR, and a high prevalence of NASH in PCOS patients. Given this high prevalence, PCOS may be a risk factor for progressive hepatic sequelae. Incidence data are of strong interest.//////////////////
Ovarian function
Follicle atresia
Comment
Changes in keratin 8/18 expression in human granulosa cell lineage are associated to cell death/survival events: potential implications for the maintenance of the ovarian reserve. Gaytan F et al. (2018) Is keratin 8/18 (K8/K18) expression linked to cell death/survival events in the human granulosa cell lineage? A close association exists between changes in K8/K18 expression and cell death/survival events along the human granulosa cell lineage lifespan. In addition to their structural and mechanical functions, K8/K18 play essential roles regulating cell death, survival and differentiation in several non-gonadal epithelial tissues. Transfection of the granulosa-like tumor KGN cells with siRNA to interfere KRT8 and KRT18 expression increases FAS-mediated apoptosis, while an inverse association between K8/K18 expression and cell death has been found in the bovine antral follicles and corpus luteum. Yet, only fragmentary and inconclusive information exists regarding K8/K18 expression in the human ovary. Expression of K8/K18 was assessed by immunohistochemistry at different stages of the granulosa cell lineage, from flattened granulosa cells in primordial follicles to fully luteinized granulosa-lutein cells in the corpus luteum (including corpus luteum of pregnancy). Immunohistochemical detection of K8/K18 was conducted in 40 archival ovarian samples from women aged 17-39 years. K8/K18 expression was analyzed at the different stages of follicle development and corpus luteum lifespan. The proportions of primordial follicles showing all K8/K18-positive, all K8/K18 negative, or a mixture of K8/K18 negative and positive granulosa cells were quantified in 18 ovaries, divided into three age groups: ≤ 25 years (N = 6), 26-30 (N = 6) and 31-36 (N = 6) years. A total number of 1793 primordial, 750 transitional and 140 primary follicles were scored. A close association was found between changes in K8/K18 expression and cell death/cell survival events in the human granulosa cell lineage. Large secondary and early antral follicles (most of them undergoing atresia) and regressing corpora lutea displayed low/absent K8/K18 expression. Conversely, early growing and some large antral follicles, functional menstrual corpora lutea, as well as life-extended corpus luteum of pregnancy, in which cell death was scarce, showed high K8/K18 expression. Three sub-populations of primordial follicles were observed with respect to the presence of K8/K18 in their flattened granulosa cells, ranging from primordial follicles showing only positive granulosa cells [P0(+)], to others with a mixture of positive and negative cells [P0(+/-)] or follicles with only negative cells [P0(-)]. Significant age-related changes were found in the proportions of the different primordial follicle types. In relation to age, a positive correlation was found for P0(+) primordial follicles (R2= 0.7883, N = 18; P < 0.001), while negative correlations were found for P0(+/-) (R2 = 0.6853, N = 18; P < 0.001) and P0(-) (R2 = 0.6725, N = 18; P < 0.001) follicles. Furthermore, an age-related shift towards greater keratin expression was found in P0(+/-) follicles (χ2 = 19.07, P < 0.05). N/A. This is a descriptive study. Hence, a cause-and-effect relationship between K8/K18 expression and cell death/survival cannot be directly established. This study describes, for the first time, the existence of sub-populations of primordial follicles on the basis of K8/K18 expression in granulosa cells, and that their proportions change with age. While a progressive increase in K8/K18 expression cannot be ruled out, our data are consistent with the hypothesis that primordial follicles expressing low levels of K8/K18 are preferentially ablated by follicle attrition, while primordial follicles showing high K8/K18 levels are those predominantly recruited into the growing pool. This suggests that K8/K18 expression could constitute a novel factor regulating primordial follicle death/survival, and raises the possibility that alterations of K8/K18 expression could be involved in the accelerated depletion of the ovarian reserve leading to premature ovarian insufficiency. This work was supported by Grants BFU2011-025021 and BFU2014-57581-P (Ministerio de Economía y Competitividad, Spain; co-funded with EU funds from FEDER Program); project PIE14-00005 (Flexi-Met, Instituto de Salud Carlos III, Ministerio de Sanidad, Spain); Projects P08-CVI-03788 and P12-FQM-01943 (Junta de Andalucía, Spain); and EU research contract DEER FP7-ENV-2007-1. CIBER Fisiopatología de la Obesidad y Nutrición is an initiative of Instituto de Salud Carlos III. The authors have nothing to disclose in relation to the contents of this study.//////////////////
Expression regulated by
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Ovarian localization
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Follicle stages
Secondary, Antral, Corpus luteum
Comment
Expression and distribution of cytokeratin 8/18 intermediate filaments in bovine antral follicles and corpus luteum: an intrinsic mechanism of resistance to apoptosis? Townson DH et al. Apoptosis is a mechanism of cell elimination during follicular atresia and luteal regression. Recent evidence suggests sensitivity to apoptosis in some cell types is partly dependent upon cytokeratin-containing intermediate filaments. Specifically, cytokeratin 8/18 (CK8/18) filaments are thought to impart resistance to apoptosis. Here, cytokeratin filament expression within bovine ovarian follicles and corpora lutea (CL) was characterized and the potential relationship between cell-specific CK8/18 expression and apoptosis explored. Immunoprecipitation and western blot analysis confirmed CK8 associates with CK18 to form CK8/18 heterodimeric filaments within bovine ovarian cells. Immunostaining revealed populations of CK18-positive (CK18+) cells in healthy growing follicles that increased in postovulatory follicles. Atretic follicles at all stages of atresia also contained some CK18+ cells. However, no CK18+ cells were detected in primordial or primary follicles. In CL, developing CL contained a higher proportion of CK18+ cells (approximately 35%, range 30-70%) than mature CL (approximately 16%) and regressing CL (approximately 5%; P<0.05, n = 3-5 CL/stage), suggesting CK8/18 filament expression diminishes over time, as luteal cells become more susceptible to apoptosis. Dual-fluorescence labeling for CK18 and a cell death marker (TUNEL labeling) confirmed this view, demonstrating less death of CK18+ than CK18- luteal cells throughout the estrous cycle (P<0.05). The results indicate differential expression of CK8/18 filaments occurs in cells of bovine ovarian follicles and CL throughout the estrous cycle. The prevalence and cell-specific pattern of cytokeratin expression in these structures is consistent with the concept these filaments might impart resistance to apoptosis in ovarian cells as is seen in other cell types.