Telomere shortening in the damaged small bile ducts in primary biliary cirrhosis reflects ongoing cellular senescence†

Abstract

Telomere shortening is a trigger of cellular senescence. Biliary epithelial cells in damaged small bile ducts in primary biliary cirrhosis (PBC) show senescent features such as the expression of senescence-associated ␤-galactosidase and the increased expression of p16 INK4a and p21 WAF1/Cip1 . We investigated whether the telomere shortening is involved in the pathogenesis of biliary cellular senescence in PBC. We analyzed the telomere length of biliary epithelial cells using quantitative fluorescence in situ hybridization in livers taken from the patients with PBC (n ‫؍‬ 13) and control livers (n ‫؍‬ 13). We also assessed immunohistochemically the prevalence of DNA damage and the expression of p16 INK4a and p21 WAF1/Cip1 . The study showed a significant decrease in telomere length in biliary epithelial cells in the damaged small bile ducts and bile ductules in PBC compared with normal-looking bile ducts and bile ductules in PBC, chronic viral hepatitis, and normal livers (P < 0.01). ␥H2AX-DNA-damage-foci were detected in biliary epithelial cells in damaged small bile ducts and bile ductules in PBC but were absent in biliary epithelial cells in chronic viral hepatitis and normal livers. The expression of p16 INK4a and p21 WAF1/Cip1 was increased corresponding to telomere shortening and ␥H2AX-DNA-damage-foci in the damaged small bile ducts in PBC. R ecent progress in the field of hepatology disclosed that cellular senescence is involved in the pathophysiology of various chronic liver diseases 1-7 and hepatocarcinogenesis. 8,9 Cellular senescence is defined as a condition in which a cell no longer has the ability to proliferate. Senescent cells are irreversibly arrested at the G1 phase of the cell cycle and do not respond to various external stimuli but remain metabolically active. Senescent cells display several characteristics, including histological changes in vitro and in vivo, 10,11 shortened telomeres, increased expression of p16 INK4 and p21 WAF1/CIP1 , and increased activity of senescence-associated ␤-galactosidase. 12 Cellular senescence can be triggered by multiple mechanisms, including telomere shortening, the epigenetic derepression of the INK4a/ARF locus, and DNA damage. 13 The telomeres at the ends of chromosomes are composed of terminal DNA sequence repeats and telomerebinding proteins. 14 These structures have an essential role in the maintenance of chromosomal stability by capping the chromosomes and protecting them from end-to-end fusion. Telomere repeat sequences are subjected to shortening with each cell division because of their incomplete replication during DNA synthesis. 15 The loss of telomere repeat sequences has been proposed as a possible mechanism of cellular senescence. 14,16 Conversely, in germ cells, most established cell lines, and more than 85% of malignant cells, the length of the telomeres is maintained or increased through the action of telomerase. 17

Autophagy mediates the process of cellular senescence characterizing bile duct damages in primary biliary cirrhosis

“…We have reported that a part of ductular cells in ductular reaction shows cellular senescence as well as the damaged small bile ducts in PBC. 22 Taken together, it is plausible that the vesicular expression of LC3 in ductular reaction may be associated with higher rate of senescent ductular cells in PBC. Further studies are needed to clarify the relation between the cell kinetics of 'bipotential stem/progenitor cells' and autophagy and/or cellular senescence in ductular reaction.…”

“…[20][21][22] A possible association of oxidative stress and decreased expression of Bmi1 polycomb ring finger oncogene (Bmi1) was suggested to be involved in the pathogenesis of cellular senescence in PBC. [20][21][22][23] Although it is well known that autophagy has important functions in clearing misfolded proteins in hepatocytes in liver diseases such as a-1 antitrypsin deficiency and hypofibrinogenemia, 24 there are few studies regarding autophagy in BECs and its relation between liver diseases to our knowledge. Given that BECs in damaged small bile ducts in PBC show senescent features, we examined an involvement of autophagy in the process of biliary epithelial senescence in PBC.…”

Primary Biliary Cirrhosis: Bad Genes, Bad Luck

“…Another study by the same group showed that Bmi1 (a suppressor of oxidative stress) was significantly reduced in damaged BEC of PBC patients [16]. Second, shortened telomere length with associated cellular senescence has been demonstrated in a study by Sasaki and colleagues [17]. In particular, they examined telomere length using quantitative fluorescent in situ hybridization of 13 patients with PBC and 13 controls and showed a significant decrease in telomere length in damaged BEC from PBC patients, in comparison to histologically normal BEC in PBC patients and controls [17].…”

“…INK4a and p21 WAF/Cip1 , and DNA damage, and demonstrated that telomere shortening is associated with DNA damage and cellular senescence as a feature of damaged BEC in PBC [17]. Third, senescent BEC seem to promote an environment that is pro-inflammatory and profibrotic through secretion of chemokines (IL-8 and MCP-1) and cytokines (such as IL-1 and IL-6) [18][19][20][21][22].…”

“…Second, shortened telomere length with associated cellular senescence has been demonstrated in a study by Sasaki and colleagues [17]. In particular, they examined telomere length using quantitative fluorescent in situ hybridization of 13 patients with PBC and 13 controls and showed a significant decrease in telomere length in damaged BEC from PBC patients, in comparison to histologically normal BEC in PBC patients and controls [17]. They also performed immunohistochemical studies for senescence markers p16…”