Primary biliary cirrhosis (PBC) is characterized by antimitochondrial autoantibodies (AMAs) in patients' sera and histologically by chronic nonsuppurative destructive cholangitis in small bile ducts, eventually followed by extensive bile duct loss and biliary cirrhosis. The autoimmune-mediated pathogenesis of bile duct lesions, including the significance of AMAs, triggers of the autoimmune process, and so on remain unclear. We have reported that cellular senescence in biliary epithelial cells (BECs) may be involved in bile duct lesions and that autophagy may precede the process of biliary epithelial senescence in PBC. Interestingly, BECs in damaged bile ducts show characteristicsof cellular senescence and autophagy in PBC. A suspected causative factor of biliary epithelial senescence is oxidative stress. Furthermore, senescent BECs may modulate the microenvironment around bile ducts by expressing various chemokines and cytokines called senescence-associated secretory phenotypes and contribute to the pathogenesis in PBC.
Primary biliary cirrhosis (PBC) is a chronic, progressive cholestatic liver disease that affects usually middle-aged women and occasionally leads to liver failure and liver transplantation [
There has been considerable progress in elucidating the immunopathological features [
We have recently reported that cellular senescence and autophagy may be involved in bile duct lesions in PBC [
Cellular senescence is defined as a condition in which a cell no longer has the ability to proliferate. Senescent cells remain metabolically active, even though they are irreversibly arrested at the G1 phase of the cell cycle and do not respond to various external stimuli. Cellular senescence can be triggered by a number of cellular stresses including telomere dysfunction. Other causes include oxidative stress, nontelomeric DNA damage, epigenetic derepression of the INK4a/ARF locus, and oncogenic activation [
Biliary epithelial senescence in PBC. BECs in small bile ducts involved in chronic nonsuppurative destructive cholangitis (CNSDC) show histological features of senescence, such as cytoplasmic eosinophilia, cellular and nuclear enlargement, and uneven nuclear spacing (a). SA-
“Chronic nonsuppurative destructive cholangitis (CNSDC)” is a characteristic bile duct lesion in PBC (Figure
BECs in damaged small bile ducts in PBC show senescent features, such as the expression of SA-
The exact mechanism how cellular senescence of BECs cause bile duct loss in PBC is not clear. Cellular senescence is supposed to impair tissue integrity and cause persistent inflammation [
Cellular senescence can be triggered by a number of cellular stresses, including telomere dysfunction, oxidative stress, nontelomeric DNA damage, epigenetic derepression of the INK4a/ARF locus, and oncogenic stress [
DR is a reactive lesion at the portal tract interface composed of increased bile ductules with an accompanying complex of stromal and inflammatory cells [
Double immunostaining for senescence markers (p16INK4a or p21WAF1/Cip1) and cell cycle markers (G1-phase, cyclin D; S-phase, cyclin A) (red) and NCAM (green) in PBC, stage 4. (a), (c) The expression of senescent markers p16INK4a and p21WAF1/Cip1 is seen in NCAM-positive ductular cells (arrows) in PBC, stage 4. (b), (d) Most NCAM-positive ductular cells (arrows) express cyclin D, whereas there is no cyclin A expression in DRs in PBC, stage 4. Original magnification ×400.
Autophagy, or cellular self-digestion, is a cellular pathway that results from various cellular stresses, such as nutrient starvation, anoxia, and activation of the endoplasmic reticulum stress pathway [
An appropriate cellular stress response is critical for maintaining tissue integrity and function and for preventing diseases [
We have reported the upregulated autophagy in the damaged small bile ducts along with cellular senescence in PBC [
Biliary epithelial autophagy in PBC. (a) The expression of autophagy marker LC3 was not observed in BECs in noninflammed bile ducts (arrow) in PBC. (b) The expression of autophagy marker LC3 was detected in intracytoplasmic vesicles (arrows) in BECs involved in inflamed and damaged small bile ducts in PBC. Immunostaining for LC3. Original magnification, ×400 (inset, ×1000).
An unsolved problem is how autophagy and cellular senescence are involved in the autoimmune-mediated processes such as AMA and other PBC-related autoantigens in PBC. Regarding apoptosis, it has been reported that BECs manifest unique features during apoptosis and that the combination of AMA and BECs apoptotic bodies (apotopes) could activate innate immune response with involvement of some inflammatory cytokines [
The dysfunctional autophagy related to the regulation of immunity may contribute also to chronic inflammatory diseases and probably autoimmune diseases. A well-characterized link is between mutations in autophagy regulators and Crohn’s disease, a chronic inflammatory bowel disease, in which autophagy proteins, ATG16L1, NOD2, and IRGM are reported as susceptibility genes [
Similar to SLE, it is possible that a dysfunctional autophagic process of BECs may play a role in autoimmune pathogenesis, for example, the immune tolerance breakdown of autoantigens, in PBC, although this is only speculative at this moment. Recent genetic studies of PBC including GWAS identified, in a reproducible fashion, genetic associations between PBC and human leukocyte antigen as well as polymorphisms in the genes encoding IL-12
An increasing body of work described the change in the cellular secretosome in senescent cells. Senescent cells play an important role in modulating the microenvironment by secreting biological active molecules, senescence-associated secretory phenotypes (SASPs). SASPs include diverse proinflammatory factors such as cytokines (IL-6, IL-1 and so on) and chemokines (CXCL8/IL-8, CCL2/monocyte chemotactic protein-1 (MCP)-1) and so on), growth factors and profibrogenic factors [
The upregulation of several cytokines and chemokines in damaged bile ducts in PBC has been reported [
Increased expression of CCL2 in inflamed and damaged bile ducts in PBC. (a) The expression of CCL2 was absent or faint in biliary epithelial cells (BECs) in the small bile duct (arrow) in normal liver (top). CCL2 was extensively expressed in the membrane and cytoplasm of damaged and senescent BECs (arrows) in the early stage of PBC (bottom). Immunostaining for CCL2. Original magnification, ×400. (b) The expression of CCL2 was significantly more frequent and intense in inflamed small bile ducts in PBC, when compared with noninflamed small bile ducts in PBC and small bile ducts in control livers (
Possible regulation of microenvironment by senescent BECs expressing SASPs in PBC. Senescent BECs may function in modulation of the inflammatory microenvironment by recruiting monocytes and possibly other inflammatory cells by secreting chemokines and cytokines as SASPs. Senescent BECs may also participate in the induction of senescence in surrounding cells and progression of fibrosis via SASPs.
The mechanisms that initiate and maintain SASPs have not been clarified, so far [
PBC is thought to result from a combination of multiple genetic factors and superimposed environmental triggers and apparently belongs to the “complex disease” category like most polygenic autoimmune diseases. Even though mitochondrial autoantigens and B-cell and T-cell autoepitopes have been well characterized in PBC, the pathogenesis of characteristic bile duct lesion and the exact role of AMA still remain to be elucidated. In this paper, we focused on a possible involvement of two novel cellular processes, autophagy and cellular senescence in BECs in bile duct lesions in PBC. Autophagy is expected to be a promising cellular mechanism involved in the autoimmune mechanism together with apoptosis. Cellular senescence may play a role in the immunopathology of BECs by expressing SASPs in PBC. Further studies are needed to disclose the autoimmune pathogenesis of PBC.
This study was supported in part by a Grant-in Aid for Scientific Research (C) from the Ministry of Education, Culture, Sports and Science and Technology of Japan (18590325 and 2590366).