Epiplakin attenuates experimental mouse liver injury by chaperoning keratin reorganization

Abstract

Background & AimsEpiplakin is a member of the plakin protein family and exclusively expressed in epithelial tissues where it binds to keratins. Epiplakin-deficient (Eppk1−/−) mice displayed no obvious spontaneous phenotype, but their keratinocytes showed a faster keratin network breakdown in response to stress. The role of epiplakin in the stressed liver remained to be elucidated.MethodsWild-type (WT) and Eppk1−/− mice were subjected to common bile duct ligation (CBDL) or fed with a 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC)-containing diet. The importance of epiplakin during keratin reorganization was assessed in primary hepatocytes.ResultsOur experiments revealed that epiplakin is expressed in hepatocytes and cholangiocytes, and binds to keratin 8 (K8) and K18 via multiple domains. In several liver stress models epiplakin and K8 genes displayed identical expression patterns and transgenic K8 overexpression resulted in elevated hepatic epiplakin levels. After CBDL and DDC treatment, Eppk1−/− mice developed a more pronounced liver injury and their livers contained larger amounts of hepatocellular keratin granules, indicating impaired disease-induced keratin network reorganization. In line with these findings, primary Eppk1−/− hepatocytes showed increased formation of keratin aggregates after treatment with the phosphatase inhibitor okadaic acid, a phenotype which was rescued by the chemical chaperone trimethylamine N-oxide (TMAO). Finally, transfection experiments revealed that Eppk1−/− primary hepatocytes were less able to tolerate forced K8 overexpression and that TMAO treatment rescued this phenotype.ConclusionOur data indicate that epiplakin plays a protective role during experimental liver injuries by chaperoning disease-induced keratin reorganization.

Keratins Are Altered in Intestinal Disease-Related Stress Responses

“…Since the overall rate of mRNA-protein turnover is rapid in the colonic epithelium which requires constant renewal, and the time point when protein changes can be detected unlikely overlaps with the peak time point for mRNA levels, the situation in the colon is somewhat different than in liver [ 23 ], kidney [ 30 ] and exocrine pancreas [ 28 ] where keratin overexpression in response to disease-related stress has been detected on both protein and mRNA levels. In the CCl 4 liver fibrosis model, a similar phenotype of unchanged mRNA levels for K8 was, however, seen in response to a chronic ongoing liver inflammation [ 25 ]. Although keratin mRNA and protein expression in diseased murine kidneys show an increase in keratin levels, the increase in protein levels are still not proportional to the mRNA-levels, suggesting an involvement of translational regulation of keratin expression [ 30 ].…”

“…HSPs are upregulated on both mRNA and protein levels upon stress [ 21 ]. IFs and keratins are similarly upregulated and modified in stress situations [ 9 , 22 ] and during recovery from stress, e.g., as seen in liver [ 23 , 24 , 25 , 26 , 27 ], pancreas [ 28 , 29 ], kidney [ 30 ], lung [ 31 ], and skin [ 32 , 33 , 34 ]. Contrary to increased hepatic K8 and K18 levels in human liver disease [ 23 ], colonic K8, K18 and K19 levels have recently been reported to decrease in human colon during inflammatory stress, as observed in ulcerative colitis [ 35 ].…”

Effects of Plectin Depletion on Keratin Network Dynamics and Organization

“…Possible reasons for the comparatively mild effects of plectin depletion on keratin cycling and network organization are differences between transformed and non-transformed cells and molecular redundancy. Multiple plakins are expressed in epithelial tissues, some of which have been implicated in keratin network organization [ 53 – 55 ]. Furthermore, other mechanisms may contribute to keratin dynamics such as disulfide bounding which has been shown to stabilize K5/K14 networks [ 11 ].…”

Keratin intermediate filaments: intermediaries of epithelial cell migration

“…Depletion of plakins affects migration albeit in different ways with loss of plectin, periplakin and desmoplakin increasing migration and loss of epiplakin decreasing it [55][56][57][58][59]. It is assumed that these effects are caused by altered KF network organization [55,56,[59][60][61][62][63]]. Yet, depletion of the major cytoskeletal cross-linker, plectin, has surprisingly little if any effect on cytoplasmic viscoelasticity and KF network dynamics [56,60,64].…”