Alterations of intermediate filaments in various histopathological conditions

Ubiquitin-proteasome-mediated degradation of keratin intermediate filaments in mechanically stimulated A549 cells

We previously reported that shear stress induces phosphorylation and disassembly of keratin intermediate filaments (IFs). Shear stress also induces a time- and strain-dependent degradation of keratin IFs, and the current study examines the mechanisms involved in degradation of keratin proteins in human A549 cells exposed to 0-24 h of shear stress (7.5-30 dynes/cm(2)). Ubiquitin was found to be covalently associated with keratin proteins immunoprecipitated from shear-stressed cells, and pretreatment with the proteasomal inhibitor MG132 prevented the degradation of the keratin IF network. Importantly, phosphorylation of K8 Ser-73 is required for the shear stress-mediated ubiquitination, disassembly, and degradation of the keratin IF network. Immunofluorescence microscopy revealed that shear stress caused the thin array of keratin fibrils observed in control cells to be reorganized into a perinuclear aggregate, known as an aggresome, and that ubiquitin was also associated with this structure. Finally, the E2 enzymes, UbcH5b, -c, and Ubc3, but not E2-25K are required for the shear stress-mediated ubiquitin-proteasomal degradation of keratin proteins. These data suggest that shear stress promotes the disassembly and degradation of the keratin IF network via phosphorylation and the ubiquitin-proteasome pathway.

Pharmacologic transglutaminase inhibition attenuates drug-primed liver hypertrophy but not Mallory body formation

Mallory bodies (MBs) are characteristic of several liver disorders, and consist primarily of keratins with transglutaminase-generated keratin crosslinks. We tested the effect of the transglutaminase-2 (TG2) inhibitor KCC009 on MB formation in a mouse model fed 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC). KCC009 decreased DDC-induced liver enlargement without affecting MB formation or extent of liver injury. TG2 protein and activity increased after DDC feeding and localized within and outside hepatocytes. KCC009 inhibited DDC-induced hepatomegaly by affecting hepatocyte cell size rather than proliferation. Hence, TG2 is a potential mediator of injury-induced hepatomegaly via modulation of hepatocyte hypertrophy, and KCC009-mediated TG2 inhibition does not affect mouse MB formation.

Identification of novel principles of keratin filament network turnover in living cells

It is generally assumed that turnover of the keratin filament system occurs by exchange of subunits along its entire length throughout the cytoplasm. We now present evidence that a circumscribed submembranous compartment is actually the main site for network replenishment. This conclusion is based on the following observations in living cells synthesizing fluorescent keratin polypeptides: 1) Small keratin granules originate in close proximity to the plasma membrane and move toward the cell center in a continuous motion while elongating into flexible rod-like fragments that fuse with each other and integrate into the peripheral KF network. 2) Recurrence of fluorescence after photobleaching is first seen in the cell periphery where keratin filaments are born that translocate subsequently as part of the network toward the cell center. 3) Partial keratin network reformation after orthovanadate-induced disruption is restricted to a distinct peripheral zone in which either keratin granules or keratin filaments are transiently formed. These findings extend earlier investigations of mitotic cells in which de novo keratin network formation was shown to originate from the cell cortex. Taken together, our results demonstrate that the keratin filament system is not homogeneous but is organized into temporally and spatially distinct subdomains. Furthermore, the cortical localization of the regulatory cues for keratin filament turnover provides an ideal way to adjust the epithelial cytoskeleton to dynamic cellular processes.

Induction of rapid and reversible cytokeratin filament network remodeling by inhibition of tyrosine phosphatases

The cytokeratin filament network is intrinsically dynamic, continuously exchanging subunits over its entire surface, while conferring structural stability on epithelial cells. However, it is not known how cytokeratin filaments are remodeled in situations where the network is temporarily and spatially restricted. Using the tyrosine phosphatase inhibitor orthovanadate we observed rapid and reversible restructuring in living cells, which may provide the basis for such dynamics. By examining cells stably expressing fluorescent cytokeratin chimeras, we found that cytokeratin filaments were broken down and then formed into granular aggregates within a few minutes of orthovanadate addition. After drug removal, gradual reincorporation of granules into the filament network was observed for aggregates that were either part of residual filaments or stayed in close apposition to remaining filaments. Even when cytokeratin filaments were no longer detectable, granules with low mobility were still able to reestablish a cytokeratin filament network. This process took less than 30 minutes and occurred at multiple foci throughout the cytoplasm without apparent correlation to alterations in the actin- and tubulin-based systems. Interestingly, the short-lived and rather small orthovanadate-induced cytokeratin granules contained the cytoskeletal crosslinker plectin but lacked the cytokeratin-solubilising 14-3-3 proteins. By contrast, the long-lived and larger cytokeratin aggregates generated after treatment with the serine/threonine phosphatase inhibitor okadaic acid were negative for plectin but positive for 14-3-3 proteins. Taken together, our observations in living orthovanadate-treated interphase cells revealed modes of cytokeratin remodeling that qualify as basic mechanisms capable of rapidly adapting the cytokeratin filament cytoskeleton to specific requirements.

Protein levels of neurofilament subunits in the hen central nervous system following prevention and potentiation of diisopropyl phosphorofluoridate (DFP)-induced delayed neurotoxicity(1)

Diisopropyl phosphorofluoridate (DFP) is an organophosphorus ester, which produces delayed neurotoxicity (OPIDN) in hens in 7-14 days. OPIDN is characterized by mild ataxia in its initial stages and severe ataxia or paralysis in about 3 weeks. It is marked by distal swollen axons, and exhibits aggregations of neurofilaments (NFs), microtubules, proliferated smooth endoplasmic reticulum, and multivesicular bodies. These aggregations subsequently undergo disintegration, leaving empty varicosities. Previous studies in this laboratory have shown an increased level of medium-molecular weight NF (NF-M) and decreased levels of high- and low-molecular weight NF (NF-H, NF-L) proteins in the spinal cord of DFP-treated hens. The main objective of this investigation was to study the effect of DFP administration on NF subunit levels when OPIDN is prevented or potentiated by pretreatment or post-treatment with phenylmethylsulfonyl fluoride (PMSF), respectively. Hens pretreated or post-treated with PMSF were killed 1, 5, 10, and 20 days after the last treatment. The alteration in NF subunit protein levels observed in DFP-treated hen spinal cords was not observed in protected hens. Estimation of NFs in the potentiation experiments, however, showed a different pattern of alteration in NF subunit levels. The results showed that an alteration in NF subunit levels in DFP-treated hens might be related to the development of OPIDN, since these changes were suppressed in PMSF-protected hens. However, results from PMSF post-treated hen spinal cords suggested that potentiation of OPIDN by PMSF was mediated by a mechanism different from that followed by DFP alone to produce OPIDN.