Purification and characterization of a vimentin-specific protease in mouse myeloid leukemia cells. Regulation during differentiation and identity with cathepsin G

Proteomic Analysis Revealed the Important Role of Vimentin in Human Cervical Carcinoma HeLa Cells Treated With Gambogic Acid

Gambogic acid (GA) is an anticancer agent in phase IIb clinical trial in China. In HeLa cells, GA inhibited cell proliferation, induced cell cycle arrest at G2/M phase and apoptosis, as showed by results of MTT assay and flow cytometric analysis. Possible target-related proteins of GA were searched using comparative proteomic analysis (2-DE) and nine proteins at early (3 h) stage together with nine proteins at late (24 h) stage were found. Vimentin was the only target-related protein found at both early and late stage. Results of both 2-DE analysis and Western blotting assay suggested cleavage of vimentin induced by GA. MS/MS analysis of cleaved vimentin peptides indicated possible cleavage sites of vimentin at or near ser51 and glu425. Results of targeted proteomic analysis showed that GA induced change in phosphorylation state of the vimentin head domain (aa51-64). Caspase inhibitors could not abrogate GA-induced cleavage of vimentin. Over-expression of vimentin ameliorated cytotoxicity of GA in HeLa cells. The GA-activated signal transduction, from p38 MAPK, heat shock protein 27 (HSP27), vimentin, dysfunction of cytoskeleton, to cell death, was predicted and then confirmed. Results of animal study showed that GA treatment inhibited tumor growth in HeLa tumor-bearing mice and cleavage of vimentin could be observed in tumor xenografts of GA-treated animals. Results of immunohistochemical staining also showed down-regulated vimentin level in tumor xenografts of GA-treated animals. Furthermore, compared with cytotoxicity of GA in HeLa cells, cytotoxicity of GA in MCF-7 cells with low level of vimentin was weaker whereas cytotoxicity of GA in MG-63 cells with high level of vimentin was stronger. These results indicated the important role of vimentin in the cytotoxicity of GA. The effects of GA on vimentin and other epithelial-to-mesenchymal transition (EMT) markers provided suggestion for better usage of GA in clinic.

Low cytoplasmic pH reduces ER-Golgi trafficking and induces disassembly of the Golgi apparatus

The Golgi apparatus was dramatically disassembled when cells were incubated in a low pH medium. The cis-Golgi disassembled quickly, extended tubules and spread to the periphery of cells within 30 min. In contrast, medial- and trans-Golgi were fragmented in significantly larger structures of smaller numbers at a slower rate and remained largely in structures distinct from the cis-Golgi. Electron microscopy revealed the complete disassembly of the Golgi stack in low pH treated cells. The effect of low pH was reversible; the Golgi apparatus reassembled to form a normal ribbon-like structure within 1-2h after the addition of a control medium. The anterograde ER to Golgi transport and retrograde Golgi to ER transport were both reduced under low pH. Phospholipase A2 inhibitors (ONO, BEL) effectively suppressed the Golgi disassembly, suggesting that the phospholipase A2 was involved in the Golgi disassembly. Over-expression of Rab1, 2, 30, 33 and 41 also suppressed the Golgi disassembly under low pH, suggesting that they have protective role against Golgi disassembly. Low pH treatment reduced cytoplasmic pH, but not the luminal pH of the Golgi apparatus, strongly suggesting that reduction of the cytoplasmic pH triggered the Golgi disassembly. Because a lower cytoplasmic pH is induced in physiological or pathological conditions, disassembly of the Golgi apparatus and reduction of vesicular transport through the Golgi apparatus may play important roles in cell physiology and pathology. Furthermore, our findings indicated that low pH treatment can serve as an important tool to analyze the molecular mechanisms that support the structure and function of the Golgi apparatus.

Effect of serine and tyrosine phosphorylation on retroviral proteinase substrates

Vimentin, a cellular substrate of HIV type 1 (HIV-1) proteinase, contains a protein kinase C (PKC) phosphorylation site at one of its cleavage sites. Peptides representing this site were synthesized in P2 Ser-phosphorylated and nonphosphorylated forms. While the nonphosphorylated peptide was a fairly good substrate of the enzyme, phosphorylation prevented hydrolysis. Phosphorylation of human recombinant vimentin by PKC prevented its processing within the head domain, where the phosphorylation occurred. Oligopeptides representing naturally occurring cleavage sites at the C-terminus of the Rous sarcoma virus integrase were assayed as substrates of the avian proteinase. Unlike the nonphosphorylated peptides, a Ser-phosphorylated peptide was not hydrolyzed by the enzyme at the Ser-Pro bond, suggesting the role of previously established phosphorylation in processing at this site. Ser-phosphorylated and Tyr-phosphorylated forms of model substrates were also tested as substrates of the HIV-1 and the avian retroviral proteinases. In contrast to the moderate effect of P4 Ser phosphorylation, phosphorylation of P1 Tyr prevented substrate hydrolysis by HIV-1 proteinase. Substrate phosphorylation had substantially smaller effects on the hydrolysis by the avian retroviral proteinase. As the active retroviral proteinase as well as various protein kinases are incorporated into mature virions, substrate phosphorylation resulting in attenuation or prevention of proteolytic processing may have important consequences in the regulation of the retroviral life cycle as well as in virus-host cell interactions.

Regulation of vimentin expression and protease-mediated vimentin degradation during differentiation of human monocytic leukemia cells

Terminal differentiation of human monocytic leukemia THP-1 cells is induced in vitro by 12-O-tetradecanoylphorbol-13-acetate (TPA). We investigated the effects of TPA on the expression of vimentin during the differentiation of THP-1 cells at both the mRNA and the protein level. On northern blotting analysis, a 2.1 kb vimentin mRNA was up-regulated by TPA. On western blotting, small vimentin molecules with a molecular mass of approximately 40 kDa were observed in the soluble fraction and increased with TPA-induction of cellular differentiation. Since larger, including intact, vimentin molecules were detectable at a high TPA dose, we assessed the possible existence of protease activity directed against vimentin in THP-1 cells. With incubation of the cellular lysates of THP-1 cells, the endogenous vimentin became increasingly smaller over time, suggesting the presence of a vimentin-degrading protease. Phenylmethylsulfonyl fluoride inhibited this apparent protease activity against vimentin, suggesting the enzyme involved to be a serine protease. Interestingly, the protease activity was down-regulated by TPA treatment. TPA-treated THP-1 cells were found to express a vimentin-filament network based on immunocytochemical analysis using an anti-vimentin monoclonal antibody, V9. Taken together, these observations suggest that post-translational mechanisms work in cooperation with transcriptional regulation to maintain the vimentin-intermediate filament structure in differentiated THP-1 cells.

Transcriptional regulation of the vimentin-encoding gene in mouse myeloid leukemia M1 cells

To investigate the regulatory mechanisms controlling expression of the vimentin-encoding gene (Vim) during mouse myeloid leukemia M1 cell differentiation, mouse Vim was cloned and the transcriptional activity of its 5' promoter region was analysed by chloramphenicol acetyltransferase (CAT) assay. Analyses of various deletion mutants revealed that a 188-bp fragment of the proximal Vim promoter (pVim) was sufficient for effective transcription in M1 cells. This 188-bp sequence is highly conserved between mouse, hamster and human. Further deletion analyses revealed that a minimum promoter element (-44 to +26) is essential for basic promoter function and could respond to cell differentiation. Detailed analyses of mutant and chimeric pVim constructs defined a CCAAT box at -89 to -84 to be an essential positive regulatory element. A G+C-rich element between the CCAAT and TATA boxes was found to act as a strong negative regulatory element in Vim transcription.

Unassembled (soluble) vimentin in human myeloid leukemia cell line HL60

The intermediate filament proteins which include vimentin, desmin, and the keratins are one of three major classes of cytoskeletal proteins in eukaryotic cells. In this study we found that most of the vimentin of undifferentiated HL60 and cells induced to differentiate either along the monocytoid pathway by 12-O-tetradecanoylphorbol-13-acetate (TPA) or along the granulocytic pathway by retinoic acid was soluble in a buffer containing 1% Triton X-100/0.6 mol/l KCl in which the intermediate filament proteins usually are not soluble. HL60 vimentin separated on polyacrylamide gel electrophoresis into two proteins of Mr 55,000 and 54,000 that we detected by immunoblotting. The Mr 55,000 species was the major form in undifferentiated HL60 cells and cells induced by retinoic acid. The distribution of both forms of vimentin changed during induction of differentiation by TPA and after 24 h the Mr 54,000 species was predominant. After an additional 24 h exposure to TPA the relative levels of the two forms of vimentin approached equivalence and a high level of vimentin degradation products was seen. These results suggest that TPA may increase vimentin degradation along a pathway that has a Mr 54,000 intermediate. In addition, the high levels of soluble vimentin in HL60 cells suggests that these cells may be a good model for studying components involved in vimentin assembly.