Proteolysis of vimentin and desmin by the Ca2 -activated proteinase specific for these intermediate filaments. Mol Cell Biol. 1983;3:1146-1156. [PMID: 6308428 DOI: 10.1128/mcb.3.6.1146]

Differential effects of calcium deprivation on the cytoskeleton of non-tumorigenic and tumorigenic rat liver cells in culture

Normal rat liver T51B epithelial cells and Morris no. 7795 hepatoma cells growing exponentially were exposed for 24 h to standard medium containing low (0.02 mM) calcium, a concentration which drastically reduces the proliferation of normal but not tumour cells. Cell surface morphology was examined by scanning electron microscopy (SEM); and the distribution and organization of microtubules, cytokeratin and vimentin filaments, and microfilaments were analysed by indirect immunofluorescence microscopy using specific antibodies. Calcium deprivation caused the loss of intercellular cohesion in both cell types and the appearance of some microvilli and blebs, particularly on tumour cells. However, marked differential (normal vs tumour cells) effects on the organizational integrity of the cytoskeleton fibrillar network were observed. Extracellular calcium deprivation led to a particular rearrangement of microtubules, and a perinuclear accumulation of cytokeratin and vimentin filaments in normal, but not in tumour cells. A massive concentration of actin-containing microfilaments was observed in the cell periphery and blebs of hepatoma cells. In the light of the possible involvement of calcium in controlling cytoskeleton assembly, the differing cytoskeletal changes of the two cell types may be linked to their different proliferative capabilities in low-calcium medium.

Kinase activities associated with calcium-activated neutral proteases

Studies on the phosphorylation of calcium-activated neutral protease (CANP) revealed the presence of kinase activities closely associated with purified CANP preparations. The kinase activity in uCANP (CANP with high affinity for calcium) was cAMP-independent whereas the kinase activity in mCANP (CANP with low affinity for calcium) was cAMP-dependent, inhibited by kinase specific inhibitor and abolished when the mCANP was preincubated in calcium. The CANP-associated kinase(s) phosphorylate uCANP and mCANP , causing modulation of their proteolytic activities.

Cysteine proteinases and metastasis

Cysteine proteinases are a subclass of endopeptidases which require activation by thiol reagents. A tumor cysteine proteinase which appears to be related to lysosomal cathepsin B has been implicated in the ability of tumor cells to invade the extracellular matrix and to metastasize to secondary sites. Lysosomal cathepsin B can degrade such components of the extracellular matrix as collagen, fibronectin and proteoglycans. Activity of this cathepsin B-like cysteine proteinase (CB) has been correlated with tumor malignancy in a number of tumor lines yet not in all tumor lines studied. CB activity in tumors seems to be associated with the viable tumor cells, probably with the plasma membrane of these tumor cells. CB activity has been measured in the sera, urine, ascites fluid and pancreatic fluid of tumor-bearing patients. CB is released from tumor explants and tumor cells in vitro as well as from normal subcutaneous tissue exposed to tumor-conditioned medium. Cathepsin B from normal tissues is rapidly inactivated above pH 7.0. Therefore, CB in tumor cell membranes or released from tumor cells (or from host cells in response to tumor cells) may not possess proteolytic activity at neutral pH and thus may not facilitate tumor cell invasion. However, CB exhibits enhanced stability at neutral or slightly alkaline pH's. There is not yet definitive proof that CB plays a role in tumor invasion and metastasis. There is, however, an increasing body of correlative evidence relating CB activity and tumor malignancy. This correlative evidence plus preliminary evidence that tumor CB can degrade components of the extracellular matrix in vitro suggests that CB may be one proteinase active in a proteolytic cascade resulting in tumor invasion and metastasis.

Large scale isolation, purification, and partial characterization of the intermediate filament-specific, Ca2+-activated proteinase from porcine kidney and Ehrlich ascites tumor cells: a comparative study

The Ca2+-activated, neutral thiol proteinase specific for intermediate filament subunit proteins was isolated at large scale from the postribosomal supernatant of a low-ionic-strength extract of porcine kidney and Ehrlich ascites tumor (EAT) cells, respectively. The purification procedure encompassed DEAE-Sephacel ion exchange chromatography of the material precipitating between 23 and 55% (NH4)2SO4 saturation, followed by hydroxylapatite chromatography and activated thiol Sepharose 4B covalent chromatography. On the average, 25 mg of 62% pure enzyme was obtained from 500 g frozen kidney and 55 mg of 51% pure enzyme from 500 g EAT cells within a week. Both enzyme preparations were free of Ca2+-independent proteolytic activities and indistinguishable with respect to their physicochemical and functional properties; their catalytic properties were indistinguishable from those of enzyme purified to homogeneity on arginine methylester Sepharose 4B. Because of this identity, porcine kidney proves to be an inexpensive source for the Ca2+-activated proteinase which had previously been isolated and purified at small scale from EAT cells (W.J. Nelson and P. Traub, (1983) J. Biol. Chem. 257, 5544-5553). Despite a 38% protein contamination, the partially purified enzyme from porcine kidney is useful for the in vitro study of structure-function relationships of intermediate filaments and their subunit proteins. During affinity chromatography of the partially purified proteinase from EAT cells on arginine methylester Sepharose 4B, a 100-kDa protein was purified which has a high affinity for arginine residues. It also occurs in porcine kidney, although at a considerably lower concentration. Its cellular localization and function remain to be determined.