Keratins 8 (K8) and 18 (K18) are major components of intermediate filaments (IFs) of simple epithelial cells and tumors derived from such cells. Structural cell changes during apoptosis are mediated by proteases of the caspase family. During apoptosis, K18 IFs reorganize into granular structures enriched for K18 phosphorylated on serine 53. K18, but not K8, generates a proteolytic fragment during drug- and UV light–induced apoptosis; this fragment comigrates with K18 cleaved in vitro by caspase-6, -3, and -7. K18 is cleaved by caspase-6 into NH2-terminal, 26-kD and COOH-terminal, 22-kD fragments; caspase-3 and -7 additionally cleave the 22-kD fragment into a 19-kD fragment. The cleavage site common for the three caspases was the sequence VEVD/A, located in the conserved L1-2 linker region of K18. The additional site for caspases-3 and -7 that is not cleaved efficiently by caspase-6 is located in the COOH-terminal tail domain of K18. Expression of K18 with alanine instead of serine at position 53 demonstrated that cleavage during apoptosis does not require phosphorylation of serine 53. However, K18 with a glutamate instead of aspartate at position 238 was resistant to proteolysis during apoptosis. Furthermore, this cleavage site mutant appears to cause keratin filament reorganization in stably transfected clones. The identification of the L1-2 caspase cleavage site, and the conservation of the same or very similar sites in multiple other intermediate filament proteins, suggests that the processing of IFs during apoptosis may be initiated by a similar caspase cleavage.

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Caspase Cleavage of Keratin 18 and Reorganization of Intermediate Filaments during Epithelial Cell Apoptosis

Entactin, a novel basal lamina-associated sulfated glycoprotein.

Properties of a basement membrane-related glycoprotein synthesized in culture by a mouse embryonal carcinoma-derived cell line

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/0014-5793%2880%2980654-5

Basement membrane glycoprotein laminin binds to heparin

Characterization of a novel serum albumin-binding glycoprotein secreted by endothelial cells in culture.

Summary The embryonal carcinoma cell line PCC4-F was shown to differentiate in vitro to a variety of cell types. A combination of light and electron microscopy and histochemical methods identifies or strongly suggests the presence of cardiac and skeletal muscle, fibroblasts, ciliated and nonciliated epithelial cells, endodermal yolk sac cells, neurally derived cells, and fat cells. Two cell lines were isolated from differentiated cultures. Preliminary comparisons have been made between these cell lines and the undifferentiated embryonal carcinoma cells. The derived cell lines produce an extracellular fibrous substance that gives a positive reaction to the periodate-Schiff reagent indicating a glycoprotein nature. There are significant differences in the incorporation of d-[3H]glucosamine and l-[3H]fucose into soluble extracellular proteins when the derived cell lines are compared with the embryonal carcinoma. These extracellular glycoproteins fall within narrow size classes. Enzyme studies indicate the presence of a new lactate dehydrogenase isozyme which is absent or greatly diminished in the derived cell lines. The specific activity of alkaline phosphatase is dramatically decreased in the derived cell lines, whereas those of acid phosphatase, lactate dehydrogenase, and glucose-6-phosphate dehydrogenase were not significantly different. Comparison of the derived lines with the properties of parietal endodermal yolk sac cells described by Lehman et al. (J. Cellular Physiol., 84: 13–28, 1974) suggests that these new lines are very similar if not identical in properties.

https://cancerres.aacrjournals.org/content/canres/37/7_Part_1/2072.full.pdf

Christine A. Chung

Papers

Morphological and biochemical observations on cells derived from the in vitro differentiation of the embryonal carcinoma cell line PCC4-F.