Cystatins--Extra- and intracellular cysteine protease inhibitors: High-level secretion and uptake of cystatin C in human neuroblastoma cells.
Cystatins are present in mammals, birds, fish, insects, plants, fungi and protozoa and constitute a large protein family, with most members sharing a cysteine protease inhibitory function. In humans 12 functional cystatins exist, forming three groups based on molecular organisation and distribution in the organism. The type 1 cystatins (A and B) are known as intracellular, type 2 cystatins (C, D, E/M, F, G, S, SN and SA) extracellular and type 3 cystatins (L- and H-kininogen) intravascular proteins. The present paper is focused on the human cystatins and especially those of type 2, which are directed (with signal peptides) for cellular export following translation. Results indicating existence of systems for significant internalisation of type 2 cystatins from the extracellular to intracellular compartments are reviewed. Data showing that human neuroblastoma cell lines generally secrete high levels, but also contain high amounts of cystatin C are presented. Culturing of these cells in medium containing cystatin C at concentrations found in body fluids resulted in increased intracellular cystatin C, as a result of an uptake process. At immunofluorescence cytochemistry a pronounced vesicular cystatin C staining was observed. The simplistic denotation of the type 2 cystatins as extracellular inhibitors is thus challenged, and possible biological functions of the internalised cystatins are discussed. To illustrate the special case of high cellular cystatin content seen in cells of patients with hereditary cystatin C amyloid angiopathy, expression vectors for wild-type and L68Q mutated cystatin C were used to transfect SK-N-BE(2) cells. Clones overexpressing the two variants showed increased secreted levels of cystatin C. Within the cells the L68Q variant appeared to mainly localise to the endoplasmic reticulum rather than to acidic vesicular organelles, indicating limitations in the transport out from the cell rather than increased uptake as explanation for the elevated cellular cystatin levels seen in hereditary cystatin C amyloid angiopathy.
Wallin
Hanna
H
Department of Laboratory Medicine, Division of Clinical Chemistry and Pharmacology, Lund University, Skåne University Hospital, SE-221 85 Lund, Sweden.
Bjarnadottir
Maria
M
Vogel
Lotte K
LK
Wassélius
Johan
J
Ekström
Ulf
U
Abrahamson
Magnus
M
D019943:Amino Acid Substitution; D000818:Animals; D045744:Cell Line, Tumor; D055316:Cystatin C / Q000235:genetics / Q000378:metabolism*; D057056:Cysteine Proteases / Q000378:metabolism*; D015853:Cysteine Proteinase Inhibitors / Q000235:genetics / Q000378:metabolism; D005110:Extracellular Space / Q000378:metabolism*; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D042541:Intracellular Space / Q000378:metabolism*; D051379:Mice; D009154:Mutation; D041681:NIH 3T3 Cells; D009447:Neuroblastoma / Q000235:genetics / Q000378:metabolism / Q000473:pathology*; D021381:Protein Transport
D055316:Cystatin C; D015853:Cysteine Proteinase Inhibitors; D057056:Cysteine Proteases