| Abstract: | We have investigated the nature and distribution of different populations of heparan sulfate proteoglycans (HSPGs) in several cell lines in culture. Clone 9 hepatocytes and NRK and CHO cells were biosynthetically labeled with 35SO4, and proteoglycans were isolated by DEAE-Sephacel chromatography. Heterogeneous populations of HSPGs and chondroitin/dermatan proteoglycans (CSPGs) were found in the media and cell layer extracts of all cultures. HSPGs were further purified from the media and cell layers and separated from CSPGs by ion exchange chromatography after chondroitinase ABC digestion. In all cell types, HSPGs were found both in the cell layers (20-70% of the total) as well as the medium. When the purified HSPG fractions were further separated by octyl-Sepharose chromatography, very little HSPG in the incubation media bound to the octyl-Sepharose, whereas 40-55% of that in the cell layers bound and could be eluted with 1% Triton X-100. This hydrophobic population most likely consists of membrane-intercalated HSPGs. Basement membrane-type HSPGs were identified by immunoprecipitation as a component (30-80%) of the unbound (nonhydrophobic) HSPG fraction. By immunofluorescence, basement membrane-type HSPGs were distributed in a reticular network in Clone 9 and NRK cell monolayers; by immunoelectron microscopy, these HSPGs were localized to irregular clumps of extracellular matrix located beneath and between cells. The cells did not produce a morphologically recognizable basement membrane layer under these culture conditions. When membrane-associated HSPGs were localized by immunoelectron microscopy, they were found in a continuous layer along the cell membrane of all cell types. The results demonstrate that two antigenically distinct populations of HSPG--an extracellular matrix and a membrane-intercalated population--are found at the surface of several different cultured cells lines; these populations can be distinguished from one another by differences in their distribution in the monolayers by immunocytochemistry and can be separated by hydrophobic chromatography; and basement membrane-type HSPGs are secreted and deposited in the extracellular matrix by cultured cells even though they do not produce a bona fide basement membrane-like layer. |
