Modification of proteoglycans during maturation of fibroblast substratum adhesion sites

Newly formed adhesion sites, left bound to the tissue culture substratum after [ethylenebis(oxyethylenenitrilo)] tetraacetic acid mediated detachment of simian virus 40 transformed Balb/c 3T3 cells, have been extracted with 0.5 M guanidine hydrochloride or Zwittergent (3-12), extractions which identify different subfractions of proteoglycans in these sites. The compositions of these extracts were then compared to similar extracts of "maturing" adhesion sites in an effort to identify structural and metabolic changes which may occur with time and which may play a role in altering adhesion during cell movement. Guanidine hydrochloride (0.5 M) extracts both hyaluronate and chondroitin sulfate proteoglycan from newly formed sites (but which are not complexed in an aggregate similar to that found in cartilage) but only hyaluronate from fully matured sites, indicating that the chondroitin sulfate proteoglycans somehow become resistant to extraction with time. Both high and low molecular weight forms of hyaluronate also accumulate in sites with time. Zwittergent 3-12 solubilizes free chains of heparan sulfate but not heparan sulfate proteoglycan from either class of sites. Most of the heparan sulfate in newly formed sites occurs as a large proteoglycan excludable from Sepharose CL-6B columns under stringent dissociative conditions; however, as adhesion sites "mature", a portion of this proteoglycan appears to be converted by some unknown mechanism to free heparan sulfate chains. This process may very well weaken the close adhesive contacts between the cell and substratum mediated by fibronectin's binding to the highly multivalent heparan sulfate proteoglycans. These studies further indicate that there is considerable metabolism and changing intermolecular associations of proteoglycans within these sites during movement of fibroblasts over this model extracellular matrix.