Stimulation of large proteoglycan synthesis in cultured smooth muscle cells from pig aorta by endothelial cell-conditioned medium.

We have previously shown (Berrou et al., J. Cell. Phys., 137:430-438, 1988) that porcine endothelial cell-conditioned medium (ECCM) stimulates proteoglycan synthesis by smooth muscle cells from pig aorta. ECCM stimulation requires protein cores for glycosaminoglycan chain initiation and is accompanied by an increase in the hydrodynamic size of proteoglycans secreted into the medium. This work investigates the mechanisms involved in the ECCM effect. 1) Control and ECCM stimulated proteoglycan synthesis (measured by a 20 min [35S]-sulfate labeling assay) was not inhibited by cycloheximide, indicating that the proteoglycans were composed of preexisting protein cores and that ECCM stimulates glycosylation of these protein cores. 2) Whereas ECCM stimulation of [35S]-methionine incorporation into secreted proteins only occurred after a 6 h incubation, the increase in [35S] methionine-labeled proteoglycans was observed after 1 h, and the increase was stable for at least 16 h. 3) As analysed by electrophoresis in SDS, chondroitinase digestion generated from [14C] serine-labeled proteoglycans 7 protein cores of high apparent molecular mass (550-200 kDa) and one of 47 kDa. The two protein cores of highest apparent molecular masses (550 and 460 kDa), but not the 47 kDa protein cores, showed increased [14C]-serine incorporation in response to ECCM (51%, as measured by Sepharose CL-6B chromatography). 4) Finally, incorporation of [35S]-sulfate into chondroitinase-generated glycosaminoglycan linkage stubs on protein cores was determined by Sepharose CL-6B chromatography: ECCM did not modify the ratio [35S]/[14C] in stimulated protein cores, indicating that ECCM did not affect the number of glycosaminoglycan chains. The results of these studies reveal that 1) endothelial cells secrete factor(s) that preferentially stimulate synthesis of the largest smooth muscle cell proteoglycans without structural modifications and 2) the stimulation proceeds via increased glycosylation of protein core through enhancement of xylosylated protein core, followed by enhanced protein synthesis.