Platelet alpha granules contain a growth factor for fibroblasts.
Platelets contain a polypeptide growth factor that stimulates the replication of normal connective tissue cells; this platelet-derived growth factor (PDGF) is released during the clotting process. Human platelets from normal volunteers were disrupted by nitrogen cavitation, and the subcellular organelles were fractionated by ultracentrifugation through a 30%--60% sucrose gradient. Electron microscopy revealed that fraction 7 (density 1.23 g/liter) contained the largest number of alpha granules. The specific activity of platelet fibrinogen, an alpha-granule marker, was also highest in this fraction. The subcellular fractions were assay for the presence of PDGF and for beta-thromboglobulin. PDGF was assayed quantitatively by the stimulation of DNA synthesis in confluent growth-arrested BALB/c-3T3 cells, whereas the concentration of beta-thromboglobulin was determined by radioimmunoassay. The highest concentrations of both PDGF and beta-thromboglobulin were found in the alpha-granule fraction. In contrast, beta-glucuronidase, a lysosomal enzyme, was more diffusely distributed and had its highest specific activity in fractions of lower density than those for PDGS, beta-thromboglobulin, or fibrinogen. The data demonstrate that the alpha granules of platelets provide a unique delivery system for PDGF, a polypeptide hormone with growth-promoting activity for connective tissue cells.
D000818:Animals; D001792:Blood Platelets / Q000378:metabolism* / Q000648:ultrastructure; D002499:Centrifugation, Density Gradient; D003594:Cytoplasmic Granules / Q000378:metabolism*; D004247:DNA / Q000096:biosynthesis; D005340:Fibrinogen; D005347:Fibroblasts / Q000378:metabolism; D005966:Glucuronidase; D006133:Growth Substances / Q000302:isolation & purification*; D006358:Hot Temperature; D006801:Humans; D051379:Mice; D008807:Mice, Inbred BALB C; D010455:Peptides / Q000302:isolation & purification; D013347:Subcellular Fractions / Q000378:metabolism
D006133:Growth Substances; D010455:Peptides; D005340:Fibrinogen; D004247:DNA; D005966:Glucuronidase