Induction of thrombospondin messenger RNA levels occurs as an immediate primary response to platelet-derived growth factor.

We have investigated the regulation of mRNA levels for thrombospondin, a platelet-derived growth factor (PDGF)-regulated secreted glycoprotein, in cultures of rat vascular smooth muscle cells (SMC). A thrombospondin cDNA hybridizes to a single 5.5-kilobase SMC message which is greatly induced by serum or PDGF. When quiescent SMC are treated with PDGF, thrombospondin levels are induced rapidly (within 15 min) and in a dose-dependent manner. The induction of thrombospondin message levels parallels PDGF-mediated mitogenesis. PDGF-mediated increases in thrombospondin mRNA can be blocked in the presence of actinomycin D, suggesting that PDGF regulates the thrombospondin message at the level of gene transcription. In the presence of cycloheximide, PDGF "super-induces" the thrombospondin message. The data establish that mRNA levels for thrombospondin are regulated by PDGF in a manner similar to that of c-myc, c-fos, and other growth-regulatory gene products and extend our previous findings that thrombospondin secretion by SMC is dependent on exposure to PDGF. These observations are strongly suggestive of an important role for thrombospondin in the growth response of mesenchymal cells.