Selective expression of PDGF A and its receptor during early mouse embryogenesis

Murine homologs of the PDGF A, PDGF B, and PDGF receptor alpha subunit genes were cloned. These were used, together with a mouse PDGF receptor beta subunit cDNA clone, to monitor gene expression in early postimplantation mouse embryos and in F9 embryonal carcinoma cells. RNAse protection analysis shows that PDGF A chain, but not B chain, mRNA is expressed in 6.5- to 8.5-day embryonic and extraembryonic tissues. Both alpha and beta receptor subunit mRNAs are expressed in early embryos, however, alpha subunit mRNA appears earlier and is more abundant than beta subunit mRNA. Undifferentiated F9 embryonal carcinoma stem cells express abundant levels of A chain, but not B chain, mRNA. Neither of the PDGF receptor genes is expressed in stem cells. Treatment with retinoic acid stimulates expression of both PDGF receptor genes. As in postimplantation mouse embryos, alpha receptor subunit mRNA appears earlier and is substantially more abundant than beta subunit mRNA. Collectively, these data demonstrate that the genes encoding the two chains of PDGF and their receptors are regulated independently during development and suggest that the two systems have some nonoverlapping functions in vivo. PDGF A, but not PDGF B, may be particularly important in modulating early events in mouse embryonic development.

Different effects of homo- and heterodimers of platelet-derived growth factor A and B chains on human and mouse fibroblasts

Binding sites for platelet-derived growth factor (PDGF) differ in their selectivity for the AA, AB and BB forms of PDGF. Human fibroblasts bind BB well and AA poorly, whereas Swiss 3T3 cells bind more similar quantities of each ligand. We found that AA PDGF was weakly mitogenic for human fibroblasts, but strongly mitogenic for 3T3 cells. Tyrosine phosphorylation of human fibroblast receptors was stimulated most by BB and least by AA, whereas the phosphorylation of 3T3 cell receptors was stimulated more uniformly by the three dimers. The receptor polypeptides that were phosphorylated were very similar. We suggest that phosphorylation of the receptor is proportional to the number of binding sites available for each ligand. Tyrosine phosphorylation of a number of other cell proteins was also proportional to receptor phosphorylation. In contrast, protein kinase C (PKC)-dependent serine and tyrosine phosphorylations were stimulated maximally by low level occupancy of PDGF binding sites, and phosphorylation of p36 required high occupancy. These data raise the possibility that differences in biological potency of AA, AB and BB forms of PDGF may be due simply to differences in the numbers of binding sites, rather than to different biochemical functions of their receptors.

The role of endothelial injury and platelet and macrophage interactions in atherosclerosis

Atherosclerotic lesions take two forms, the fatty streak and the fibrous plaque or complicated lesion. The former is a flat, lipid-rich lesion containing variable numbers of foam cells in the form of macrophages and/or smooth muscle. In contrast, the fibrous plaque is a proliferative lesion of smooth muscle containing variable numbers of macrophages. Associated with the smooth muscle proliferative response is the formation of connective tissue matrix and the accumulation of intracellular and extracellular lipid. The response to injury hypothesis of atherogenesis provides and approach to exploring the interactions among substances that alter endothelial function and/or structure and to investigating the relationships among endothelium, smooth muscle, platelets, macrophages, and plasma constituents. Studies of molecules potentially important in lesion development, such as mitogens derived from platelets, macrophages, and endothelium, and of the potential roles of lipids in generating these lesions and in inducing injury to the overlying endothelial cells, have begun to shed light on how these processes may occur and on approaches that may be taken to better understand and possibly prevent them. We still lack an understanding of one of the key events, endothelial injury. The many ways by which endothelial cells may be altered in relation to the hypercholesterolemic environment and to the underlying macrophages in the fatty streaks remain to be explored. Nevertheless, these approaches point the way to the development of new means for prevention, intervention, and diagnosis of atherosclerosis.

Growth factors from platelets, monocytes, and endothelium: their role in cell proliferation

Of the various biological roles assigned to growth factors at the beginning of this article, the factors described here are largely associated with the response to injury. These represent a special type of factor since two of them, PDGF and MDGF, are carried in the circulation by the platelet and the monocyte respectively, and can therefore be delivered to sites where a proliferative response would be an important event in the restitution of tissue continuity. The role of the endothelial-derived growth factor in these phenomena is not clear at present. Atherosclerosis has been suggested to represent a protective proliferative response that has gone awry and become disease. In this instance both PDGF and MDGF could play important roles, since platelets have been associated with the early injury phenomenon and macrophages appear to be present in virtually all phases of the development of the lesions of atherosclerosis from the fatty streak to the fibrous plaque and the complicated lesion. In each of these circumstances the macrophage may be important in lesion progression and possibly in lesion initiation. PDGF may also be important in initiation of some lesions, and in some instances would undoubtedly participate in the fibroproliferative response that occurs during organization of a thrombus.