Induction of macrophage growth by the lipid moiety of lipoprotein and its augmentation by denaturation of the lipoproteins.

Lipoprotein (d less than 1.21) isolated from mouse tumor ascitic fluid or mouse-serum induced growth of peritoneal macrophages in vitro. Lipoprotein fractions that stimulated macrophage growth were the chylomicron, very-low-density lipoprotein (VLDL), and low-density lipoprotein (LDL), whereas the high-density lipoprotein (HDL) fraction did not. Lipids extracted from total lipoprotein also showed significant macrophage-growth-stimulating activity and lost this activity when hydrolyzed. The macrophage-growth-stimulating activity of the lipoprotein (d less than 1.21) was increased about ten times by heat treatment of the lipoprotein (100 degrees C, 30 min). The HDL fraction that had no activity in the native form also showed activity after heat treatment. Lipoprotein-depleted ascitic fluid and simple proteins such as bovine serum albumin (BSA) had no activity even after heat treatment. These results show that the lipid moiety of lipoproteins caused proliferation of macrophages and that denatured lipoproteins were more effective than native ones.