Rat hematopoietic cell receptors for very low density lipoprotein (VLDL) growth inhibitor

Physiologic concentrations of rat plasma very low density lipoproteins (VLDL) have profound inhibitory effects in tissue culture on the proliferation of hormonally stimulated bone marrow granulocytic and erythrocytic cells and mitogen-stimulated spleen cells. In this study we have evaluated the in vitro uptake and binding of 125I-VLDL by rat marrow cells and spleen lymphocytes. To this end, VLDL were isolated from rat plasma by sequential ultracentrifugation flotation and radioiodinated using iodine monochloride. Biological activity of 125I-VLDL was ascertained by demonstrating that 125I-VLDL and native VLDL had comparable proliferative inhibitory effects when added to erythropoietin-stimulated marrow cells and PHA-stimulated lymphocytes. After 1 h exposure of marrow to VLDL, exhaustive cell washing did not reverse the lipoprotein growth inhibitory effect. The cell uptake of VLDL was evaluated by adding 125I-VLDL to marrow or spleen cells. Uptake of 125I-VLDL by rat cells showed a preference for binding of VLDL as compared to chylomicrons, LDL, or HDL. Based on Scatchard plot analysis of 125I-VLDL binding at 37 degrees C, the approximate number of saturable VLDL receptors available per marrow or spleen cell during a 3 h incubation was 34,000 and 63,000, respectively. We conclude that rat marrow cells and lymphocytes have specific receptors for plasma VLDL and that receptor binding of VLDL is an initial step in its growth inhibitory effect. The physiologic role of plasma lipoprotein cell growth inhibitors will remain speculative, however, until the in vivo distribution of the biologically active lipoprotein moiety to extravascular sites of hematopoiesis has been determined.