Production of TCII (Vitamin B12 Transport Protein) by Mouse Mononuclear Phagocytes

Abstract

The role of the mononuclear-macrophage system in the production of transcobalamm II (TCII), the vitamin B12 transport protein that delivers the vitamin to the tissues, was investigated in mice. Of all the organs examined for TCII content in unstimulated mice, highest TCll levels were found in the bone marrow. A considerable amount of TCll was present in peripheral blood monocytes. Small amounts of TCII were present in peritoneal exudate cells (PEC) and in the spleen. TCII was undetectable in the thymus and in lymph nodes. Following a single intraperitoneal injection of thioglycolate (TG), a significant increase of TCII concentration was observed in PEC, with a concomitant drop of TCII in the bone marrow. Cultures of PEC harvested from unstimulated and TG-stimulated mice synthesized and secreted considerable amounts of TCll into the medium. After a lag period of several hours the concentration of TCll in the culture mcdi urn increased constantly throughout the entire period of incubation (5-7 days). PEC from stimulated and unstimulated mice were separated into adherent and nonadherent populations. Only the adherent cells, i.e., the macrophages, were consistently found to produce TCII in vitro. These findings show that macrophages produce and secrete TCII. Macrophages, however, did not contain R binders (TCI and TCIII). The observation that concomitant with the rise of TCll in PEC following stimulation there was a marked fall of TCII in the bone marrow indicates that TCII is produced by precursors of mononuclear-macrophage cells in the bone marrow that migrate to the periphery. OF THE THREE vitamin B12-binding proteins, the transcobalamins (TC) I, II and III, TCII has been shown to transport vitamin B,2 and to deliver it to the tissues.’-2 The site of TCII synthesis is not yet established. Indirect evidence from animal experiments indicates that the liver and possibly other organs may be involved in the synthesis of TCII.3 The transport function of TCII has been shown in vitro on cell systems such as Ehrlich ascites cells, reticubocytes, HeLa cells, transformed lymphocytes, fibroblasts, and mouse leukemic lymphobbasts.6-9 In man the role of TCII in delivering vitamin B,2 to tissues is evidenced by the observation that hereditary TCII deficiency leads to lack of cellular maturation of the hemopoietic system and neonatal megaloblastic anemia in spite of normal serum concentration of vitamin B,2.’#{176} 2 Increased serum TCII levels have been reported in acute leukemia and lymphoma in the active stage of the disease’3”4 as well as in Gaucher disease,’5”6