Growth factor-dependent regulation of transferrin receptor in proliferating and quiescent macrophages

Proteomic Analysis Reveals Distinct Metabolic Differences Between Granulocyte-Macrophage Colony Stimulating Factor (GM-CSF) and Macrophage Colony Stimulating Factor (M-CSF) Grown Macrophages Derived from Murine Bone Marrow Cells

Macrophages are crucial in controlling infectious agents and tissue homeostasis. Macrophages require a wide range of functional capabilities in order to fulfill distinct roles in our body, one being rapid and robust immune responses. To gain insight into macrophage plasticity and the key regulatory protein networks governing their specific functions, we performed quantitative analyses of the proteome and phosphoproteome of murine primary GM-CSF and M-CSF grown bone marrow derived macrophages (GM-BMMs and M-BMMs, respectively) using the latest isobaric tag based tandem mass tag (TMT) labeling and liquid chromatography-tandem mass spectrometry (LC-MS/MS). Strikingly, metabolic processes emerged as a major difference between these macrophages. Specifically, GM-BMMs show significant enrichment of proteins involving glycolysis, the mevalonate pathway, and nitrogen compound biosynthesis. This evidence of enhanced glycolytic capability in GM-BMMs is particularly significant regarding their pro-inflammatory responses, because increased production of cytokines upon LPS stimulation in GM-BMMs depends on their acute glycolytic capacity. In contrast, M-BMMs up-regulate proteins involved in endocytosis, which correlates with a tendency toward homeostatic functions such as scavenging cellular debris. Together, our data describes a proteomic network that underlies the pro-inflammatory actions of GM-BMMs as well as the homeostatic functions of M-BMMs.

In vitro and in vivo variation in transferrin receptor expression on a human medulloblastoma cell line

The poor prognosis associated with pediatric central nervous system tumors such as medulloblastoma has led to the development and investigation of a variety of new treatment techniques. Therapeutic agents include targeted-toxin conjugates or immunotoxins that show significant in vitro activity against many brain tumors. Transferrin receptors (TRs) are specific, cell-surface antigens that are expressed preferentially on brain tumors rather than on normal human brain tissue. This antigen has been successfully targeted in human and nonhuman brain tumors in vitro and in vivo. In this study, when TRs were used as a target in the DAOY human medulloblastoma-derived cell line in vitro, a significant level of expression was confirmed by testing the sensitivity to different immunotoxins. To ensure the relevance of the in vitro data to the in vivo situation, we also analyzed TR expression in DAOY tumors growing in athymic mice and rats. Immunocytochemistry, immunohistochemistry, immunobead binding, immunofluorescence, 125iodine-transferrin binding, and Northern blot analysis were used to compare TR expression in DAOY cells in vitro and in vivo. All in vitro assays demonstrated significant TR expression, whereas in vivo, the TR expression was negligible in the DAOY tissue. The results caution against extrapolating in vitro antigen and receptor expression data directly to the in vivo situation. Using a transferrin-toxin conjugate in a nude rat model of leptomeningeal carcinomatosis, we achieved therapeutic efficacy, despite demonstrating reduced TR expression on tumor tissue. With respect to clinical efficacy, the reduced expression of TR on DAOY medulloblastoma in vivo may be less significant than expected because of the extreme potency of immunotoxins observed in central nervous system tumors.

Macropinosome maturation and fusion with tubular lysosomes in macrophages

Macropinosomes formed by addition of recombinant macrophage colony-stimulating factor (rM-CSF) to mouse macrophages migrate centripetally and shrink, remaining detectable by phase microscopy for up to 15 min. This longevity allowed us to study how macropinosomes age. Macropinosomes were pulse labeled for 1 min with fixable fluorescein dextran (FDx10f), a probe for fluid phase pinocytosis, and chased for various times. To quantify changes in their antigenic profile, pulse-labeled macropinosomes of different ages were fixed and stained for immunofluorescence with a panel of antibodies specific for the transferrin receptor (TfR), the late endosome-specific, GTP-binding protein rab 7 or lysosomal glycoprotein A (lgp-A), and the percentage of antibody positive, FDx10f-labeled macropinosomes was scored. Some newly formed macropinosomes were positive for TfR, but few were rab 7 or lgp-A-positive. With intermediate chase times (2-4 min), staining for rab 7 and lgp-A increased to > 60%, while TfR staining declined. After a long chase (9-12 min), rab 7 staining returned to low levels while lgp-A staining remained at a high level. Thus, macropinosomes matured by progressive acquisition and loss of characteristic endocytic vesicle markers. However, unlike a maturation process, their merger with the tubular lysosomal compartment more nearly resembled the incorporation of a transient vesicle into a pre-existing, stable compartment. Shortly after their formation, FDx10f-labeled macropinosomes contacted and merged with Texas red dextran (TRDx10)-labeled tubular lysosomes. This occurred in two steps: macropinosomes acquired lgp-A first, and then several minutes later the cation-independent mannose-6-phosphate receptor (CI-MPR) and markers of lysosomal content (cathepsin L or pre-loaded TRDx10), all apparently derived from tubular lysosomes. Thus, macropinosome progress through macrophages showed features of both the maturation and vesicle shuttle models of endocytosis, beginning with a maturation process and ending by merger into a stable, resident lysosomal compartment.

Transferrin uptake by bone marrow macrophages is independent of the degree of iron saturation

The uptake of transferrin by macrophages was studied in relation to the degree of iron saturation. Rat bone marrow derived macrophages were incubated with transferrin labelled with 59Fe and 3H. At 37 degrees C the amount of 59Fe incorporated by macrophages was dependent on the time of incubation. 3H labelled transferrin was found degraded in the supernatants of the cell culture (material not precipitated by trichloroacetic acid) in a time dependent fashion. Taking into account the specific activity of 59Fe-3H labelled transferrin, we found that 95% of the transferrin uptake was degraded. This suggests that most of the uptake of transferrin was not mediated by a receptor-dependent mechanism, but by a phase fluid endocytosis. 3H-labelled apotransferrin appears in the supernatant of the cell culture at the same rate as 59Fe-3H labelled diferric transferrin, showing an identical uptake for the two types of transferrin. Uptake of apo- or diferric transferrin by macrophages was identical in relation to time of incubation and the amount of transferrin used. These studies suggest that most of the transferrin uptake by bone marrow macrophages (non-activated or non-elicited cells) is mediated by a non-receptor mechanism that is independent of the degree of transferrin saturation.