Maturation‐associated loss and incomplete de novo synthesis of the transferrin receptor in peripheral sheep reticulocytes: Response to heme and iron

Abstract

Hemin, but not iron, in the culture medium stimulates the maturation-associated loss of the transferrin receptor from sheep reticulocytes (t1/2 for loss approximately 6 hr) and its appearance in a population of externalized vesicles. A similar pattern is seen with nucleoside binding (a measure of the nucleoside transporter), where hemin increases the loss of binding activity from the cells during culture, concomitant with an increase in nucleoside binding in the externalized vesicles. Sheep reticulocytes retain the ability to synthesize the transferrin receptor, but the 35S-labeled receptors are not detected in released vesicles. Whereas hemin stimulates the loss of 35S-labeled transferrin receptors from the cell (t1/2 for loss approximately 20 hr), nonheme iron is more effective than heme. This difference in response of native and 35S-labeled receptor to hemin and iron supplements appears to be related to the differences in the two classes of receptors. Although the 35S-labeled receptor binds transferrin and both native and 35S-labeled peptides comigrate after chemical deglycosylation, the 35S-receptor is approximately 2 kD smaller than the native receptor and fails to acquire its complete size even when chased for up to 24 hr. Moreover, the 35S-labeled receptor is not expressed at the cell surface, but is retained in a nonrecycling compartment, where it is insensitive to digestion by trypsin at both 0 degrees C and 37 degrees C.

Transgenic rescue of erythroid 5‐aminolevulinate synthase‐deficient mice results in the formation of ring sideroblasts and siderocytes

“…Therefore, one attractive hypothesis is that TfR expression is sensitive to heme‐deficiency in these cells. In contrast, the heme in sheep reticulocytes stimulates the loss of TfR associated with maturation (Ahn & Johnstone 1989). This observation rather suggests that the heme‐deficiency in siderocytes might act to sustain the high‐level of TfR expression.…”

Regulation of transferrin receptor mRNA expression

“…In contrast to uninduced cells, in hemoglobinsynthesizing MEL cells, iron appears to have less influence on TfR mRNA expression (see Figs 7 and 10). In this context, it is of interest to mention that FePIH has no effect on synthesis of TfR in sheep reticulocytes (Ahn and Johnstone, 1989). Several lines of evidence suggest that the differences in responsiveness to iron in these two cell types are related to intrinsic properties of TfR gene expression during MEL cell differentiation.…”