Effects of overexpression of the transferrin receptor on the rates of transferrin recycling and uptake of non-transferrin-bound iron

COPII-Golgi protein interactions regulate COPII coat assembly and Golgi size

Under experimental conditions, the Golgi apparatus can undergo de novo biogenesis from the endoplasmic reticulum (ER), involving a rapid phase of growth followed by a return to steady state, but the mechanisms that control growth are unknown. Quantification of coat protein complex (COP) II assembly revealed a dramatic up-regulation at exit sites driven by increased levels of Golgi proteins in the ER. Analysis in a permeabilized cell assay indicated that up-regulation of COPII assembly occurred in the absence GTP hydrolysis and any cytosolic factors other than the COPII prebudding complex Sar1p–Sec23p–Sec24p. Remarkably, acting via a direct interaction with Sar1p, increased expression of the Golgi enzyme N-acetylgalactosaminyl transferase-2 induced increased COPII assembly on the ER and an overall increase in the size of the Golgi apparatus. These results suggest that direct interactions between Golgi proteins exiting the ER and COPII components regulate ER exit, providing a variable exit rate mechanism that ensures homeostasis of the Golgi apparatus.

Stratum-specific expression of human transferrin receptor increases iron in mouse epidermis

Epidermal desquamation accounts for 20% of the body's iron loss each day. Yet, little is known about how iron content in epidermis is regulated. To test the importance of the transferrin receptor in regulating iron content in epidermis, we created transgenic mice that have stratum-specific expression of the human transferrin receptor. The keratin 14 promoter targeted the receptor primarily to basal, proliferating keratinocytes; the involucrin (Inv) promoter targeted the receptor to suprabasal, differentiating keratinocytes. There were age- and site-dependent differences in iron content in the epidermis and hair. In both types of transgenic mice, epidermal iron content increased with age and at 8 weeks was 2-3-fold greater in transgenic mice compared to littermate controls. Iron was increased up to 2-fold in hair of keratin 14-human transferrin receptor (hTfR) transgenics and 30% in Inv-hTfR transgenics. No gross or histological changes were seen in transgenic animals with increased iron in the epidermis. Ferritin expression, which was low in normal epidermis, was greatly increased in both transgenic lines, indicating that it is the likely depot for the extra iron in these animals. These data show that control of transferrin receptor expression is sufficient to regulate iron content in proliferating or differentiating keratinocytes in the epidermis.

Functional ability of tendinocytes to take up Fe substances in an inflamed tendon

The fate of iron (Fe) after bleeding has been analyzed in various connective tissues, but there have been only a few inconclusive studies on Fe in the tendon. In this study, energy dispersive X-ray microanalysis and an iron staining method were used to determine the localization of Fe in cells of the equine superficial digital flexor tendon. In inflamed tendons, Fe was detected in tendinocytes as well as macrophages. In contrast, negative or weak reactions were observed in many cells in the normal tendon. Although the accepted theory states that the reticuloendothelial system (macrophages and reticuloendothelial cells) is mainly responsible for the uptake and decomposition of erythrocytes, and tendon cells under conditions of inflammation show a functional ability to take up Fe substances. It has been reported that tendinocytes have multiple functions, including synthesis and secretion of collagen, cytokines, and matrix metalloproteinases. Taking these functions into consideration, indicates that the tendinocyte is not only an active cell but also a multi-functional cell.

Transferrin and transferrin receptor gene expression and iron uptake in hepatocellular carcinoma in the rat

Iron plays an important role in cell growth and metabolism. In preneoplastic liver nodules, a rise in the number of transferrin receptors (Tf-R) is associated with decreased endocytosis of the Fe2-Tf/Tf-R complex. Because nodules are precursors of hepatocellular carcinoma (HCC), the question arises whether changes in iron uptake by nodules persist in HCC. Current work showed up-regulation of Tf messenger RNA (mRNA) production in preneoplastic nodules, 12 to 37 weeks after initiation, and down-regulation in atypical nodules (at 45 and 50 weeks) and HCCs, induced in rats by the "resistant hepatocyte" model. Tf-R gene expression increased in nodules and HCCs. Tf-R numbers increased, without changes in affinity constant, in HCC. Iron uptake was higher in HCC than in normal liver, 5 to 40 minutes after injection of 59Fe2-Tf, with preferential accumulation in cytosol of tumor cells and in microsomes of normal liver. Purification through Percoll gradient of mitochondria plus lysosomes allowed the identification in liver and HCC of an endosomal compartment sequestering injected 125I-Tf. This subfraction was not seen when 59Fe2-Tf was injected into rats, and 59Fe was found in particulate material of both tissues. Liver and HCC exhibited comparable basal activities of plasma membrane NADH oxidase, an enzyme involved in iron uptake and cell growth. Stimulation of this activity by Fe2-Tf was higher in HCC than in normal liver. These results indicate that Tf expression may be a marker of preneoplastic liver progression to malignancy. Differently from nodules, HCC may sequester relatively high iron amounts, necessary for fast growth, both through the endocytic pathway and the reduced form of nicotinamide adenine dinucleotide (NADH) oxidase system.

Regulation of mammalian iron metabolism: current state and need for further knowledge

Due to its character as an essential element for all forms of life, the biochemistry and physiology of iron has attracted very intensive interest for many decades. In more recent years, the ways that iron metabolism is regulated in mammalian and human organisms have been clarified, and many aspects of iron metabolism have been reviewed. In this article, some newer aspects concerning absorption and intracellular regulation of iron concentration are considered. These include a sorting of possible models for intestinal iron absorption, a description of ways for membrane passage of iron after release from transferrin during receptor-mediated endocytosis, a consideration of possible mechanisms for non-transferrin bound iron uptake and its regulation, and a review of recent knowledge on the properties of iron regulatory proteins and on regulation of iron metabolism by these proteins, changes of their own properties by non-iron-mediated influences, and regulatory events not mediated by these proteins. This somewhat heterogeneous collection of themes is a consequence of the intention to avoid repetition of the many aforementioned reviews already existing and to concentrate on newer findings generated within the last couple of years.

Transferrin receptor functions as a signal-transduction molecule for its own recycling via increases in the internal Ca2+ concentration

Transferrin binding to its receptor modulates transferrin receptor (Tf-R) recycling rates in several cells [Klausner, R. D., Van Renswoude, J., Ashwell, G., Kempf, C., Schechter, A., Dean, A. & Bridges, K. R. (1983a) J. Biol. Chem. 258, 4715-4724; Gironès, N. & Davis, R. J. (1989) Biochem. J. 264, 35-46; Sainte-Marie, J., Vidal, M., Bette-Bobillo, P., Philippot, J. R. & Bienvenüe, A. (1991) Eur. J. Biochem. 201, 295-302]. To delineate the mechanism of this regulation, we hypothesized that the binding of the ligand to its receptor could lead to activation of several second-messenger pathways, which may redundantly stimulate recycling of the receptor. The effects of different regulators of Ca2+ flux or concentrations were investigated on the Tf-R-recycling pathway; these studies were carried out in two cell types. Perhexiline, a calcium antagonist, slowed receptor recycling in comparison with the control by more than 80% in L2C cells and by 60% in Jurkat cells (B and T lymphoblasts, respectively) but did not affect their internalization rate. Perhexiline thus trapped considerable amounts of Tf-R in the internal compartment. Ca2+ chelators, such as EGTA or 1,2-bis(2-aminophenoxy)ethane-N,N,N,N'-tetraacetic acid, and a Ca2+-channel inhibitor (Ni2+) decreased drastically the recycling rate of Tf-R. Tf-R recycling was shown to be slowed by a calmodulin antagonist. Conversely, artificial elevation of free internal Ca2+ in L2C cells, using lectin, accelerated the recycling rate. These results suggest that the intracellular Ca2+ concentration plays an important role in the outward flow of transferrin receptors. Consequently, we examined the role of transferrin in internal free Ca2+ regulation. The addition of transferrin or anti-(Tf-R) Ig specifically elicited a rise in [Ca2+], as demonstrated by inefficacy of apotransferrin or irrelevant antibodies. These results suggest that Ca2+ is a regulator of Tf-R recycling and that Tf-R seems to function as a signal-transduction molecule (perhaps in conjunction with other membrane proteins) rather than merely as an endocytic receptor.