Biology Reference
In-Depth Information
It is unlikely that PrP facilitates export of iron from neuroblastoma cells based on our
observations.
At the plasma membrane, PrP
C
could take up iron directly from the extra-cellular
milieu and deliver to an endosomal compartment as suggested for copper [34]. How-
ever, this seems unlikely for three reasons; (1)
59
Fe-labeled PrP
C
could not be detected
in radiolabeled cells although labeled recombinant PrP is easily detected using the
same procedure [17], (2)
59
Fe-labeled recombinant PrP loses its label to Tf when added
to cells, indicating lower affi nity for iron relative to Tf (unpublished observations),
and (3) intracellular LIP is high in cells expressing anchorless PrP
231stop
despite low fer-
ritin iron content, indicating effi cient uptake of iron in the absence of cell surface PrP
C
.
It remains plausible, though, that PrP
C
modulates iron uptake by the Tf/TfR pathway
at the plasma membrane or in an endosomal compartment [35].
It is also possible that extra-cellular iron induces the movement of PrP
C
from deter-
gent insoluble membrane domains where it normally resides to the proximity of TfR in
a similar manner as in the presence of copper [34]. Here, it may enhance the binding
of iron loaded Tf to its receptor, or stimulate the endocytosis of Tf/TfR complex by a
direct or an indirect interaction. In this context, it is interesting to note that PrP
C
under-
goes endocytosis through clathrin coated pits after associating with a transmembrane
protein through its N-terminal domain [36], suggesting that the reported co-localiza-
tion of PrP
C
with Tf and TfR within endosomes may refl ect a functional association
rather than co-residence due to a common mode of endocytosis [37]. Assuming this
scenario, the increase in TfR levels by stimulation of PrP endocytosis by 3F4 and the
differential effect of mutant PrP forms on ferritin iron content may be explained by
a change in the rate of endocytosis, or altered interaction of normal and mutant PrP
forms with Tf or TfR due to misfolding [35-37]. We have previously reported in-
creased endocytosis and defective recycling of mutant PrP
102L
in neuroblastoma cells
[38], a fact that may account for increased ferritin iron in these cells. Though attrac-
tive, this model fails to explain decreased ferritin iron in the presence of signifi cantly
high LIP in cells expressing anchorless PrP
231stop
and by cross-linking PrP at the plasma
membrane, indicating a role downstream from the plasma membrane. The up-regula-
tion of PrP
C
at the transcriptional and translational level when cells are exposed to ex-
cess extra-cellular iron (supporting information) perhaps refl ects its function as an iron
regulatory protein, though a protective response to oxidative stress cannot be ruled out
under these experimental conditions [39]. However, since all cell lines display similar
differences in
59
Fe-ferritin content when labeled with
59
FeCl
3
or purifi ed
59
Fe-Tf (un-
published observations), it is likely that PrP
C
functions downstream of the iron uptake
pathways specifi c for free and Tf bound iron, perhaps in an endosomal compartment.
Keeping the above facts in mind, it is plausible that PrP
C
functions as a ferric
reductase along with Steap3 to facilitate the transport of ferric iron released from Tf
across the endosomal membrane to cytosolic ferritin. This assumption is supported
by the fact that PrP
C
functions as a copper transport protein by reducing copper (II)
prior to transfer to copper (I) specifi c traffi cking proteins within cells [34]. Such a
function would explain the low ferritin iron content in cells expressing mutant PrP
lacking the octapeptide region responsible for reducing copper (II) [34], the observed
