Unconventional endocytosis and trafficking of transferrin receptor induced by iron.

The posttranslational regulation of transferrin receptor (TfR1) is largely unknown. We investigated whether iron availability affects TfR1 endocytic cycle and protein stability in HepG2 hepatoma cells exposed to ferric ammonium citrate (FAC). NH4Cl and bafilomycin A1, but not the proteasomal inhibitor MG132, prevented the FAC-mediated decrease in TfR1 protein levels, thus indicating lysosomal involvement. Knockdown experiments showed that TfR1 lysosomal degradation is independent of 1) endocytosis mediated by the clathrin adaptor AP2; 2) Tf, which was suggested to facilitate TfR1 internalization; 3) H-ferritin; and 4) MARCH8, previously implicated in TfR1 degradation. Notably, FAC decreased the number of TfR1 molecules at the cell surface and increased the Tf endocytic rate. Colocalization experiments confirmed that, upon FAC treatment, TfR1 was endocytosed in an AP2- and Tf-independent pathway and trafficked to the lysosome for degradation. This unconventional endocytic regulatory mechanism aimed at reducing surface TfR1 may represent an additional posttranslational control to prevent iron overload. Our results show that iron is a key regulator of the trafficking of TfR1, which has been widely used to study endocytosis, often not considering its function in iron homeostasis.

Y-box Binding Protein-1 Is Part of a Complex Molecular Network Linking ΔNp63α to the PI3K/akt Pathway in Cutaneous Squamous Cell Carcinoma.

Cutaneous squamous cell carcinomas (SCCs) typically lack somatic oncogene-activating mutations and most of them contain p53 mutations. However, the presence of p53 mutations in skin premalignant lesions suggests that these represent early events during tumor progression and additional alterations may be required for SCC development. SCC cells frequently express high levels of ΔNp63α and Y-box binding 1 (YB-1 or YBX1) oncoproteins. Here, we show that knockdown of YB-1 in spontaneously immortalized HaCaT and non-metastatic SCC011 cells led to a dramatic decrease of ΔNp63α, cell detachment and death. In highly metastatic SCC022 cells, instead, YB-1 silencing induces PI3K/AKT signaling hyperactivation which counteracts the effect of YB-1 depletion and promotes cell survival. In summary, our results unveil a functional cross-talk between YB-1, ΔNp63α and the PI3K/AKT pathway critically governing survival of squamous carcinoma cells.