Cytoskeletal Protein 4.1R in Health and Diseases

The protein 4.1R is an essential component of the erythrocyte membrane skeleton, serving as a key structural element and contributing to the regulation of the membrane's physical properties, including mechanical stability and deformability, through its interaction with spectrin-actin. Recent research has uncovered additional roles of 4.1R beyond its function as a linker between the plasma membrane and the membrane skeleton. It has been found to play a crucial role in various biological processes, such as cell fate determination, cell cycle regulation, cell proliferation, and cell motility. Additionally, 4.1R has been implicated in cancer, with numerous studies demonstrating its potential as a diagnostic and prognostic biomarker for tumors. In this review, we provide an updated overview of the gene and protein structure of 4.1R, as well as its cellular functions in both physiological and pathological contexts.

A Recap of Heme Metabolism towards Understanding Protoporphyrin IX Selectivity in Cancer Cells

Mitochondria are essential organelles of mammalian cells, often emphasized for their function in energy production, iron metabolism and apoptosis as well as heme synthesis. The heme is an iron-loaded porphyrin behaving as a prosthetic group by its interactions with a wide variety of proteins. These complexes are termed hemoproteins and are usually vital to the whole cell comportment, such as the proteins hemoglobin, myoglobin or cytochromes, but also enzymes such as catalase and peroxidases. The building block of porphyrins is the 5-aminolevulinic acid, whose exogenous administration is able to stimulate the entire heme biosynthesis route. In neoplastic cells, this methodology repeatedly demonstrated an accumulation of the ultimate heme precursor, the fluorescent protoporphyrin IX photosensitizer, rather than in healthy tissues. While manifold players have been proposed, numerous discrepancies between research studies still dispute the mechanisms underlying this selective phenomenon that yet requires intensive investigations. In particular, we wonder what are the respective involvements of enzymes and transporters in protoporphyrin IX accretion. Is this mainly due to a boost in protoporphyrin IX anabolism along with a drop of its catabolism, or are its transporters deregulated? Additionally, can we truly expect to find a universal model to explain this selectivity? In this report, we aim to provide our peers with an overview of the currently known mitochondrial heme metabolism and approaches that could explain, at least partly, the mechanism of protoporphyrin IX selectivity towards cancer cells.

The protein 4.1R downregulates VEGFA in M2 macrophages to inhibit colon cancer metastasis

M2 macrophages are crucial components of the tumour microenvironment and have been shown to be closely related to tumour progression. Co-culture with 4.1R^-/- M2 macrophages enhances the malignancy of colon cancer (CC), but the mechanism remains unclear. Here, we report that protein 4.1R knockout reduced the phagocytosis of M2 macrophages (M-CSF/IL-4-treated bone marrow cells) and promoted MC38 colon cancer cell proliferation, migration, invasion, tumour formation and epithelial-mesenchymal transition (EMT), which are regulated by M2 macrophages. Further mechanistic dissection revealed that the 4.1R knockout upregulated vascular endothelial growth factor A (VEGFA) secreted by M2 macrophages and promoted colon cancer progression by activating the PI3K/AKT signalling pathway. In summary, our present study identified that 4.1R downregulates VEGFA secretion in M2 macrophages and delays the malignant potential of colon cancer by inhibiting the PI3K/AKT signalling pathway.

Protein 4.1R affects photodynamic therapy for B16 melanoma by regulating the transport of 5-aminolevulinic acid

Melanoma is the most aggressive malignant tumor of skin cancer as it can grow rapidly and metastasize. Photodynamic therapy (PDT) is a promising cancer ablation method for skin tumors, although it lacks efficiency owing to factors such as tumor characteristics, delivery of photosensitizers, immune response in vivo etc. Extensive investigation of molecules that can potentially modulate treatment efficacy is required. Protein 4.1R is a cytoskeletal protein molecule. Previous studies have shown that protein 4.1R knockdown reduces PDT sensitivity in mouse embryonic fibroblast cells. However, the functional role of protein 4.1R in melanoma is unclear. In this study, we aimed to elucidate the effect of protein 4.1R on PDT for melanoma in mice and the mechanism of anti-tumor immunity. Our results indicated that CRISPR/Cas9-mediated protein 4.1R knockout promotes the proliferation, migration, and invasion of B16 cells. We further investigated the potential mechanism of protein 4.1R on tumor cell PDT sensitivity. Our results showed that protein 4.1R knockout reduced the expression of membrane transporters γ-aminobutyric acid transporter (GAT)-1 and (GAT)-2 in B16 cells, which affected 5-ALA transmembrane transport and reduced the efficiency of PDT on B16 cells. Protein 4.1R knockout downregulated the anti-tumor immune response triggered by PDT in vivo. In conclusion, our data suggest that protein 4.1R is an important regulator in PDT for tumors and may promote the progress and efficacy of melanoma treatment.

Aggregation of protein complexes with porphyrins under light irradiation

The interaction of meso-tetra-([Formula: see text]-methyl-3-pyridyl)bacteriochlorin tetra 4-methylbenzenesulfonate, meso-tetra-([Formula: see text]-methyl-4-pyridyl)porphine tetraiodide and meso-tetra-([Formula: see text]-methyl-3-pyridyl)porphine tetraiodide with protein was studied. The localization of macrocycles in the protein and their influence on the processes of protein aggregation were established. The aggregation process is initiated by the transition of alpha structures into beta folds, caused by the binding of porphyrins at the sites of protein IB and IIA. The complexes were irradiated with blue and green light. The data obtained showed that by varying the intensity of the light exposure, one can influence the shift of the aggregation equilibrium, obtaining predominantly monomeric or aggregated structures. Thus, photoexposure can be used as an alternative method of treating diseases caused by amyloidosis and for regulating the state of the protein in the pharmacological preparations.

Protein 4.1R negatively regulates CD8+ T-cell activation by modulating phosphorylation of linker for activation of T cells

Protein 4.1R, an 80 000 MW membrane skeleton protein, is a vital component of the red blood cell membrane cytoskeleton that stabilizes the spectrin-actin network and regulates membrane properties of deformability and mechanical stability. It has been shown that 4.1R is expressed in T cells, including CD8⁺ T cells, but its role in CD8⁺ T cells remains unclear. Here, we have explored the role of 4.1R in CD8⁺ T cells using 4.1R^-/- mice. Our results showed that cell activation, proliferation and secretion levels of interleukin-2 and interferon-γ were significantly increased in 4.1R^-/- CD8⁺ T cells. Furthermore, the phosphorylation levels of linker for activation of T cells (LAT) and its downstream signaling molecule extracellular signal-regulated kinase were enhanced in the absence of 4.1R. In vitro co-immunoprecipitation experiments showed a direct interaction between 4.1R and LAT. Moreover, 4.1R^-/- CD8⁺ T cells and mice exhibited an enhanced T-cell-dependent immune response. These data enabled the identification of a negative regulation function for 4.1R in CD8⁺ T cells by a direct association between 4.1R and LAT, possibly through inhibiting phosphorylation of LAT and then modulating intracellular signal transduction.