Flotillin and AP2A1/2 Promote IGF-1 Receptor Association with Clathrin and Internalization in Primary Human Keratinocytes

Flotillin-1 palmitoylation is essential for its stability and subsequent tumor promoting capabilities

Flotillin-1 contributes to invasion and metastasis in triple negative breast cancer (TNBC) and is modified post-translationally through palmitoylation. Palmitoylation, the process of conjugating palmitoyl-CoA to proteins, plays an essential role in protein stability and trafficking. Thus far, there has not been any investigation into the role of flotillin-1 palmitoylation in the context of metastasis in vivo. To address the role of flotillin-1 palmitoylation in metastasis, MDA-MB-231 cells expressing palmitoylation defective flotillin-1 constructs were used as models. Compared to flotillin-1 WT expressing tumors, flotillin-1 palmitoylation defective displayed abrogated tumor progression and lung metastasis in vivo in both spontaneous and experimental models. Further mechanistic investigation led to the identification of zDHHC5 as the main palmitoyl acyltransferase responsible for palmitoylating endogenous flotillin-1. Modulation of flotillin-1 palmitoylation status through mutagenesis, zDHHC5 silencing, and 2-bromopalmitate inhibition all resulted in the proteasomal degradation of flotillin-1 protein. To assess if flotillin-1 palmitoylation can be inhibited for potential clinical relevance, we designed a competitive peptide fused to a cell penetrating peptide sequence, which displayed efficacy in blocking flotillin-1 palmitoylation in vitro without altering palmitoylation of other zDHHC5 substrates, highlighting its specificity. Additionally, TNBC xenograft tumor models expressing a doxycycline inducible flotillin-1 palmitoylation inhibiting peptide displayed attenuated tumor growth and lung metastasis. Collectively, these results reveal a novel palmitoylation dependent mechanism which is essential for the stability of flotillin-1 protein. More specifically, disruption of flotillin-1 palmitoylation through mutagenesis or competitive peptide promoted flotillin-1 protein degradation, subsequently impeding its tumor promoting and metastasis-inducing effects in TNBC tumor models.

The roles of FLOT1 in human diseases (Review)

FLOT1, a scaffold protein of lipid rafts, is involved in several biological processes, including lipid raft protein‑-dependent or clathrin‑independent endocytosis, and the formation of hippocampal synapses, amongst others. Increasing evidence has shown that FLOT1 can function as both a cancer promoter and cancer suppressor dependent on the type of cancer. FLOT1 can affect the occurrence and development of several types of cancer by affecting epithelial‑mesenchymal transition, proliferation of cancer cells, and relevant signaling pathways, and is regulated by long intergenic non‑coding RNAs or microRNAs. In the nervous system, overexpression or abnormally low expression of FLOT1 may lead to the occurrence of neurological diseases, such as Alzheimer's disease, Parkinson's disease, major depressive disorder and other diseases. Additionally, it is also associated with dilated cardiomyopathy, pathogenic microbial infection, diabetes‑related diseases, and gynecological diseases, amongst other diseases. In the present review, the structure and localization of FLOT1, as well as the physiological processes it is involved in are reviewed, and then the upstream and downstream regulation of FLOT1 in human disease, particularly in different types of cancer and neurological diseases are discussed, with a focus on potentially targeting FLOT1 for the clinical treatment of several diseases.

An overview of receptor endocytosis and signaling

Endocytosis is a cellular process which mediates receptor internalization, nutrient uptake, and the regulation of cell signaling. Microorganisms (many bacteria and viruses) and toxins also use the same process and enter the cells. Generally, endocytosis is considered in the three forms such as phagocytosis (cell eating), pinocytosis (cell drinking), and highly selective receptor-mediated endocytosis (clathrin-dependent and independent). Several endocytic routes exist in an analogous, achieving diverse functions. Most studies on endocytosis have used transformed cells in culture. To visualize the receptor internalization, trafficking, and signaling in subcellular organelles, a green fluorescent protein-tagged receptor has been utilized. It also helps to visualize the endocytosis effects in live-cell imaging. Confocal laser microscopy increases our understanding of receptor endocytosis and signaling. Site-directed mutagenesis studies demonstrated that many short-sequence motifs of the cytoplasmic domain of receptors significantly play a vital role in receptor internalization, subcellular trafficking, and signaling. However, other factors also regulate receptor internalization through clathrin-coated vesicles. Receptor endocytosis can occur through clathrin-dependent and clathrin-independent pathways. This chapter briefly discusses the internalization, trafficking, and signaling of various receptors in normal conditions. In addition, it also highlights the malfunction of the receptor in disease conditions.

Emerging tools for studying receptor endocytosis and signaling

Ligands, agonists, or antagonists use receptor-mediated endocytosis (RME) to reach their intracellular targets. After the internalization of ligand-receptor complexes, it traffics through different subcellular organelles such as early endosome, recycling endosome, lysosome, etc. Further, after the ligand binding to the receptor, different second messengers are generated, such as cGMP, cAMP, IP₃, etc. Several methods have been used, such as radioligand binding assay, western blotting, co-immunoprecipitation (co-IP), qRT-PCR, immunofluorescence and confocal microscopy, microRNA/siRNA, and bioassays to understand the various events, such as internalization, subcellular trafficking, signaling, metabolic degradation, etc. This chapter briefly discusses the key principles and methods used to study internalization, subcellular trafficking, signaling, and metabolic degradation of numerous receptors.

Molecular characterization and function of the lipid raft protein Lvflotillin-1A from Litopenaeus vannamei

White spot syndrome virus (WSSV) can cause a contagious, high virulent and pandemic disease for crustaceans, especially shrimps. However, the molecular mechanism of WSSV pathogenesis remains unclear. Flotillins are lipid raft-associated proteins, which mainly include flotillin-1 and flotillin-2. They are involved in the formation of large heteromeric protein complexes engaged in diverse signalling pathways at the membrane-cytosol interface. They defined a clathrin-independent endocytic pathway in mammalian cells. Our previous studies suggested that shrimp flotillin-2 might mediate endocytosis involved in WSSV infection. To further explore the function of shrimp flotillin, a flotillin-1 homologous, Lvflotillin-1A was identified and characterized in Litopenaeus vanamei. The transcription of Lvflotillin-1A showed a significant decline at 12h post-infection, followed by complete recovery and a slight up-regulation after the WSSV challenge. Gene silencing revealed that inhibition of Lvflotillin-1A raised the virus infection, suggesting Lvflotillin-1A might play an important role in shrimp immunity. Furthermore, co-immunoprecipitation and immunofluorescence illustrated that Lvflotillin-1A and Lvflotillin-2 could form hetero-oligomers, and co-expression promoted the accumulation of intracellular vesicles. The study revealed that WSSV might up-regulate Lvflotillin-2 expression and alter the subcellular location of Lvflotillin-1 protein to facilitate virus infection. These results will provide information for understanding the interaction between WSSV and shrimp.

MicroRNA‑485‑5p suppresses the progression of esophageal squamous cell carcinoma by targeting flotillin‑1 and inhibits the epithelial‑mesenchymal transition

As esophageal squamous cell carcinoma (ESCC) is one of the most frequently diagnosed cancers in Asia, it is crucial to uncover its underlying molecular mechanisms that support its development and progression. Several articles have reported that microRNA (miR)-485-5p inhibits the malignant phenotype in a number of cancer types, such as lung, gastric and breast cancer, but to the best of our knowledge, its function in ESCC has not been studied in depth until the present study. It is of great significance to probe the regulatory action and underlying mechanism of miR-485-5p in ESCC. In brief, this study identified that miR-485-5p expression in ESCC tissues was significantly lower than that in normal tissues. The decrease in miR-485-5p expression was associated with a larger tumour size and poor histology and stage. The expression of miR-485-5p was relatively high in Eca 109 and TE-1 cells, but relatively low in KYSE 30. The overexpression of miR-485-5p inhibited cell proliferation, migration and invasion in vitro, whereas miR-485-5p knockdown did the opposite. Flotillin-1 (FLOT-1) can facilitate the malignant phenotype in various cancer types. The present study found that in ESCC tissue, the protein expression of FLOT-1 was negatively correlated with miR-485-5p expression. Further experiments showed that miR-485-5p directly targeted the 3′-untranslated region of FLOT-1. The overexpression of miR-485-5p significantly suppressed the mRNA and protein expression levels of FLOT-1, whereas knockdown had the reverse effects. Furthermore, overexpression of miR-485-5p restrained epithelial-mesenchymal metastasis (EMT)-related factors at both the mRNA and protein levels. At the same time, it also inhibited the growth of ESCC and restrained the EMT in vivo. In summary, miR-485-5p was found to be an inhibitor of ESCC and may have potential as a novel target candidate for ESCC treatment.