Fra-1 in Cancer A Key Driver of Tumor Invasion

What is Fra-1?

Fra-1, also known as Fos-related antigen 1, is a pivotal member of the transcription factor activator protein 1 (AP-1). This protein plays a critical role in a wide range of biological processes, including cell proliferation, apoptosis, differentiation, inflammation, oncogenesis, and tumor metastasis. Emerging evidence strongly suggests that targeting Fra-1 directly can have a significant impact on the malignancy and invasiveness of tumors. Moreover, Fra-1's influence extends beyond cancer, with its effects becoming increasingly evident in immune and inflammatory conditions, such as arthritis, pneumonia, psoriasis, and cardiovascular disease. These intricate regulatory mechanisms, which coordinate both immune and non-immune cells, position Fra-1 as a promising therapeutic target for numerous human diseases.

The Structure of Fra-1

Initially discovered as being highly expressed in various cancer cells, FOSL1 was classified as a proto-oncogene. It's located at locus 11q13.1 and encodes a mature mRNA with a length of 1.7kb. The FOSL1 gene encodes a 271 amino acid protein called Fra-1, with a relative molecular mass of 29,000. Its genome structure comprises four exons and three introns[1].

Fra-1 belongs to the FOS subfamily within the nuclear transcription factor AP-1 family. It shares common structural characteristics with other AP-1 members. Like its counterparts, Fra-1 is categorized as a basic Leucine zipper (bZIP) protein. The leucine zipper motif (LZ) within Fra-1 is crucial for dimerization, and the basic region (BR) facilitates binding to specific DNA motifs. This domain is highly conserved within the AP-1 family, and Fra-1's bZIP domain spans amino acid sites 107-161. The Leucine zipper is composed of extended amino acids, with one leucine occurring every 6 amino acids. The dimer formed by the interaction between Fra-1 and JUN family members creates the characteristic "zipper"[2].

The two subunits constituting the dimer form a continuous α-helix. The carboxy-terminal region forms asymmetric helical coils, while the amino terminal region engages in base-specific binding to DNA within the main groove[3]. Unlike JUN family members, the amino acid composition in the bZIP domain of FOS family members differs slightly, preventing them from forming homologous dimers with their own members. Instead, they combine with JUN family members to form heterodimers and exert their function. Additionally, Fra-1 can form heterodimers with other ubiquitous bZIP transcription factors.

Furthermore, studies have shown that an extensive electrostatic interaction network exists between subunits in the α-helix. This results in Fos : Jun heterodimers having a stronger affinity for DNA compared to Jun : Jun homodimers, making them more stable and demonstrating more robust transcriptional stimulation activity[4].

The dimer composed of Fra-1 and Jun has a preference for binding to the DNA motif known as the TPA response element (TRE; also referred to as the AP-1 motif). It can also bind to the cAMP-responsive element (CRE), although the latter exhibits slightly lower affinity. The consensus sequences for TRE/AP-1 and CRE are 5′-TGA(C/G)TCA and 5′-TCACGTCA motifs, respectively. In addition to the bZIP structure, FOS family members possess a second highly homologous region at the C-terminal. This region, containing 30-40 residues, is where one of the two instability factors resides. The C-terminal destabilizing domain (DEST) of the Fra-1 protein is essential for its transformative activity and contributes to its intracellular instability.

Fra-1 in Tumor Growth, Invasion, and Metastasis

Fra-1 plays a pivotal role in cancer cell invasion across a spectrum of solid tumors, including adenocarcinomas (such as breast, lung, colon, pancreas, and thyroid), squamous cell carcinomas, and non-epithelial cancers like melanoma, malignant mesothelioma, and GBM.

Fra-1 orchestrates a series of changes in cellular structure and function, leading to increased cancer cell motility and invasiveness. These changes are context-dependent and encompass various degrees of mesenchymal transformation, from partial to complete epithelial-mesenchymal transition (EMT)[6]. In breast and colorectal adenocarcinoma cell lines, Fra-1's influence extends to well-known EMT inducers, including tyrosine kinase receptors (AXL), EMT-triggering cytokines (TGF-beta and IL-6), EMT transcription factors (ZEB1 and ZEB2), and chromatin components (HMGA1). Beyond its involvement in EMT-related pro-invasive programs, Fra-1's target genes also regulate cell proliferation, survival, and resistance to anoikis[7].

Fra-1 contributes to both autocrine and paracrine mechanisms in EMT and tumor angiogenesis. It does so by stimulating the production of multiple cytokines, including TGF-beta in breast and colorectal cancer cells, and IL-6 and VEGF in tumor-associated macrophages (TAMs) recruited to the tumor microenvironment[8].

Non-coding transcripts are among Fra-1's downstream effectors. For instance, Fra-1 regulates the transcription of the widely expressed oncogenic microRNA miR-21, which, in turn, establishes positive feedback loops with AP-1 in RAS-transformed cancer cells. Another feedback mechanism involves Fra-1's control of miR-134 in ovarian cancer. miR-134 inhibits the Protein Phosphatase-1 (PP1) regulatory subunit SDS22, enhancing ERK and JNK MAPK signaling, Fra-1 accumulation, and driving cancer cell proliferation, migration, and invasion. Non-coding RNAs also play a role in Fra-1's control of the Epithelial to Mesenchymal Transition. For example, Fra-1-induced miR-221/222 regulates the miR-221/222-TRPS1-ZEB2 pathway, promoting EMT in breast cancer cells[9].

Fra-1's significance extends to the dynamic balance between cancer and non-cancer stem cells (CSCs). In breast cancer cells, induction of FOSL1 by Twist and Snail results in Fra-1 accumulation, driving the transition from non-CSCs to CSCs associated with EMT. In colorectal cancer cells, IL-6 amplifies Fra-1 activity by inducing HDAC6-mediated Fra-1 deacetylation and accumulation. This leads to the acquisition of stem-like characteristics, partially through Fra-1-mediated transactivation of the NANOG promoter. In NF1-mutant GBM tumors and cell lines, FOSL1 overexpression has recently been linked to the control of the mesenchymal subtype and the acquisition of stem-like features. In a mouse model of GBM, FOSL1 deletion induces a shift from mesenchymal to proneural transcriptional signatures, along with reduced stemness and tumor growth[11].

Signaling Pathway of Fra-1

Effects of FRA-1 overexpression on proliferation, apoptosis and aging of tumor cells and its signal transduction mechanism

FRA-1, known for its overexpression in tumor cells, wields significant influence over cell signal transduction by engaging with various molecules. This interaction ultimately drives either cell proliferation, apoptosis, or aging. For instance, in gastric cancer cells, the overexpression of FRA-1 curbs apoptosis and fuels proliferation. This effect is attributed to the upregulation of PI3K, AKT, and MDM2, alongside the downregulation of the p53 tumor suppressor gene. Essentially, FRA-1's overexpression activates the PI3K/AKT pathway while inactivating the p53 tumor suppressor gene, thereby inhibiting apoptosis and promoting proliferation[13].

Moreover, research reveals that the upregulation of FRA-1 following Helicobacter pylori infection in gastric cancer cells elevates the Wnt/β-Catenin level and activates the transforming growth factor-beta (TGF-β) signaling pathway. Activation of the Wnt/β-Catenin pathway contributes to the upregulation of genes related to cell proliferation, apoptosis, and invasion. Simultaneously, the TGF-β signaling pathway influences crucial cell activities like growth, mobility, and invasion, ultimately facilitating the development of gastric cancer. Additionally, the high expression of FRA-1 induced by H. pylori hampers the apoptosis of MGC-803 cells through the PI3K/AKT and RAS/ERK signaling pathways[14]. Consequently, the FRA-1 molecule emerges as a pivotal player in the initiation of gastric cancer.

Furthermore, in RAS-transformed thyroid cells, the expression and phosphorylation of FRA-1 are regulated during the cell cycle, with peaks occurring at the G2/M phase. Cells lacking FRA-1 predominantly accumulate in the G2 phase, while a few proceed to an abnormal cell division phase, marked by micronuclei, lagging chromosomes, and anaphase bridges. Knocking out FRA-1 leads to cell proliferation arrest and apoptosis. The connection between FRA-1 and cell cycle mechanisms is underscored by its identification as a new transcription target of cyclin A. Throughout the cell cycle, FRA-1 is recruited to the cyclin A promoter, where it binds to previously unidentified AP-1 sites and CRE. FRA-1 also induces the expression of Jun B, which, in turn, interacts with the cyclin A promoter, promoting cell proliferation. FRA-1's role in regulating cyclin expression and advancing the cell cycle progression significantly contributes to the development of thyroid tumors.

Effect of FRA-1 overexpression on invasion, migration and transformation of tumor cells and its signal transduction mechanism

The expression of FRA-1 is intricately linked to the invasion and migration capabilities of tumor cells. In colorectal cancer (CRC) cells, research has revealed that ubiquitin-specific protease 21 (USP21) enhances FRA-1 stability and the expression of AP-1 target genes by deubiquitinating FRA-1, thereby promoting CRC cell migration. This discovery underscores the pivotal role of USP21 as a cysteaminase for FRA-1, ultimately driving CRC metastasis. It also provides a foundation for identifying new therapeutic targets for metastatic CRC[16].

Moreover, a signaling pathway plays a crucial role in the liver metastasis of colorectal cancer. Sublethal heat shock (HS) resulting from incomplete radiofrequency ablation (ICR) significantly elevates FRA-1 downstream of PKCα-ERK1/2 signal transduction. FRA-1/AP-1, in response, induces gene transcription, upregulating the expression of proliferation and tumorigenesis inducer c-MYC, as well as the tumor invasion inducer matrix metalloproteinase-1 (MMP-1). These events promote liver metastasis in colorectal cancer (CRC), contributing to the development of malignancies associated with colorectal cancer[16]. Notably, FRA-1 levels positively correlate with local invasion depth, lymph node involvement, and liver metastases in CRC patients.

In the tumor microenvironment, interleukin-6 (IL-6) is a pro-inflammatory cytokine believed to be a significant factor in promoting colorectal cancer's initiation and progression. Activation of STAT3 during IL-6-driven Epithelial-Mesenchymal Transition (EMT) in colorectal cancer cell lines leads to a substantial upregulation of the FRA-1 gene. This, in turn, increases the expression of EMT-promoting factors such as ZEB1, Snail, Slug, MMP-2, and MMP-9, ultimately enhancing CRC cell invasiveness. The IL-6/STAT3/FRA-1 signaling axis thus plays a crucial role in inducing EMT and driving CRC invasiveness[17].

Furthermore, microRNA-34a (miR-34a), a transcriptional target of p53 and a tumor suppressor gene, hampers the migration and invasion of colon cancer cells. This effect is mediated by miR-34a's ability to inhibit FRA-1 expression, resulting from the DNA activity of p53. Reduced FRA-1 levels lead to decreased expression of matrix metalloproteinases MMP-1 and MMP-9, ultimately suppressing colon cancer cell migration and invasion. Additionally, the ectopic expression of SIRT1 significantly enhances the migration and invasion of colorectal cancer cells. In contrast, SIRT1 gene knockout disrupts colorectal cancer metastasis in vivo. SIRT1 expression promotes colon cancer cell migration and invasion by regulating FRA-1 expression and inducing Epithelial-Mesenchymal Transition (EMT).

Fra-1 Protein

Recombinant Human FRA1 Protein (His Tag)

Synonym : FRA; fra-1; FRA1 and FOS-like antigen 1.

References:

 

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