ARA55 in Cancers: A Double-Edged Sword

Androgen receptor coactivator 55kDa protein (ARA55) was initially isolated and identified from mouse MC3T3-E1 osteoblasts treated with transforming growth factor-β1 (TGFβ1). So, it is also named as TGFβ1-induced transcript 1 protein (TGFβ1I1). Thus far, the gene or amino acid sequence, structure, biological properties as well as peptide synthesis of ARA55 have been well studied. Current studies suggest that ARA55 functions as a molecular adapter in coordinating multiple Protein-Protein Interactions (PPIs) in Focal Adhesion (FA) complexes and the nucleus. Besides, accumulating evidence has shown that ARA55 is also implicated in tumorigenesis and the progression of cancers. However, there are a lot of disputes on the functional role of ARA55 in different cancers. This review summarized the recent findings on ARA55 with particular emphasis on its role in tumors.

With an Open Reading Frame (ORF) of 1386bp in length, it can encode a protein polypeptide composed of 461 amino acids with a molecular weight of 55 kDa. Allowing for the similar 4 LD domains in the N-terminal region and 4 LIM zinc finger domains in the C-terminal segment along with 55% amino acid sequence consistent with that of paxillin, and the molecular structure of ARA55 has 72% similarity to paxillin [2]. Specifically, the LIM zinc- The fourth LIM zinc-binding singly interacts with homooligomerization and targets to the nuclear matrix. Therefore, the unique protein structure endows distinctive biofunctions of ARA55. Recently, it has been found that ARA55 as a coactivator interacts with multiple targets and enhance the transcriptional activity of AR--a transcription factor which is activated by androgen and essential in male sexual differentiation [1,2]. Despite the role as a molecular adapter coordinating multiple PPIs in FA complexes and the nucleus, it is also involved in the cell growth, proliferation, migration, differentiation and senescence particularly in cancers [2]. In this review, we have summarized the distribution of ARA55 in tissues and its role in Epithelial-To-Mesenchymal Transitions (EMT) and given high priority to its biofunctions in cancers.

The Low Expression of Ara55 In Somatic Tissues and Cell Lines
ARA55 has a low tissue specificity with a low-to-medium expression in the cytoplasm in most tissues including smooth muscle ( Figure 1). ARA55 also reveals a low cell line specificity. The immunofluorescence staining shows that ARA55 protein is mainly localized to FA sites and the cytosol in both U-2 OS and U-251MG cell lines, and to the cytosol of A-431 cells ( Figure 2A). The RNA cell line category shows that cell lines derived from brain, skin, breast, sarcoma, fibroblast, endothelial and miscellaneous retain a medium level of ARA55 RNA ( Figure 2B).

Figure 1:
Immunohistochemistry assay reveals that ARA55 has low tissue specificity in most tissues. A.
The expression and distribution of ARA55 in colon, liver, kidney, testis, cerebral cortex, and lymph node.

B.
The expression of ARA55 protein in normal tissues. Note: Data were downloaded from the HUMAN PROTEIN ATLAS.
In the nucleus, ARA55 regulates the transcription activity of corticosteroids, androgens, mineralocorticoids and progesterone receptors as a receptor coactivator through phosphorylation or mediating PPIs [4][5][6]. Combining with Hsp27, it is also involved in the TGFβ-induced adhesion of vascular smooth muscle cells [14]. In

The Pathological Significance of Varied Ara55 Levels in Cancers
ARA55 is up regulated in several types of cancers compared with normal tissues. In cancer tissues, the expression peaks of ARA55 RNA are higher in several cases of prostate, pancreatic, colorectal, breast, liver and lung carcinomas, etc. (Figure 3). ARA55 serves as an activator of steroid hormone receptor in the occurrence of prostate cancer, showing the potential to promote tumor metastasis [17][18][19]. In breast cancer, ARA55 has been found to be involved in the regulation of tumor adhesion, invasion and migration [20].

A.
Moderate granular cytoplasmic staining is adopted in colorectal; prostate, breast, liver, and lung carcinomas and the moderate granular cytoplasmic staining is confirmed.

B.
Several cases of prostate colorectal, breast, endometrial, pancreatic cancers and squamous cell carcinomas reveals the moderate granular cytoplasmic staining by immunohistochemistry, and the remaining cancers are weakly stained or negative.
Note: Data were downloaded from the HUMAN PROTEIN ATLAS. Besides, ARA55 as a scaffold protein contributes to the formation of abdominal aortic aneurysm through MKK4 / p54 / JNK signaling pathway [21]. Qian, et al. [22] have found that ARA55 is highly expressed in pancreatic cancer, and the higher the expression level of ARA55 is, the worse the prognosis of patients will be. The downregulation of ARA55 in pancreatic cancer

cells (PCCs) significantly inhibits cell proliferation, invasion and
metastasis, and induces tumor cell apoptosis. Noguchi, et al. [23] confirm that there is a high expression of ARA55 in B16-F1 mouse melanoma cells. When the expression of ARA55 has been silenced by shRNA, the cell growth, proliferation, invasion and migration are inhibited to different degrees. In Hepatocellular Carcinoma (HCC), the expression of ARA55 is closely associated with tumor progression [24]. Data from the HUMAN PROTEIN ATLAS reveal that ARA55 can be rated as a prognostic biomarker in renal cancer and urothelial cancer (Figure 4), as those cancer patients with high protein levels of ARA55 have yielded unfavourable prognosis.
Therefore, it can be inferred that ARA55 acts as an oncogene in these tumors. Nevertheless, others suggest that ARA55 acts as a tumor suppressor gene.

Evidences show that the expression level of ARA55 in colorectal
cancer is significantly lower than that in adjacent tissues; and the overexpression of ARA55 in colorectal cancer LoVo cells triggers endogenous mitochondrial apoptosis [25]. In prostate cancer, the expression of ARA55 significantly decreases and is found to be negatively correlated with the degree of tumor differentiation.
The silenced expression of ARA55 has also been found in some prostate cancer cell lines [17,18].

Summary
ARA55 as a multi-domain protein can bind to a variety of structural proteins and signal proteins, which means it is involved in the formation of FAs in extracellular matrix and bridges the upstream and downstream proteins in a signal pathway. It also plays as a co-promoter of nuclear receptors in the activation of target genes. Like a double-edged sword, the biofunction of ARA55 in malignant tumors varies according to the tumor type.
Advancing research on the biofunctions and mechanisms of ARA55 in malignant tumors will unveil the key link in this variation.