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Activated Cdc42-binding protein kinase 1(ACK1) has been found to be over-activated in many types of tumors as a signaling protein. ACK1 protein belongs to a family of non-receptor tyrosine kinases that specifically bind to Cdc42 and inhibit its GTPase activity. The ACK1 gene is located on the human chromosome 3q29.ACK1 gene-translated protein molecule and is modified by cleavage, tyrosine phosphorylation, etc. The final molecule has a total length of 1038 amino acids and a molecular weight of about 140 kDa. The ACK1 protein has its unique structural features different from other tyrosine kinases: the SH3 domain of its protein sequence is located on the carboxyl side of the kinase domain while the SH3 domain in the protein sequence of other non-receptor tyrosine kinases is usually located on the amino side of the kinase domain.
As a tyrosine kinase, ACK1 mainly functions as a phosphorylation of a substrate tyrosine residue in a cell, that is, phosphorylation of a tyrosine residue of a substrate protein. In the cellular signaling pathway, ACK1 transmits signals from receptor tyrosine kinases (RTKs) to downstream effector proteins. The upstream of the MHR domain of the ACK1 molecule has a proline-rich residue sequence that interacts with the methionine 409 residue in the SH3 domain to promote binding of the MHR domain to the kinase domain, inhibits kinase activity, and makes ACK1 inactive. When the ligand activates RTKs, RTKs act on the MHR domain of ACK1, releasing the inhibition of kinase by MHR, and making ACK1 active. ACK1 also has another activation mode. Recent studies have found that the SAM domain in the ACK1 molecule contains a membrane-directed signal that causes ACK1 to localize to the cell membrane. At the same time, the ACK1 molecule is symmetrically homodimerized. Finally, ACK1 is in an activated state. Therefore, ACK1 can change the activity of its own kinase through the allosteric effect, so that the cells can quickly adapt to changes in the surrounding environment.
Multiple ligands can activate ACK1. For example, epidermal growth factor (EPF), growth arrest specific gene 6, modulating protein, insulin-like growth factor (IGF), and insulin, suggesting that ACK1 is the main transmitter of RTK signal. Jones et al. found that the UBD domain of ACK1 interacts with the autophagy receptor p62/SQSTM1, revealing that ACK1 interacts with p62/SQSTM1, NBR1 and Atg16L to induce activation of EGFR degradation. Buchwald et al. found that ACK1 interacts with SIAH ubiquitin ligase to induce activation of EGFR degradation. SIAH ubiquitin ligase inhibits tumor formation. In summary, in addition to the EGFR activation pathway, other pathways can also induce tumor formation.
Carcinogenic Mechanism of ACK1
The discovery of the carcinogenic properties of the ACK1 protein was obtained by studying the activation of ACK1 to promote the growth of non-adherent cells and the growth of tumors in vivo. There are at least three mechanisms for abnormally activated ACK1 tumorigenic properties: (1) continuous activation of RTK and continuous activation of ACK1; (2) ACK1 gene amplification; (3) ACK1 gene somatic cell mismutation.ACK1 is characterized by rapid activation. Phosphorylation of tyrosine is used as an activation standard. When growth factors stimulate cells, not only rapid activation of RTKs but also rapid activation of ACK1 is observed. The activation process is time-dependent. At present, the study found that there are two main mechanisms for the activation of extracellular signal-regulated ACK1: (1) RTKs directly activate ACK1; (2) cell type-dependent specific ligand/RTK regulates ACK1 activation. In addition, multiple RTKs interact with ACK1 to induce their activation. Due to different cell types, there is a significant difference in the duration of ACK1 activation in various cells.
Gene sequencing suggested that the ACK1 gene was significantly amplified in various types of tumor cells and induced ACK1 mRNA overexpression, and activation of ACK1 by gene amplification was an RTK-independent pathway. The study found that ACK1 gene amplification can be found in cervical cancer, ovarian cancer, lung squamous cell carcinoma, head and neck squamous cell carcinoma, breast cancer, prostate cancer, and gastric cancer.
In addition, the study found that ACK1 directly interacts with the oncogenic factor AKT and phosphorylates AKT tyrosine 176. Tyrosine 176 phosphorylated AKT selectively binds to phospholipids or phosphatidic acid on the plasma membrane, followed by threonine 308 and serine 473 phosphorylation, AKT activation. Activated AKT is transferred into the nucleus, which inhibits the expression of pro-apoptotic genes and cell cycle arrest genes, promotes cell mitosis, inhibits apoptosis, and promotes cell proliferation. ACK1 gene may be somatic mutations E346K stimulate tyrosine 176 phosphorylation of AKT and AKT activation, suggesting that the tumor cells in the carcinogen led by ACK1 can be suppressed to avoid RTK / PI3K-dependent activation of AKT pathways.
Figure 1. ACK1 signaling network. (Mahajan, et al. 2013).
ACK1 and Tumor
Lei et al. first screened isolated large liver cancer (SLHCC, single nodule, tumor diameter >5 cm) and nodular liver cancer (NHCC, number of nodules ≥2) by cDNA microarray. The results suggest that the expression level of ACK1 in liver cancer tissues was significantly higher than that of normal liver tissue. Then they confirmed this conclusion by real-time fluorescent quantitative (qRTPCR), and confirmed by immunohistochemistry that ACK1 protein was mainly distributed in cytoplasm. ACK1 is overexpressed in liver cancer cells.
Xie et al. used qRT-PCR and immunohistochemistry to analyze the expression level of ACK1 in liver cancer tissues and paracancerous tissues of 150 patients with liver cancer, and analyzed the expression level of ACK1 and the clinicopathological characteristics of the corresponding patients. The expression level of ACK1 was positively correlated with the pathological grade, number, vascular invasion and TNM stage of the tumor, but not with the gender, age, HBsAg, alpha-fetoprotein, tumor number, cirrhosis and tumor capsule. They also followed up patients and found that patients with high ACK1 expression group had significantly lower overall survival and disease-free survival than patients with low ACK1 expression. It has also been found that overexpression of ACK1 in liver cancer patients predicts tumor recurrence and poor survival. Therefore, overexpression suggests a poor clinical outcome for liver cancer patients.
Epithelial to mesenchymal transition (EMT) is considered to be one of the important mechanisms leading to tumor cell metastasis. It is found that ACK1 may play a certain role in the process of EMT. After overexpression of ACK1, epithelial cell markers such as E-cadherin were significantly reduced in liver cancer cells, but interstitial cell markers such as vimentin, fibronectin and N-cadherin were significantly increased in the cells. It is suggested that ACK1 may be involved in the EMT of hepatoma cells. This study also found that ACK1 activates AKT by phosphorylating the AKT serine 473 pathway, instead of phosphorylating the AKT threonine 308 pathway.