Cell surface transmembrane proteins are mainly composed of G-protein-coupled receptors (GPCRs), ion channels and transporters, which play an important role in neuronal signal processing and plasticity in the brain. The function of GPCRs in cell motility, growth differentiation and gene expression is closely related to tumors. The GPCRs are the largest family of cell-surface receptors and are ubiquitous on the surfaces of various eukaryotic cells. Conjugated GPCR proteins also mediate different signaling pathways
[36]. As shown in
Figure 3, GPCRs include a variety of neurotransmitters, peptide hormones and C-X-C chemokine receptor (CXCR), and receptors that accept exogenous physical and chemical factors in the senses of taste, vision and smell. CXCR4 plays a key role in tumor invasion and metastasis. CXCR can induce neutrophils, lymphocytes, monocytes and fibroblasts to aggregate and activate the inflammatory sites and participate in tissue injury repair. CXCR is important member of the GPCR family, it mainly causes downstream signal transmission. G proteins regulate ion channels through second messengers, and the activity of many ion channels is influenced by specific GPCR activation. Phosphatidylinositol signaling pathway is an extracellular signal molecule that binds to GPCRs on the cell surface, activates phospholipase C on the plasma membrane, hydrolyzes phosphatidylinositol bisphosphate (PIP
2) on the plasma membrane into the inositol 1,4,5-trisphosphate (IP
3) and diacylglycerol (DG), and converts extracellular signals into intracellular signals. The phosphatidylinositol-3-kinase/protein kinase B/mammalian target of rapamycin (PI3K/Akt/mTOR) signaling pathway is considered to be involved in the regulation of cellular physiological processes through the activation of downstream effector factors, and is directly related to cell growth, proliferation, cancer and longevity. This pathway is involved in the occurrence of human diseases and can regulate many biological functions in the body
[37]. cGMP activates cGMP-dependent protein kinase PKG and downstream MAPK pathways, resulting in tumor cell dryness and metastasis
[38]. Structurally, these receptors are monomer proteins with the amino terminus on the outer surface of the cell and the carboxyl terminus on the inner membrane. The complete peptide chain crosses the membrane seven times and so this type of receptor is also called a sevenfold transmembrane receptor. As the peptide chain repeatedly crosses the membrane, several loop structures are formed on the outer and inner sides of the membrane. These are responsible for binding to ligands (chemical and physical signals) and intracellular signal transmission, respectively. The cytoplasmic portion interacts with a GTP-binding protein (G protein), which is the first signaling molecule in the pathway
[39]. G protein-coupled receptors regulate a variety of intracellular signaling cascades including G protein-dependent and G protein-independent pathways
[40].
Figure 3. The structure of G protein-coupled receptors and downstream signal transduction pathways. It shows that the agonist binds to activate the receptor by inducing a conformational change in the transmembrane domain (TM6, blue). The activated receptor binds to a variety of intracellular signaling proteins, including G proteins (light purple), and GRKs (light orange), that are active (yellow) and inactive (dark orange). The coupling of heterotrimer G proteins to the receptor initiates nucleotide exchange, and then the G proteins dissociate into the Gα, Gβ and Gγ subunits. Both subunits regulate different downstream effector proteins. The GTP-bound Gα subunit regulates the activity of adenylate cyclase (AC, dark purple), whilst the Gβ and γ subunits interact with the g-protein-coupled internal rectifying potassium channel (GIRK, represented by cylindrical TM, green). The G protein-mediated signaling pathway is terminated by GTP hydrolysis and the recombination of Gα with Gβ and γ to form inactive heterotrimers. Activation of the receptor also leads to phosphorylation of GRKs and subsequent coupling of statin. Statin-coupled receptors lead to desensitization and statin-mediated activation of downstream effector proteins, such as mitogen-activated protein kinases (MAPKs) or SRC kinases. Statin activation also promotes receptor internalization into the endosome and subsequent degradation or circulation into the plasma membrane.