IL-6 is an inflammatory cytokine with multiple biological effects; it is composed of 184 amino acids, with a molecular weight of 21–28 kDa. IL-6 has a 4-helix bundle structure consisting of 4 long α-helices (73–75). It has been reported that enhancing the production of IL-6 stimulates the expression of proinflammatory factors, such as IL-1, TNF-α, interferons, bacterial endotoxin and lipopolysaccharide or viral infection (76,77). Numerous studies have explored the role of IL-6 in kidney functions. Renal cortical tissue and kidney cancer tissue can produce IL-6 (78). IL-6 is expressed in most RCC cell lines and in patient tissues, and it plays an important role in the proliferation of RCC cells (78). Mechanistic studies showed that IL-6 activates IL-6 receptor (IL-6R) and glycoprotein 130 (gp130), causing phosphorylation of the tyrosine kinases JAK1, JAK2 and TYK2. This results in phosphorylation of STAT3 (79). Pathophysiological conditions play an important role in the regulation of IL-6. The transcriptional signal of IL-6 appears to be activated only during immune stress, and the IL-6/soluble IL-6R complex is usually upregulated in pathophysiological conditions (80). Matsumoto et al (81) indicated that tumor endothelial cells upregulate the expression of gp130 and downregulate the expression of membrane-bound IL-6R through the IL-6/IL-6sR complex, leading to cell proliferation, inhibition of apoptosis and enhancement of tumor development. IL-6 inhibits STAT1 phosphorylation, enhances STAT3 phosphorylation, promotes RCC proliferation and blunts the antitumor effect of IFN (82). Meanwhile, studies have shown that IL-6-activated plasma membrane-associated sialidase (NEU3) may also contribute to the expression of malignant phenotypes in RCC (83) (Fig. 3).

TNF is a potent pro-inflammatory cytokine that mediates complex biological responses, including inflammation, antiviral response, septic shock and apoptotic cell death (84). TNF can trigger apoptosis through the caspase-protease pathway and the NF-κB pathway. In the NF-κB pathway, TNF binds to TNFR to activate atypical protein kinase C and phosphorylates inhibitor of nuclear factor-κB kinase subunit β (IKKβ). Subsequently, the activated IKKβ phosphorylates inhibitor of NF-κB (IκB), resulting in ubiquitin-mediated degradation of IκB. NF-κB is released after IκB degradation and translocated into the nucleus where it activates the transcription of a number of anti-apoptotic genes, including the c-FLIP and c-IAP families (85,86). It has been shown that TNF-α regulates RCC cells growth by modulating the NF-κB-mediated anti-apoptotic pathway (55). Harrison et al (87) used an anti-TNF monoclonal antibody, infliximab, in two phase II clinical trials in patients with locally advanced and metastatic RCC. It was found that targeting TNF may be a beneficial therapeutic approach for cancer management. A previous study has reported that patients with high serum inflammatory cytokine levels in RCC have a poor prognosis, thus, TNF-α can be used as an independent prognostic indicator. Normal TNF-α plasma levels are high predictors of good prognosis in untreated patients (88).

TRAIL is a molecule belonging to the TNF superfamily; it is an effective anticancer agent as it specifically targets cancer cells while retaining normal cells, thereby inducing apoptosis (89). TRAIL can bind to death receptor DR4 and DR5, and assembles a death-inducing signaling complex by recruiting FAS-associated protein associated with death domain and caspase-8. Autocatalytic activation of caspase-8 leads to caspase cascade activation, ultimately leading to cell death (90). In cancer treatment, normal cells highly express decoy receptors DcR1 and DcR2, while cancer cells highly express death receptors DR4 and DR5 (89). TRAIL can bind to DR4 and DR5 and ultimately target cancer cell death (91). The complex formed by the binding of TRAIL to the DcR does not activate the apoptotic signaling pathway. Therefore, there is a weaker influence of TRAIL on the apoptosis of normal cells than cancer cells. Furthermore, TRAIL produced by immune cells has been shown to limit the progression of RCC (92).

CSF-1 is an important regulator of macrophage homeostasis (93). Co-expression of CSF-1 and CSF-1 receptor (CSF-1R) promotes proliferation and anti-apoptosis during regeneration of renal TECs. The CSF-1-dependent autocrine pathway is an important pathway for RCC growth. High levels of CSF-1 and tumor-associated macrophages (TAMs) are associated with a poor cancer prognosis. Co-expression of CSF-1 and CSF-1R on RCCs and adjacent TECs promotes tumor proliferation and tumor metastasis (94). The paracrine interaction between tumor cells and TAMs promotes tumor cell migration, invasion and metastasis, accelerating the spread of tumors in the host (95,96). Tumor-derived CSF-1 regulates TAMs, while CSF-1 and CSF-1R co-expression-regulated macrophages affect the TME of the host (97,98). A study by Dosquet et al (88) indicated that the autocrine CSF-1-dependent RCC mechanism is the core of RCC growth, and that the binding of CSF-1 to its unique cognate receptor CSF-1R promotes the survival and proliferation of RCC, and reduces apoptosis. Moreover, EGF induces CSF-1 and CSF-1R on RCC, thereby promoting tumor cell proliferation and inhibiting tumor cell apoptosis (99). This indicates that CSF-1R signaling promotes the growth of RCC.

Chemokines are a subfamily of cytokines that contain ~50 different signaling proteins. Similar to other cytokines, chemokines affect cell behavior through both autocrine and paracrine modes (100). Chemokines and their receptors are involved in the regulation of growth, angiogenesis and metastasis of RCC (101). It has been shown that CXCR4 is an ideal target for tumor diagnosis and treatment (102). The expression of CXCR4 in most cases is associated with tumor-specific survival in ccRCC with VHL mutations. A close association between VHL and CXCR4 has been observed. The VHL disease tumor suppressor (pVHL) is a protein that negatively regulates CXCR4. pVHL can target the degradation of HIF under normoxic conditions to prevent CXCR4 expression. This process is inhibited under hypoxic conditions (103–105). A study by Ieranò et al (106) indicated that the CXCL12-CXCR4-CXCR7 axis activates mTOR signaling in human renal cancer cells, resulting in enhanced RCC cell growth and invasion (106). Other studies have shown that the CXCR4-CXCL12-CXCR7 axis also plays a key role in RCC. The activity of CXCR4 is mainly γ-mediated; however, CXCR7 is considered to be an atypical G protein-coupled receptor, as it does not cause intracellular Ca²⁺ release when bound to a ligand (107). Some studies have shown that CXCR7 is a DcR that isolates extracellular CXCL12 or regulates the CXCR4 signaling pathway by forming a CXCR7-CXCR4 heterodimer (108,109). High expression of CXCR7 in human cancer types such as bladder cancer, glioma, colorectal cancer, ovarian cancer and breast cancer, and in tumor-associated blood vessels, may be critical for tumor cell survival, adhesion and growth (110–115). Overall, CXCR4 and CXCR7 are potential molecules affecting the prognosis of RCC (116).

MMPs are members of the zinc-dependent endopeptidase family; they regulate signaling pathways that control cell growth, inflammation or angiogenesis. Hence, MMPs participate in molecular communication between tumors and stroma, mediating microenvironmental changes during tumor progression (117,118). Compelling evidence from knockout mouse experiments has shown that MMPs play an important role in acute and chronic inflammation (119). Numerous studies have demonstrated that MMPs can aggregate leukocytes to cause tumor-related inflammation (120,121). Studies have confirmed that MMPs affect the early proliferation of metastatic tumor cells, while tissue inhibitors of MMP (TIMPs) inhibit MMP activity to indirectly regulate tumor cell proliferation (122,123). Among these MMPs, MMP-9 has the most important function in tumors. MMP-9 exhibit higher expression in malignant tumors than in benign or non-invasive tumors, and it also exhibits high expression in RCC; it can denature type I, II and IV collagen, enabling tumor cells to penetrate the basement membrane (124,125). Additionally, membrane type 1 MMP (MT1-MMP/MMP-14) is involved in tumor invasion; it is widely expressed in most malignant tumors, and its overexpression enhances cell invasion ability (126). MT1-MMP not only degrades extracellular matrix molecules such as type I, II and III collagen, vitronectin, laminin-1 and −5, fibronectin and aggrecan (127), but it also recruits pro-MMP-2 to the cell surface and causes the activation of MMP-2 by cleaving the propeptide sequence (128). A study by Petrella and Brinckerhoff (129) showed that MT1-MMP is the main trigger of type I collagen degradation and the invasiveness of VHL RCC cells expressing MT1-MMP or HIF-2α. In addition, in the absence of VHL, the protein and gene levels of MT1-MMP are significantly upregulated in RCC (130).

TAMs are derived from peripheral blood mononuclear cells. Macrophages usually undergo M1 or M2 activation under inflammatory stimuli (131). In RCC, M1 cells produce inflammatory cytokines such as TNF-α, IL-6, IL-12 and IL-23, while M2 cells produce anti-inflammatory cytokines such as IL-10, thereby promoting RCC-related immune dysfunction (132). Santoni et al (133) showed that high TAM infiltration in the RCC microenvironment promotes tumor progression and metastasis by stimulating angiogenesis, tumor growth and cell migration. RCC is a typical hemangioma in which VEGF is significantly upregulated. VEGF, considered to be a chemokine of TAM, supports tumor proliferation. TAM can self-produce VEGF, which increases the accumulation of TAM in tumor sites (134). TAM plays a key role in RCC immunosuppression and T-cell tolerance by secreting immunosuppressive factors and inducing T-cell immunity without response (135).

MDSCs are a group of heterogeneous cells derived from the bone marrow, which are preferentially expanded in cancer and have a significant ability to suppress immune cell responses; they primarily inhibit T-cell proliferation and NK-cell activation, and induce differentiation and proliferation of regulatory T cells (136). MDSCs have the ability to inhibit T-cell activation through upregulation of arginase-1 (Arg1) and inducible nitric oxide synthase in monocytic MDSCs, and Arg1 and reactive oxygen species in granulocytic MDSCs (137). The TME affects the progression and metastasis of solid tumors, which consist of tumor cells and other primitive stromal cells (138,139). MDSCs are the main components of the TME, and the increase in blood volume is associated with a shorter patient survival time. Mechanistic studies have shown that MDSCs promote tumor cell survival, associated angiogenesis, invasion and metastasis (140,141). MDSCs also protect tumor cells from immune-mediated killing, establish a TME and interact with tumor cells to promote epithelial-mesenchymal transition to support tumor growth and metastasis (142). A close association between MDSCs and clinical outcomes of cancer patients has been established. MDSCs hold great promise as novel biomarkers for tumor prognosis (143).

Simvastatin is a cholesterol-lowering drug for the prevention of cardiovascular disease, and is involved in tumor growth, spread and endothelial function (149). A previous study has reported that the extracellular signal-regulated kinase (ERK)1/2 signaling pathway is a statin-dependent pro-apoptotic mediator (150). Knockdown of ERK can make RCC cells sensitive to simvastatin-induced anticancer effects. Simvastatin can inhibit the proliferation and migration of RCC cells by inhibiting the phosphorylation of AKT, mTOR and ERK; it also exhibits antitumor effects by inhibiting the IL-6-induced phosphorylation of JAK2 and STAT3 (151).

ATRA is the active metabolite of vitamin A, involved in cell proliferation, differentiation and apoptosis; its role is mediated by nuclear flavonoid receptors, MAPK and cAMP/cAMP-dependent protein kinase signaling pathways (152). ATRA reduces cell proliferation and alters gene expression through nuclear receptor- and non-receptor-mediated pathways, thereby accelerating cell differentiation and apoptosis. In genomic action, the function of ATRA is mediated by nuclear receptors, particularly retinoic acid receptors (RARs) (α, β and γ). Nuclear RAR acts as a retinoid-inducible transcription factor, and regulates cell cycle arrest, cell differentiation and cell regulation through heterodimer formation with retinoid X receptor (153). Among the RARs, RARβ is a tumor suppressor that is expressed at low levels in a number of cancer types, such as breast and prostate cancer, and whose expression is regulated by ATRA (154). In the non-genomic pathway, ATRA independently activates the transcription of genes involved in the PI3K/AKT pathway in cells, reversing the dysregulation of the PI3K/AKT pathway in most human cancer types. In more detail, this process entails the activation of PI3K by ATRA through G-protein coupled receptors and multiple receptor tyrosine kinases. Activated PI3K catalyzes the production of phosphatidylinositol-3,4,5-triphosphate to promote aggregation and activation of AKT on the membrane (155).

Nivolumab is a programmed death 1 (PD-1) checkpoint inhibitor that selectively blocks the interaction between PD-1 and PD ligand (PD-L)1/PD-L2 expressed on activated T cells, thereby preventing T-cell inactivation (156). Expression of PD-L1 as a remote immunomodulator occurs in 20–50% of human cancer types. A variety of cancer immunotherapies targeting the interaction between PD-L1 and PD-1 have been developed (157,158). PD-L1 effectively inhibits the tumor-killing ability of T cells. Once the PD-L1:PD-1 interaction is blocked, T cells can quickly restore their effector function. PD-L1 expressed on tumor cells binds to PD-1 on activated effector T cells, and phosphatase SHP-2 is recruited, resulting in inactivation of the PI3K signaling cascade (159). It has been found that PD-L1 or PD-1 inhibitors have a positive effect on the treatment of cancer. Nivolumab exerts antitumor activity in metastatic RCC (160). The immunotherapeutic agent ipilimumab is a human cytotoxic T-lymphocyte antigen-4 (CTLA-4) immune checkpoint inhibitor antibody, which can prevent CD80 and CD86 ligands on antigen-presenting cells from binding to the CTLA-4 receptor on activated T cells, thereby preventing the downregulation of antitumor T cell activity (161). CTLA-4 plays a significant role in early immune response, primarily occurring in lymphoid tissues, while PD-1, whose expression is upregulated after T-cell activation in peripheral tissues, is more involved in late immune response (162). The combined application of CTLA-4 and PD-1 blockers can synergistically activate the antitumor immune response and increase the response rate of patients (163). Thus, a combination of nivolumab and ipilimumab will effectively control tumor development with a good safety profile (164).

Emerging clinical data reveals that immunotherapy has great potential in the treatment of cancer. Associated studies have shown that a series of gradual and repeated immune response events, defined as the cancer-immune cycle, can effectively kill cancer cells. In the first step of this process, tumor antigens are captured by dendritic cells (DCs) for processing (165). Proinflammatory cytokines and factors released by dying tumor cells can be used as a designated immune signal to avoid induction of tumor tolerance antigens (step 1) (166,167). DCs then present the captured antigen on the major histocompatibility complex I (MHCI) and MHCII molecules to the T cells (step 2) (168). This initiates and activates the effector T-cell response (step 3) (169). This is followed by the entry (step 4) and invasion (step 5) of the tumor bed by the activated effector T cells (170), which then recognize and bind to specific cancer cells (step 6) and kill them (step 7) (171). Subsequently, the tumor-associated antigens released from the dying cells amplify the cycle of immune response and make it more widespread and repetitive (172).

The entire immune cycle is enhanced through the positive regulatory signal or suppressed via the negative regulatory signal in the aforementioned steps during therapeutic treatments. The first phase of treatment corresponds to chemotherapy, radiotherapy and targeted therapy. These treatments induce immune cell death by activating the immune system (173). The second phase corresponds to use of cancer vaccines, which rely on tumor cell-associated antigens to awaken the immune system against cancer (174). The third phase is mainly associated with CTLA-4 inhibitors. CTLA-4 is a receptor found on the surface of activated T-cells, which predominantly acts by competing with CD28 receptors for binding to B7 ligands on antigen presenting cells (APCs). In the process of T cell activation, CD28 receptors on the T-cells bind to the B7 ligands on APCs and provide the essential second activation signal for T-cell. However, CTLA-4 receptors can competitively bind to B7 ligands with higher affinity, resulting in the lack of second activation signal. Lack of the second activation signal in the presence of CTLA-4 receptors would lead to anergy in T-cells (163,175). The fourth phase involves the transport of T lymphocytes, and no intervention is available at this stage. The fifth stage is predominantly through anti-VEGF treatment to enhance T cell transport and tumor bed infiltration. The transfer of activated T cells from the lymph nodes into circulation and then to the tumor requires a series of steps. VEGF promotes the formation of abnormal tumor blood vessels, which can negatively affect the transmigration of T-cells from lymph nodes to the tumor bed (176). In addition, the blockade of VEGF can increase the expression of E-selectin on tumor vascular endothelium and down-regulate the Fas ligand on vascular endothelial cells, ultimately promoting the increased of T-cell tumor infiltration (177). The sixth phase involves CAR-T-cell therapy, which is to generate a powerful immune-mediated anti-tumor response through the in vitro manipulation of T-cells (178). This treatment is achieved through the selection and expansion of tumor-infiltrating lymphocytes (TILs), or through gene transfer of a synthetic TCR (sTCR) or a chimeric antigen receptor (CAR) into T-cells (179). The seventh stage corresponds to PD-1/PD-L1 inhibitor treatment (180). Immunotherapy has successfully been applied to RCC in recent years (172).

In addition, it has been found that inhibiting the inflammatory pathway is an effective approach to suppress the progression of RCC. Thus agents, such as LY294002, that inhibit the PI3K/AKT signaling cascade may benefit patients with RCC (181). Activation of the 15-lipoxygenase 2/15(S)-hydroxyeicosatetraenoic acid pathway increases the metabolism of arachidonic acid in the RCC TME, compromising the function of the recruited immune cells, thereby promoting local immunosuppression and tumor escape (182).