Renal cell carcinoma (RCC), or renal cancer, originates from the epithelial cells of the renal tubules and accounts for 2-3% of malignancies in adults and is the most lethal urological tumor. It is the 6th and 8th most common malignant tumor in men and women, and the incidence of kidney cancer is increasing at a rate of about 2.5% per year. There are no obvious symptoms in the early stage of kidney cancer, and about 30% of patients have metastatic kidney cancer by the time they are diagnosed.
For early stage limited kidney cancer, surgical resection is the best treatment option. However, about 30% of patients with limited kidney cancer will develop local recurrence or distant metastasis after surgery. Metastatic kidney cancer has a poor prognosis and is insensitive to radiotherapy and chemotherapy, with a 5-year survival rate of less than 10%.
In recent years, the emergence of molecularly targeted drugs, represented by sunitinib, has brought new hope to patients with advanced kidney cancer. Because of this, urologists should not only strengthen the improvement of kidney cancer treatment technology, but also pay attention to the experimental progress related to kidney cancer and understand the translational medicine research in this field.
I. Molecular mechanism of kidney cancer development
There are several pathological types of kidney cancer, among which clear cell carcinoma (ccRCC) is the most common, accounting for 70%-80% of the total, and ccRCC has been studied most intensively and extensively. Mutations in the oncogene Von Hippel-Lindau syndrome (VHL) have been shown to play a key role in the development of ccRCC. Under normal conditions VHL protein binds to and degrades hypoxia-inducible factor (HIF-α), maintaining low levels of HIF-α.
When VHL proteins are inactivated by hypoxia or VHL gene mutations, HIF-α accumulates in the cytoplasm without the pathway of VHL protein-mediated ubiquitin degradation. Subsequently, HIF-α enters the nucleus and binds covalently with HIF-β to form a transcriptionally active dimer that upregulates the expression of a series of downstream target genes, including vascular growth factor (VEGF) and platelet-derived growth factor (PDGF), which promote vascular neogenesis, cell proliferation, and energy metabolism.
About 60% of ccRCC patients have VHL gene mutations, resulting in a large accumulation of HIF-α in the cytoplasm and a sustained activation of the VHL-HIF signaling pathway, which releases large amounts of VEGF, PDGF, and other cytokines.
These growth factors bind to the VEGF receptor (VEGFR) and PDGF receptor (PDGFR) on the cell membrane and initiate the receptor tyrosine kinase signaling system, which continuously activates the mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt) signaling pathways, leading to the development and progression of kidney cancer.
Recently, Brannon et al. classified ccRCC into ccA and ccB subtypes by large-scale gene expression analysis, and further results showed significant differences in molecular phenotype and clinical prognosis between these two subtypes.
Although HIF-α accumulation in the cytoplasm due to VHL mutations plays an important role in ccRCC development, a proportion of patients (30% – 40%) do not have VHL mutations, suggesting that other mechanisms are involved in renal carcinogenesis. Current studies suggest that in addition to the classical changes in the VHL-HIF signaling pathway, mechanisms involved in ccRCC pathogenesis include aberrant activation of signaling pathways such as Notch, nuclear factor KB (NF-KB), MAPK, and PI3K/Akt.
Our results show that Jaggedl ligands and Notchl receptors in the Notch family also play an important role in the development of renal cancer. The expression of Jaggedl was significantly higher in kidney cancer than in the corresponding normal kidney tissue, and the expression level of Jaggedl was closely related to the size, grade, and stage of the tumor as well as the prognosis of the patients.
Further studies showed that there was aberrant activation of Jaggedl/Notchl/Hesl signaling in kidney cancer and that this aberrantly activated Notchl signaling could promote tumor growth by activating PI3 K/Akt signaling to promote proliferation, adhesion-independent growth, and cell cycle progression in G1-S phase of kidney cancer cells.
We also found that the expression level of Klotho protein, which has anti-aging effects, was significantly lower in kidney cancer than in normal kidney tissues; Klotho exerts anti-cancer effects by inhibiting PI3 K/Akt/GSK3B/Snail signaling pathway to inhibit epithelial mesenchymal transformation, migration and invasion of kidney cancer cells. These studies have enriched our understanding of the molecular mechanisms of kidney cancer pathogenesis.
Immunotherapy of kidney cancer
Since kidney cancer is a highly immunogenic tumor, immunotherapy is often used for metastatic kidney cancer, i.e., cytokine injection [IFN-α or IL-2] or tumor vaccination to enhance the body’s anti-tumor immune response. Although cytokines have been used in clinical practice for many years, the response rate for IFN-α treatment is only 6-15%; the response rate for IL-2 treatment is only 7-27%.
Tumor vaccines for kidney cancer are still in clinical trials. Antigen vectors can be RNA, DNA, peptides, or whole cells. The most promising clinical applications are cellular vaccines, including autologous tumor cell vaccines, genetically modified tumor cell vaccines, and dendritic cell vaccines. Dendritic cells are the most powerful antigen-presenting cells in the body, and mature dendritic cells can induce anti-tumor immune responses in the body.
Dendritic cell-based tumor immunotherapy is a hot research topic in tumor immunotherapy today. Granulocyte-macrophage colony-stimulating factor (GM-CSF) can induce the maturation of dendritic cells and thus enhance their antigen-presenting ability, and GM-CSF-modified tumor cell vaccines have shown promising antitumor effects and are already in clinical trials.
Our results show that the efficacy of tumor cell vaccines can be significantly enhanced by the interaction between the tumor/testis antigen NY-ESO-1 and dendritic cell surface receptors. Programmed cell death receptor 1 (PD-1) is an inhibitory receptor expressed on the surface of T cells. Recent studies have shown that neutralizing antibodies against PD-1 or its ligand PD-L1 can significantly improve the body’s anti-tumor immune response and thus can be used in the treatment of advanced kidney cancer.
Molecular targeted therapy for kidney cancer
In recent years, the emergence of molecularly targeted drugs, represented by sunitinib, has brought new hope to patients with advanced kidney cancer. Currently, there are two main categories of FDA-approved targeted drugs for kidney cancer: VEGF/VEGFR inhibitors and mammalian target of rapamycin (mTOR) inhibitors. Sunitinib inhibits tumor angiogenesis and causes hypoxic necrosis of tumor cells mainly by inhibiting VEGFR/PDGFR receptor tyrosine kinase activity.
Our study showed that sunitinib, in addition to inhibiting tumor angiogenesis, has a direct mechanism of action on the tumor cells themselves by inhibiting the NF-KB signaling pathway to upregulate p53/Decl activity, which causes tumor cell senescence. However, with the widespread use of sunitinib, its drug resistance problem has gradually emerged. Clinically, complete or long-term response to sunitinib is rare; and most patients develop resistance or treatment resistance 6-15 months after dosing.
This is manifested by temporary tumor growth arrest or tumor shrinkage after treatment, but followed by continued tumor growth, distant metastasis and disease progression. Therefore, deciphering the resistance mechanism of molecularly targeted drugs has become a hot research topic to solve this clinical dilemma.
Current studies have shown that after the application of anti-angiogenic targeted drugs such as sunitinib, tumors will initiate other pathways so that they no longer rely on VEGFR/PDGFR signaling, but regain the ability of angiogenesis, tumors continue to grow, and gain stronger invasive and metastatic abilities, possible mechanisms include: (1) upregulation of other pro-angiogenic cytokines or signaling pathways, such as IL 8, fibroblast growth factor (FGF), and angiopoietins; (2) recruitment of bone marrow-derived vascular progenitor cells and pro-angiogenic monocytes/macrophages to build neovascularization required for tumor growth; (3) increase in pericytes covering the tumor vasculature to support tumor neovascularization; and (4) invasion of tumor cells into adjacent normal tissues to obtain oxygen and metastasis. adjacent normal tissues to obtain oxygen and nutrients, i.e., the tumor acquires a stronger ability to invade and metastasize.
IV. Kidney cancer stem cells
Tumor stem cells have the characteristics of self-renewal, differentiation potential, high tumorigenicity and multi-drug resistance, which are the root cause of tumor formation and continuous growth of tumors with different degrees of differentiation, and are the “starting cells” and “driving cells” for tumor development, metastasis and recurrence. Currently, it is believed that stem cells can be identified by stem cell surface molecules, and common kidney cancer stem cell markers include CD44, CD105, CD133, etc.; kidney cancer stem cells expressing these molecules are involved in tumor formation, invasion and metastasis.
Tumor stem cells have many similarities with adult stem cells and maintain their stem cell properties mainly through activation of four signaling pathways, namely Notch, Hedgehog, Wnt and bone morphogenetic protein (BMP). It has been demonstrated that tumor hypoxia can lead to enrichment of tumor stem cells, and this phenomenon is achieved through the interaction between HIF-1 and Notchl signaling pathways.
We suggest that the use of sunitinib for the treatment of advanced kidney cancer inhibits tumor growth mainly through inhibition of tumor neovascularization, but also exacerbates tumor hypoxia, and continues to activate the Notch1 signaling pathway through the cross talk between HIF-1 and Notchl signaling pathway, which mediates the enhancement of kidney cancer stem cell-like phenotype, invasive metastatic ability, and treatment resistance of sunitinib. Therefore, targeted inhibition of the Notch1 signaling pathway may enhance the efficacy of sunitinib or reverse drug resistance.
V. Renal cancer micro RNA micro
MicroRNAs (miRNAs) are evolutionarily conserved endogenous non-coding small molecule RNAs. mature miRNAs function as ribonucleoprotein complexes, degrading mRNA by complementary base-pairing with the non-coding region at the 3′ end of target mRNAs, inhibiting protein translation, mediating post-transcriptional regulation of genes, and leading to silencing of specific silencing of specific genes.
It is now recognized that miRNAs affect a wide range of physiological and pathological processes in the body through post-transcriptional regulatory mechanisms. Juan et al. analyzed miRNAs in 28 cases of ccRCC and normal kidney tissues using a real-time fluorescent quantitative nucleic acid amplification assay (QPCR) and found that 26 miRNAs were down-regulated in kidney cancer tissues and 9 miRNAs were up-regulated, including miR-34a, which was specifically up-regulated in kidney cancer. These included miR-34a, which was specifically upregulated in kidney cancer.
Jung et al. found that the combination of up-regulated miR-155 and down-regulated miR-141 was 97% accurate in distinguishing normal kidney tissues from kidney cancer tissues. The results showed that miR-155 could directly target the VHL gene to promote the activity of HIF signaling pathway and angiogenesis. Therefore, miRNA has the potential to become a molecular marker for kidney cancer diagnosis and a new target for kidney cancer treatment.
VI. Prognostic assessment of kidney cancer impact
The most important factor in the prognosis of kidney cancer is the pathological stage, however, the prognosis of patients with the same stage still varies greatly, which requires the establishment of a perfect prognostic judgment model or the discovery of new biomarkers. The UISS model proposed by UCLA and the SSIGN model proposed by Mayo Medical Center are the most widely used models for prognosis of kidney cancer.
The former integrates 3 indicators, TNM staging, Fuhrman grading and ECOG physical status score, and divides the risk of recurrence of kidney cancer into 5 groups after surgery; the latter integrates 4 pathological parameters, TNM staging, tumor size, Fuhrman grading and tumor necrosis, to stratify the prognosis of patients. The predictive accuracy of these models can be further improved by introducing new molecular markers.
The results of Jensen et al. showed that the level of tumor-associated neutrophils (CD66b+) was an independent predictor of prognosis in limited renal carcinoma, and Schutz et al. showed that polymorphisms in the MET gene were strongly associated with postoperative recurrence in patients with renal cancer. In metastatic kidney cancer, polymorphisms in the signal transducer of transcription and activator 3 (STAT3) gene were effective in predicting the response rate of patients to IFN-α therapy.
Our results showed that both M1 (CD68+CDllc+) and M2 (CD68+ CD206+) tumor-associated macrophages (TAM) with two polarization states were present in the kidney cancer microenvironment, and the relative number between M1 and M2 could suggest the prognosis of patients. When Ml-type TAM predominates, patients have a good prognosis and tend to survive for a long time after surgery; conversely, when M2-type TAM predominates, patients generally have a poor prognosis and are more likely to have local recurrence and distant metastasis of the tumor after surgery.
VII. Conclusion
In recent years, as the experimental research of kidney cancer continues to progress, the understanding of the molecular mechanism of kidney cancer pathogenesis has been gradually improved, and new targeted drugs and immunotherapy have emerged. However, kidney cancer of the same pathological type has great heterogeneity in terms of gene and protein levels; the prognosis of patients with the same pathological stage is also different. Therefore, molecular staging of kidney cancer and the prognostic assessment and individualized treatment based on it will be the focus of future experimental research.