Renal cell carcinoma (RCC) is the third most common genitourinary malignancy worldwide (1, 2). In 2022, it is estimated that 79,000 new cases are diagnosed as RCC and 13,920 related deaths in the United States (3). Pathologically, RCC incorporates three main subtypes, including clear cell RCC (ccRCC), papillary RCC, and chromophobe RCC (4). Clear-cell RCC is the most common type of RCC with high aggressiveness, accounting for approximately 80% of all RCC pathology types (5). Around 30% of RCC patients are diagnosed as having advanced RCC, and the five-year survival rate is 23% (6). Hence, there is an urgent need to discover the underlying mechanisms of high invasiveness and high metastatic potential to find more reliable biomarkers that could assist in diagnosing and predicting prognosis.

Metabolic reprogramming is widespread in malignant tumors, the most well-known of which is glucose metabolic reprogramming that is termed the “Warburg effect” (7). This inefficient form of energy metabolism supplies the need for new proliferating cancer cells and enables the use of intermediate products to yield biomolecules, such as amino acids, and nucleotides (8). Previous studies revealed that amino acids could have impacts on cell proliferation, the tumor microenvironment, epigenetic modification, and drug resistance (9–14). Previous studies also revealed that amino acid aberrant metabolism was associated with tumor progression and immune infiltration in ccRCC and other cancers (15–18). Therefore, to better understand the profound mechanisms, studies are in demand to identify key amino acid metabolism-related genes and transfer them to drug targets.

L-dopa decarboxylase (DDC) locates at chromosome 7p and encodes a protein that catalyzes the decarboxylation process of L-3,4-dihydroxyphenylalanine (DOPA), L-5-hydroxytryptophan, and L-tryptophan to dopamine, serotonin, and tryptamine, respectively (19). Our previous proteomic analysis demonstrated that L-dopa decarboxylase was significantly downregulated in ccRCC (15). The regional dopamine of the kidney plays a potential role in regulating blood pressure, and the dysregulation of DDC might lead to hypertension, which is a common symptom of RCC (20). Tremmel et al. found that DDC was a favorable prognostic factor in breast cancer (21). However, in prostate cancer, the higher expression of DDC was associated with advanced stages, higher Gleason scores, biochemical recurrence, and short disease-free survival (DFS) (22). Also, the role of DDC has been investigated in the development of colorectal cancer (23), small cell lung cancer (24), and gastric cancer (25). However, the prognostic value and underlying mechanism caused by aberrant L-dopa decarboxylase expression have not been systematically elucidated in ccRCC.

In this study, we thoroughly performed DDC-related bioinformatics analysis in ccRCC and validated conclusions using external cohorts from multi-omics data. We found the downregulation of DDC in ccRCC was significantly associated with worse outcomes. Furthermore, DDC expression showed close relationships with clinicopathologic features and prognosis. We also revealed that DDC was correlated with immune cell infiltration and expressions of immune checkpoint genes. In order to boost the knowledge of basic cancer biology, our study sought to identify the underlying mechanisms of DDC in carcinogenesis and provided a new therapeutic target for ccRCC patients.

Kidney cancer is a highly genetically heterogeneous malignant tumor, which may cause patients from different races and regions to carry different gene mutations and genetic phenotypes, which in turn cause the biological behavior of tumor cells and different sensitivity to treatment (32, 33). Therefore, molecular characteristics and subtypes based on multi-omics data are essential for improving treatment efficacy and promoting the achievement of precision medicine in cancer (34, 35). Although there is a growing interest in the function of amino acid metabolism-related genes in cancer, little is known about how DDC proteins work in ccRCC, and it is uncertain whether DDC expression may be used as diagnostic or prognostic markers. Here, we assessed the diagnostic and prognostic value of DDC mRNA and protein expression in external ccRCC cohorts and found potential associations between DDC expression and clinicopathological features. We also explored functional analysis and found aberrant enrichment in the PI3K-Akt signaling pathway. Analysis of immune cell infiltration and ICP expression revealed the underlying regulatory effects of DDC on the tumor microenvironment (TME) and immune system.

In our study, we discovered that DDC mRNA and protein expression were downregulated in ccRCC compared to adjacent normal tissue. There were potential correlations between DDC expression and higher grade, advanced stages. The survival analysis from external validation cohorts revealed that low DDC expression correlated with worse OS. The results above indicate that DDC expression level might be a reliable biomarker assisting in diagnosis and predicting prognosis in ccRCC. To further investigate the potential functions of DDC, we employed KEGG and GO analyses. The findings reveal that DDC protein could possibly enrich the PI3K-Akt signaling pathway, amino acid metabolism, extracellular matrix organization, and so on. The following subgroup analysis identified Treg as being significantly upregulated in the DDC^High subgroup, while M1 macrophage was significantly upregulated in the DDC^Low subgroup. There were significant differences in ICP gene expressions between the two DDC subgroups, which might eventually contribute to the different TIDE scores and prognosis.

DDC expression has been investigated in several malignant tumors. But in contrast, high DDC expressions are found more frequently in high Gleason’s score and advanced stage, and the underlying mechanism could be attributed to that DDC could co-activate androgen receptor (AR)–ligand transcriptional activity without affecting AR protein expression (36, 37). The following research tested whether the DDC enzymatic inhibitor, carbidopa, would suppress prostate cancer cell proliferation (38). Carbidopa could significantly restrict AR transactivation and PSA upregulation. The cell and castrated mice experiments demonstrated significant tumor growth suppression and decreased serum PSA effects of carbidopa. However, in ccRCC, the opposite strategy should be taken into consideration because of the unique genetic backgrounds between prostate cancer and ccRCC. In breast cancer, DDC upregulation was associated with a longer OS. The two breast cancer cells treated with epinephrine demonstrated contrary results in DDC expression and cell viability (21). Although the previous studies provided a novel insight that antitumor treatments could be combined with endocrine-related therapy strategies, targeted therapy combined with immunotherapy has become the first line of treatment for advanced ccRCC patients and ccRCC patients usually demonstrate relatively fixed types of mutations unlike prostate cancer or breast cancer. The underlying correlations between DDC and mutations such as VHL, BAP1, SETD2, and PBRM1 and whether DDC could serve as a new drug target for treating ccRCC patients or boosting immunotherapy response should be investigated in future studies to better guide treatments.

Dopamine (DA), catalyzed by the DDC protein, plays a role in the normal activities of human lives. It is also an important ingredient in orepinephrine and epinephrine (39). Previous studies found that dopamine receptors could be a biomarker for several malignant tumors (40), which highlights the important role of dopamine that depends on DDC activity in carcinogenesis. Chakroborty et al. found that a low dosage of DDC could restrict tumor angiogenesis via inhibiting VEGFR phosphorylation and was correlated with growth restriction in vitro (41). Dopamine could significantly promote the efficacy of anti-cancer drugs. The replenishments caused a low proliferation rate and metastatic potential that might be attributed to decreased phosphorylation levels of VEGF receptor-2, mitogen-activated protein kinase, and focal adhesion kinase. Angiogenesis is also one of the major characteristics of ccRCC, and several targeted therapies such as sunitinib, axitinib, and other drugs inhibit ccRCC progression by targeting VEGF targets. Supplementation with dopamine or increasing the activity of DDC enzymes may have a synergistic effect in combination with targeted therapy, significantly inhibiting tumor growth and progression. Moreover, a previous study found that mice with daily stress contributed to increased tumor growth compared to those without daily stress, which could be blocked by dopamine replenishment (42). This study also highlighted the importance of the tumor microenvironment in dopamine deletion and high-stress conditions. Dopamine could activate resting effector T cells (Teffs) and suppress regulatory T cells (Tregs) (43, 44). It also affects helper T-cell differentiation, inhibits Treg activation, takes part in antigen presentation processes, and modulates intracellular signaling pathways, suggesting that dopamine plays an important regulatory role in affecting the tumor microenvironment (45). Dopamine improved the efficacy of chemotherapy in vivo and in vitro experiments by inhibiting the M2 characteristics of tumor-associated macrophages (TAMs) (46). Qin et al. attempted to re-polarize M2 macrophages to M1 macrophages, and they found that dopamine could upregulate M1-polarized markers and downregulate M2-polarized markers, which could transfer the tumor microenvironment from “cold” to “hot” (47, 48). The M1 macrophage exerted anti-tumor effects and correlated with the immunotherapy response (49, 50). PD-L1 expression (CD274) was previously approved by the FDA as a predictive biomarker for ICI (50, 51). Thus, the differentially expressed PD-L1 and different proportions of M1 macrophages may influence the efficacy of immunotherapy. Future studies should focus on the effects of dopamine catalyzed by the DDC protein on the TME and the underlying mechanisms.

The findings of this study contribute to our knowledge of the function of DDC and recognize it as a potential diagnostic and prognostic factor in ccRCC. However, our study has certain limitations. First, although we utilize several external cohorts to validate DDC expression, large cohorts are needed to validate our conclusions. Second, the diagnostic and prognostic significance of DDC expression has been defined, although the underlying processes regulating its expression levels are still unknown. This will be made clearer by additional functional enrichment and annotation analysis. Third, the DDC protein could influence the percentage of M1 macrophages within the tumor microenvironment, and the underlying mechanisms are needed to be explored in future studies.

In conclusion, our study first systematically identified and assessed DDC expression and its potential functions in the regulation of metabolism and tumor microenvironment of ccRCC. DDC might function as a tumor suppressor protein and has been markedly linked to cancer progression and a worse prognosis in ccRCC.

The original contributions presented in the study are included in the article/ Supplementary Material . Further inquiries can be directed to the corresponding authors.

Conceptualization: KC, JS, CL, and AA. Data curation and formal analysis: KC, WX, JS, AA, WL, and YQ. Funding acquisition: WX, YQ, HZ, and DY. Investigation and methodology: KC, WX, JS, AA, and WL. Resources and software: WL, WX, YQ, HZ, and DY. Supervision: YQ, HZ, and DY. Validation and visualization: WX, WL, KC, and AA. Original draft: KC, JS, and CL. Editing: YQ, HZ, and DY. All authors listed have made a substantial, direct, and intellectual contribution to the work and approved it for publication.

This work was supported by grants from the National Natural Science Foundation of China (nos. 81802525 and 82172817), the Natural Science Foundation of Shanghai (no. 20ZR1413100), the Beijing Xisike Clinical Oncology Research Foundation (no. Y-HR2020MS-0948), the National Key Research and Development Project (no. 2019YFC1316005), the Shanghai “Science and Technology Innovation Action Plan” Medical Innovation Research Project (no. 22Y11905100), and the Shanghai Anti-Cancer Association Eyas Project (nos. SACA-CY21A06 and SACA-CY21B01).

We thank all the writers who gave precious advice to this article.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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