Dipeptidase 1 promotes ferroptosis in renal tubular epithelial cells in diabetic nephropathy via inhibition of the GSH/GPX4 axis

Renal tubular injury is an important pathological change associated with diabetic nephropathy (DN), in which ferroptosis of renal tubular epithelial cells is critical to its pathogenesis. Inhibition of the glutathione/glutathione peroxidase 4 (GSH/GPX4) axis is the most important mechanism in DN tubular epithelial cell ferroptosis, but the underlying reason for this is unclear. Our biogenic analysis showed that a zinc-dependent metalloproteinase, dipeptidase 1 (DPEP1), is associated with DN ferroptosis. Here, we investigated the role and mechanism of DPEP1 in DN tubular epithelial cell ferroptosis. DPEP1 upregulation was observed in the renal tubular epithelial cells of DN patients and model mice, as well as in HK-2 cells stimulated with high glucose. Furthermore, the level of DPEP1 upregulation was associated with the degree of tubular injury in DN patients and HK-2 cell ferroptosis. Mechanistically, knocking down DPEP1 expression could alleviate the inhibition of GSH/GPX4 axis and reduce HK-2 cell ferroptosis levels in a high glucose environment. HK-2 cells with stable DPEP1 overexpression also showed GSH/GPX4 axis inhibition and ferroptosis, but blocking the GSH/GPX4 axis could mitigate these effects. Additionally, treatment with cilastatin, a DPEP1 inhibitor, could ameliorate GSH/GPX4 axis inhibition and relieve ferroptosis and DN progression in DN mice. These results revealed that DPEP1 can promote ferroptosis in DN renal tubular epithelial cells via inhibition of the GSH/GPX4 axis.

DPEP1 promotes drug resistance in colon cancer cells by forming a positive feedback loop with ASCL2

BACKGROUND

Drug resistance is an important factor affecting the efficacy of chemotherapy in patients with colon cancer. However, clinical markers for diagnosing drug resistance of tumor cells are not only a few in number, but also low in specificity, and the mechanism of action of tumor cell drug resistance remains unclear.

METHODS

Dipeptidase 1 (DPEP1) expression was analyzed using the cancer genome atlas (TCGA) and genotype-Tissue Expression pan-cancer data. Survival analysis was performed using the survival package in R software to assess the prognostic value of DPEP1 expression in colon cancer. Correlation and Venn analyses were adopted to identify key genes. Immunohistochemistry, western blot, qRT-PCR, Co-immunoprecipitation, and dual-luciferase reporter experiments were carried out to explore the underlying associations between DPEP1 and Achaete scute-like 2 (ASCL2). MTT assays were used to evaluate the role of DPEP1 and ASCL2 in colon cancer drug resistance.

RESULTS

DPEP1 was highly expressed in colon cancer tissues. DPEP1 expression correlated negatively with disease-specific survival but not with overall survival. Bioinformatics analysis and experiments showed that the expressions of DPEP1 and ASCL2 in colon cancer tissues were markedly positively correlated. Mechanistic research indicated that DPEP1 enhanced the stability of protein ASCL2 by inhibiting its ubiquitination-mediated degradation. In turn, ASCL2 functioned as a transcription factor to activate the transcriptional activity of the DPEP1 gene and boost its expression. Furthermore, DPEP1 also could enhance the expression of colon cancer stem cell markers (LGR5, CD133, and CD44), which strengthened the tolerance of colon cancer cells to chemotherapy drugs.

CONCLUSIONS

Our findings reveal that the DPEP1 enhances the stemness of tumor cells by forming a positive feedback loop with ASCL2 to improve resistance to chemotherapy drugs.

DPEP1 promotes the proliferation of colon cancer cells via the DPEP1/MYC feedback loop regulation

DPEP1 is highly expressed in the colorectal carcinoma tissues and colon cancer cells. However, the function and underlying mechanism of DPEP1 in the colon cancer cells are still poorly understood. Here, we found that transcription factor MYC could occupy on the DPEP1 promoter and activate its activities, and DPEP1 was up-regulated by MYC proteins in mRNA and protein levels in a dose-dependent manner in colon cancer cells. The expression levels of DPEP1 were positively correlated with that of MYC in colorectal tumor tissues. Moreover, Laser confocal images and Co-immunoprecipitation (Co-IP) revealed that DPEP1 and MYC proteins could bind to each other in the colon cancer cells. In turn, DPEP1 could enhance the stability of MYC proteins by extending the half-life of MYC proteins in colon cancer cells. Thus, DPEP1 and MYC proteins might form a positive feedback loop to maintain their high expression levels in colon cancer cells. In function, the MTT, EdU, Clone Formation assays and xenograft tumors assays demonstrated that DPEP1 could boost the proliferation of colon cancer cells through the DPEP1/MYC positive feedback loop in vitro and in vivo. Theoretically, DPEP1 may serve as a colon cancer biomarker and a novel target of colorectal carcinogenesis therapy.

Selection of a malignant subpopulation from a colorectal cancer cell line

Colorectal cancer (CRC) is a leading cause of cancer-associated mortality worldwide; therefore, there is an emerging need for novel experimental models that allow for the identification and validation of biomarkers for CRC-specific progression. In the present study, a repeated sphere-forming assay was used as a strategy to select a malignant subpopulation from a CRC cell line, namely HCT116. The assay was validated by confirming that canonical stemness markers were upregulated in the sphere state at every generation of the selection assay. The resulting subpopulation, after eight rounds of selection, exhibited increased sphere-forming capacity in vitro and increased tumorigenicity in vivo. Furthermore, dipeptidase 1 (DPEP1) was identified as the major differentially expressed gene in the selected clone, and its depletion suppressed the elevated sphere-forming capacity in vitro and tumorigenicity in vivo. Overall, the present study established an experimental strategy to isolate a malignant subpopulation from a CRC cell line. Additionally, results from the present model revealed that DPEP1 may serve as a promising prognostic biomarker for CRC.

DPEP1 Balance GSH Involve in Cadmium Stress Response in Blood Clam Tegillarca granosa

The blood clam, Tegillarca granosa, is a benthic filter feeder with a strong capacity to accumulate and tolerate cadmium (Cd). In our previous study, DPEP1 was shown to be significantly up-regulated under Cd stress based on proteomic analysis. To investigate whether DPEP1 is involved in Cd-induced response, the function of DPEP1 in T. granosa was investigated by integrated molecular and protein approaches. Rapid amplification cDNA end (RACE) assay was established to achieve the complete cDNA sequence of DPEP1 from T. granosa. The full-length cDNA of DPEP1 was 1811 bp, and it contained a 1359-bp open reading frame (ORF), including a 22-amino acid signal peptide. qRT-PCR analysis revealed that DPEP1 was expressed in all examined tissues with the highest expression in gills. At the same time, we investigated DPEP1 gene expression changes after Cd stress at different time points over 96 h. We found that the expression of DPEP1 increased upon initial Cd stress, then it was inhibited, and finally, it was maintained at a low level. Moreover, recombinant DPEP1 showed that higher glutathione (GSH) hydrolysis activity in the temperature range of 30–40°C, and its maximum activity was at pH = 6. Additionally, the results of immunohistochemistry also confirmed that DPEP1 protein was expressed in all test tissues with the highest expression in gills. In addition, there was a positive correlation between QRT-PCR and immunohistochemistry. These results suggested that DPEP1 is probably involved in Cd-induced response by balancing GSH.

Reintroduction of a Homocysteine Level-Associated Allele into East Asians by Neanderthal Introgression

In this study, we present an analysis of Neanderthal introgression at the dipeptidase 1 gene, DPEP1. A Neanderthal origin for the putative introgressive haplotypes was demonstrated using an established three-step approach. This introgression was under positive natural selection, reached a frequency of >50%, and introduced a homocysteine level- and pigmentation-associated allele (rs460879-T) into East Asians. However, the same allele was also found in non-East Asians, but not from Neanderthal introgression. It is likely that rs460879-T was lost in East Asians and was reintroduced subsequently through Neanderthal introgression. Our findings suggest that Neanderthal introgression could reintroduce an important previously existing allele into populations where the allele had been lost. This study sheds new light on understanding the contribution of Neanderthal introgression to the adaptation of non-Africans.