Sphingosine-1-Phosphate Catabolizing Enzymes Predict Better Prognosis in Triple-Negative Breast Cancer Patients and Correlates With Tumor-Infiltrating Immune Cells

Abstract

Background: Triple-negative breast cancer (TNBC) is associated with a poor prognosis. Sphingosine-1-phosphate (S1P), a potent sphingolipid metabolite, has been implicated in many processes that are important for breast cancer (BC). S1P signaling regulates tumorigenesis, and response to chemotherapy and immunotherapy by affecting the trafficking, differentiation or effector function of tumor-infiltrating immune cells (TIICs). Objective: In this study, using bioinformatics tools and publicly available databases, we have analyzed the prognostic value of S1P metabolizing genes and their correlation with TIICs in BC patients. Methods: The expression of S1P metabolizing genes and receptors was evaluated by the UALCAN cancer database. The correlation between mRNA expression of S1P metabolizing genes and receptors and survival outcome of breast cancer patients was analyzed by the Kaplan-Meier plotter database. The association between the gene expression and infiltration of immune cells in the tumors was analyzed by "Tumor-Infiltrating Immune Estimation Resource (TIMER). In silico protein expression analysis was done using the Human Protein Atlas" database. Results: TNBC patients with lower expression of S1P phosphatase 1 (SGPP1) or lipid phosphate phosphatase 3 (PLPP3) have much shorter relapse-free survival than the patients with a higher expression of these genes. SGPP1 and PLPP3 expression show a strong positive correlation with tumor-infiltrating dendritic cells (DCs), CD4+ and CD8+ T cells, neutrophils, and macrophages in the TNBC subtypes. In addition, S1P receptor 4 (S1PR4), an S1P receptor exhibit a strong positive correlation with DCs, CD4+ and CD8+ T cells and neutrophils in TNBC. We, therefore, conclude that low expression of SGPP1 and PLPP3 may hinder the recruitment of immune cells to the tumor environment, resulting in the blockage of cancer cell clearance and a subsequent poor prognosis.

Keywords: PLPP3; S1PR4; SGPP1; breast cancer; sphingosine-1-phosphate; tumor-infiltrating immune cells.

FIGURE 1

Prognostic role of mRNA expression of S1P-metabolizing enzymes (A–H) in breast cancer patients. Kaplan–Meier survival curves (RFS) were plotted for S1P-metabolizing genes from the publicly available KM plotter database (N = 3955 BC).

FIGURE 2

Prognostic role of mRNA expression of S1P receptors (A–E) in breast cancer patients. Kaplan-Meier survival curves (RFS) were plotted for S1P receptors from the publicly available KM plotter database (N = 3955 BC).

FIGURE 3

High expression of sphingosine-1-phosphate phosphatase1 (SGPP1) is highly significantly associated with relapse-free survival (RFS) of patients with HER2+ and basal-like BC (intrinsic subtypes). (A–D), Kaplan-Meier survival curves (RFS) plotted for SGPP1 for intrinsic subtypes of BC.

FIGURE 4

mRNA expression was analyzed in normal tissue (N = 114) and primary tumors from breast cancer (N = 1,097) patients from the publicly available UALCAN database. (A–C), SGPP1 (where A = normal vs. tumors, B = normal vs. cancer subtypes and C = normal vs. tumors of different stages). *p < 0.01 Stage I vs Stage IV, Stage II vs Stage IV and Stage III vs Stage IV. (D–F), PLPP3 (where D = normal vs. tumors, E = normal vs. cancer subtypes and F = normal vs. tumors of different stages). Data are shown as average number of transcripts per million. p values are calculated for primary tumors vs. normal tissues in corresponding genes.

FIGURE 5

The SGPP1 gene with invasive breast carcinoma, basal, luminal and HER2+ and SGPP1is significantly associated with basal-like tumor-infiltrating immune cells according to correlation via TIMER. (A–D), SGPP1 (where A = invasive breast carcinoma, B = basal-like breast carcinoma, C = luminal breast carcinoma, and D = HER2+ breast carcinoma). High expression of integrin subunit alpha X (ITGAX) is significantly associated with relapse-free survival (RFS) of breast cancer patients. (E), Kaplan-Meier survival curves (RFS) were plotted for ITGAX for breast cancer.

FIGURE 6

Correlation of S1PR4 expression with immune infiltration level in breast cancer patients. SGPP1 expression is significantly correlated with infiltrating levels of CD8+ T cells, CD4+ T cells (A), neutrophils and dendritic cells in basal-like BC, but not with macrophage cells (B).