Regulation of CD137 expression through K‐Ras signaling in pancreatic cancer cells

Abstract

Background The interaction between CD137 and its ligand (CD137L) plays a major role in the regulation of immune functions and affects cancer immunotherapy. CD137 is a cell surface protein mainly located on activated T cells, and its regulation and functions in immune cells are well established. However, the expression of CD137 and its regulation in cancer cells remain poorly understood. The main purposes of this study were to examine the expression of CD137 in pancreatic cancer cells and to investigate its underlying mechanisms. Methods Cells containing inducible K-Ras G12V expression vector or with different K-Ras mutational statuses were used as in vitro models to examine the regulation of CD137 expression by K-Ras. Various molecular assays were employed to explore the regulatory mechanisms. Tumor specimens from 15 pancreatic cancer patients and serum samples from 10 patients and 10 healthy donors were used to test if the expression of CD137 could be validated in clinical samples. Results We found that the CD137 protein was expressed on the cell surface in pancreatic cancer tissues and cancer cell lines. Enzyme-linked immunosorbent assay revealed no difference in the levels of secreted CD137 in the sera of patients and healthy donors. By using the K-Ras inducible cell system, we further showed that oncogenic K-Ras up-regulated CD137 through the activation of MAPK (mitogen-activated protein kinases) and NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells) pathways, as evidenced by significantly reduced CD137 mRNA expression led by genetic silencing of MAPK1 and p65, the key proteins involved in the respective pathways. Furthermore, we also found that the NF-κB pathway was mainly stimulated by the K-Ras-induced secretion of interleukin-1α (IL-1α) which promoted the transcription of the CD137 gene in pancreatic cancer cell lines. Analysis of the TCGA (the cancer genome atlas) database also revealed a significant correlation between IL-1α and CD137 expression ( r = 0.274) in tumor samples from pancreatic cancer patients ( P < 0.001). Conclusions The present study has demonstrated that the CD137 protein was expressed on pancreatic cancer cell surface, and has identified a novel mechanism by which K-Ras regulates CD137 in pancreatic cancer cells through MAPK and NF-κB pathways stimulated by IL-1α.

Mechanisms of T-Cell Exhaustion in Pancreatic Cancer

“…Choi et al reported that antitumor immunity is enhanced in 4-1BBDKO mice as a result of the increase in NK cells due to the shift of the dominant type of immune cells from the innate NK cell to the adaptive Tc via 4-1BB signaling [ 167 ]. It is known that constitutive activation of oncogenic KrasG12D, which approximately 90% of pancreatic cancer incidences exhibit, upregulates 4-1BB in tumor cells through MAPK and NF-κB signaling [ 168 , 169 ]. Therefore, many clinical CAR-T cell trials in pancreatic cancer are also using the 4-1BB receptor, targeting mesothelin, MUC1, CD19, CD133 ( Table 1 ).…”

CD137 expression in cancer cells: regulation and significance

“…However, the regulatory mechanisms and the significance of CD137 expression in cancer cells remain largely unknown. A recent study has shown that CD137 expression in pancreatic cancer cells might be positively regulated by oncogenic K-ras [9]. Using a K-ras-inducible cell system and cancer cell lines with various K-ras status, the authors demonstrated that K-ras could induce CD137 expression through mitogen-activated protein kinases (MAPK) and NF-κB signaling pathways.…”

“…The NF-κB pathway seems mainly activated by extracellular IL-1α (interleukin-1 alpha), whose expression and secretion are enhanced by K-ras [12]. In contrast, the MAPK pathway is stimulated by K-ras activation without the involvement of IL-1α [9]. It is currently unclear how the activation of K-ras could lead to increased expression of IL-1α, or which transcription factor(s) might bind and regulate the CD137 transcription in cancer cells.…”

“…It is currently unclear how the activation of K-ras could lead to increased expression of IL-1α, or which transcription factor(s) might bind and regulate the CD137 transcription in cancer cells. The relative contributions of the MAPK and NF-κB pathways in promoting the CD137 expression in cancer are likely cell type-dependent, as evinced by the findings that specific suppression of each pathway affected CD137 expression differently in different cell types [9]. Both pathways could be active in certain cancer cells, while in other cancer cells one of the pathways could be dominant.…”

“…Both pathways could be active in certain cancer cells, while in other cancer cells one of the pathways could be dominant. An interesting finding was that IL-1α could stimulate the expression of CD137 in multiple cancer cell lines, whereas neutralization of the IL-1α by antibody failed to decrease the CD137 expression in most of these cell lines [9]. This suggests that these cancer cells might have more than one pathway to promote CD137 expression and thus are not exclusively dependent on IL-1α for stimulation.…”