Inhibition of IL‐6‐JAK/Stat3 signaling in castration‐resistant prostate cancer cells enhances the NK cell‐mediated cytotoxicity via alteration of PD‐L1/NKG2D ligand levels

Abstract

To investigate whether IL‐6 signaling affects the susceptibility of castration‐resistant prostate cancer (CRPC) cells to cytotoxic action of natural killer (NK) cells, CRPC cell lines (having different IL‐6 levels) were developed by lentiviral transduction. While observing no secreted IL‐6 level in parental C4‐2 and CWR22Rv1 cells, we found the IL‐6 expression/secretion in these cells was induced after the transduction process and the IL‐6 level difference in C4‐2siIL‐6/sc and CWR22siIL‐6/sc cell CRPC cell sets could be detected. We then found that IL‐6‐knockdown cells were more susceptible to NK cell cytotoxicity than control cells due to lowered programmed death receptor ligand 1 (PD‐L1) and increased NK group 2D (NKG2D) ligand levels. In animal studies, to concur with the in vitro results, we found that IL‐6‐expressing cell‐derived tumors were more resistant to NK cell action than the tumors of IL‐6‐knockdown cells. Further, we discovered that JAK‐Stat3 is the most critical IL‐6 downstream signaling that modulates PD‐L1/NKG2D ligand levels in CRPC cells. Furthermore, inhibition of the JAK or Stat3 signaling effectively increased the susceptibility of C4‐2sc and CWRsc cells to NK cell cytotoxicity. We observed the most effective cytotoxicity when the PD‐L1 Ab and JAK inhibitor (or Stat 3 inhibitor) were used together. These results suggest that the strategy of targeting IL‐6 signaling (or its downstream signaling) may enhance the NK cell‐mediated immune action to CRPC tumors, thus yielding clinical implications in developing future immunotherapeutics of exploiting this strategy to treat patients with CRPC.

Adipocytes affect castration‐resistant prostate cancer cells to develop the resistance to cytotoxic action of NK cells with alterations of PD‐L1/NKG2D ligand levels in tumor cells

“…In this context, improving the NK cell‐mediated cytotoxicity, which is independent of tumor mutation burden and antigen presentation, may be an alternative immunotherapeutic approach to treat CRPC patients. In our previous in vitro and in vivo studies, we have demonstrated a significant antitumor NK cell activity toward PCa cells, supporting our hypothesis. Therefore, we speculate that discovering the adipocyte role in affecting CRPC cells and developing the resistance to NK cell cytotoxicity may be clinically important.…”

“…In previous studies, we found JAK‐Stat3 signaling was critical signaling pathway that is responsible for the up‐regulation of PD‐L1/down‐regulation of NKG2D ligands in CRPC cells . We questioned whether this signaling pathway is also critical in triggering the adipocyte CM effect.…”

“…Zhang et al suggested that IL‐6 and leptin are major adipokines that are positively associated with aggressiveness of PCa. The IL‐6 role in controlling the susceptibility of CRPC cells to NK cell cytotoxicity has already suggested in our previous studies . Leptin, which is a protein hormone and is expressed predominantly by adipocytes, has an important role in regulating body weight, metabolism, and reproductive function .…”

“…Our previous study results showed that the interactions of tumor cells and NK cells through the PD‐L1/PD‐1 and NKG2D ligand/NKG2D axis are important in determining the susceptibility of CRPC cells to NK cell action . We tested whether treating adipocyte CM alters the PD‐L1/NKG2D ligand levels in CRPC cells.…”

“…It was suggested that the down‐regulated NKG2D in NK cells prevented recognition by tumor cells . Our previous studies on lung cancer and PCa also showed importance of up‐regulation of PD‐L1/down‐regulation of NKG2D ligands in developing resistance to NK cell cytotoxicity. Nevertheless, whether adipocytes can affect the PD‐L1/NKG2D ligand levels in CRPC cells has not been studied before.…”