Overexpression of inositol 1,4,5-trisphosphate receptor type 1 (ITPR1), mediated by hypoxia-inducible factor 2-α (HIF2-α), protects renal cancer cells from lysis by natural killer (NK) cells through induction of autophagy, according to a new study published in Cancer Research.

HIF expression is normally controlled by the von Hippel–Lindau (VHL) pathway, but in many renal cell carcinomas (RCCs) the VHL gene is mutated or functionally inactivated. The resulting HIF overexpression causes induction of genes that can promote tumour growth. In addition, immune therapy using NK cells in patients with renal tumours seems to be hampered by cancer-cell-inherent mechanisms of resistance that have been linked to HIF1-α expression. Now, Yosra Messai and Muhammad Noman from the Gustave Roussy Cancer Campus, France, and colleagues show that HIF2-α is directly regulating the susceptibility of renal cancer cells to NK-cell-mediated lysis.

Credit: NPG

First, the team demonstrated that cell lines with mutated VHL constitutively express HIF2-α and are resistant to lysis, but still form stable conjugates with NK cells. When silencing HIF2-α with small interfering RNAs the cells were again susceptible to lysis. Gene expression analysis using DNA microarray technology identified three genes involved in cell death and survival that were downregulated in the absence of HIF2-α expression. In the presence of HIF2-α, direct silencing of one of those genes, ITPR1, restored lysis susceptibility, pointing towards a central role of this gene in the resistance of RCCs with dysfunctional VHL to NK-cell-mediated lysis.

Next, as ITPR1 had previously been shown to be involved in autophagy regulation, the researchers investigated how this gene was connected to lysis-resistance. In the presence of NK cells, silencing of ITPR1 in HIF2-α-expressing cells prevented the formation of autophagosomes and the cancer cells were killed by NK cells. Further experiments in these cells demonstrated that ITPR1 knockdown increased the activity of granzyme B, a serine protease that is released by NK cells to mediate lysis. Hence, the researchers hypothesize that the induction of autophagy, instead of lysis, in cells expressing HIF2-α and ITPR1 might be caused by decreased activity of NK-cell-released granzyme B.

...ITPR1 could be a potential novel therapeutic target

Finally, the team investigated HIF2-α and ITPR1 expression in tissue sections from 235 patients with RCC using immunohistochemistry and found a significant correlation of the expression of these two markers, indicating that ITPR1 could be a potential novel therapeutic target. In a mouse model of RCC expressing HIF2-α and ITPR1, administration of digoxin, a HIF2-α inhibitor, decreased expression of HIF2-α and ITPR1 in a dose-dependent manner in vivo. In addition, when ITPR1 was stably silenced using lentiviral expression of short hairpin RNAs, tumour growth was significantly decreased in the presence of NK cells.