The role of HIF-1 as a positive regulator of the TH
17 response has been established by the work of several groups including our own.8,12,13
Studies using cell lineage specific HIF-1 deficient mice or T cells from these animals have been central to these discoveries. Genetic ablation of HIF-1 function, however does not speak to the suitability of HIF-1 modulation as a therapy to counter undesirable TH
17 responses. Work concerning HIF-1’s role in cancer biology, however does suggest that the downstream effects of HIF-1 signaling can be modulated with inhibitor compounds.
HIF-1 is known to play a major role regulating several aspects of cancer cell biology. HIF-1 targets include genes crucial for cell immortalization, vascularization and glyolytic metabolism. Since hypoxia is a common element of the tumor microenvironment, and pronounced HIF-1 expression during cancer has been linked to poorer prognoses and patient survival, it is not surprising that efforts to identify inhibitors of HIF-1 function have been taken up in earnest by cancer biologists. Indeed, a number of inhibitors with diverse mechanisms of action have been identified and tested in vitro and in vivo for their effectiveness at counteracting HIF-1 mediated tumor processes.14
Among the compounds shown to have potent HIF-1 antagonizing properties is a drug known as, digoxin. This cardiac glycoside was identified by the Semenza group to be a potent inhibitor of HIF-1 function.15
Recently, the Littman group found that this compound and its derivatives were also potent antagonists of TH
Specifically, treatment of mice with digoxin inhibited the generation of a TH
17 response in mice subjected to EAE. As a result, these mice were protected from the immune mediated neuropathology seen in this model of human MS. While the authors attribute the action of digoxin to inhibition of RORγt, a key regulator of TH
17 development, it is reasonable to suspect that the drug may have impacted HIF-1 levels, as has been previously reported. It is also noteworthy that the Treg/TH
17 imbalance seen in digoxin treated cells mirrors that seen upon genetic ablation of HIF-1.12
Other characterized HIF-1 inhibitors include a range of agents with distinct modes of action.14
Unpublished findings from our group corroborate those of the aforementioned drug study and suggest that using other HIF-1 inhibitors can also suppress in vivo TH
17 mediated pathology as well (FP and DP unpublished results). These findings strongly suggest that such molecules, due to their ability to dramatically reduce the severity of TH
17 responses, are a pool of drugs with a great deal of potential as treatments for inappropriate or excessive T-cell responses of this kind.
While the effectiveness of HIF-1 inhibition as a therapy for autoimmune diseases has been demonstrated (at least in animal models), it remains to be determined which consequence of HIF-1 functional ablation is chiefly responsible for this protection from autoimmune disease; the reduced TH
17 response or the enhanced presence of Treg cells. The findings of Korn et al. that antigen specific Tregs accumulating in the CNS are insufficient to control EAE suggest that reducing the development of a robust TH
17 response in the first place may be more beneficial in EAE.17
Indeed, further study may shed light on this question. The treatment of other autoimmune diseases having a strong IL-17 component may benefit from including HIF-1 inhibition in the treatment arsenal. It is reasonable to expect that inflammatory bowel disease (IBD) may also be ameliorated upon HIF-1 inhibition since the balance between TH
17 and Tregs has been shown to impact disease severity. Of course, future work is will be needed to validate this notion. While it seems likely that HIF-1 inhibition is well suited as an anti-autoimmune intervention, the setting of cancer, however, presents a more complicated scenario in regards to the suitability of HIF-1 targeting as a treatment method.
In the marshalling of an effective anti-tumor response, the proinflammatory, TH
1-associated cytokines IFNγ and IL-12 play an undeniable role in promoting the killing of cancer cells. CD8+
cytotoxic T lymphocytes also are important for the destruction of tumor cells. On the other hand, the action of Tregs in the anti-tumor response has been shown repeatedly to be negatively associated with the effectiveness of the response and overall disease outcome.18
Tregs are known to accumulate in tumor tissues and depleting them or inhibiting their suppressive function increases the effectiveness of certain cancer vaccines. For this reason, the sabotaging of Tregs has become an aim of many developing immunotherapy approaches.
The impact of TH
17 cells in the tumor setting is less cut-and-dry. While they have been observed to accumulate in many cancer patients often along side Tregs, their contributions to either tumor progression or tumor eradication is the subject of some debate. TH
17 cells have been reported by some to be efficient participants in the anti-tumor response.19
Furthermore, in a recent study, adoptive transfer TH
17 cells slowed the growth of established ovarian tumors in immunodeficient mice. Co-transfer of these cells with tumor antigen specific CD8+
T cells had an even greater effect on tumor growth.13
Not only does this study suggest that, at least after tumor initiation, TH
17 cells have anti-tumor capacities, it also raises the possibility that cooperation with other effector T cells may be key for this function. On the other hand, mounting numbers of studies suggest that IL-17 and the cells producing it promote tumor growth and progression. In particular, TH
17 and IL-17 producing T cells have been linked to poorer prognoses in cancer patients or numerous types.20
IL-17 more clearly contributes to the progression of inflammation associated tumors. In colorectal cancer, IL-17 producing Foxp3+
are thought to be important instigators of disease—suggesting that Treg cells may play a more active role in tumor progression, beyond dampening a desirable anti-tumor response. Moreover it was found that exposure to hypoxia could bring about IL-17 expression by Treg cells showing that this condition, which almost certainly involves upregulation of HIF-1, can also drive Treg cells, under the right conditions, to become active participants in an IL-17 driven response. Another strong link between TH
17 cells and tumor progression was seen in a model of colon cancer induced by a wide spread human commensal organism. Experimental colonization of mice predisposed to intestinal tumors with an enterotoxigenic strain of Bacteriodes fragilis
results in the aggressive development of large bowel tumors.21
In these studies, deletion of STAT3 in the CD4+
compartment or anti-IL-17 antibody treatment greatly reduced cancer severity providing another example of IL-17’s tumor promoting capacity.
The underlying mechanisms by which IL-17 and the cells that produce it influence tumor formation and progression remain to be completely defined. However, recently, several studies have linked TH
17 cells or the cytokines they are known to produce to the promotion of angiogenesis- a process both characteristic and necessary for tumor development. In gastric cancers, vascularization of tumors is positively correlated with the levels of IL-17 and TH
17 associated cytokine mRNA in the tumor tissue.22
IL-17 producing cells which are enriched in colorectal cancer (CRC) tumor tissues are associated with poor prognosis at least in part due to the induction of the infamous pro-angiogenic factor, VEGF in the cancer cells. Indeed, HIF-1 within the cancer cell itself has been clearly shown to be important for regulating genes important for angiogenesis.9
All the same, the possibility that IL-17 producing T cells influence their intra-tumor neighbors should be considered. Interestingly, Hot and Miossec have reported that the TH
17-associated cytokines can induce the expression of genes linked to the hypoxic response23
suggesting that TH
17 responses may be subject to positive feed back loop regulation. Therefore, one wonders if tumor infiltrating IL-17+
T cells might perpetuate a pro-angiogenic chain reaction through interaction with other cells of the tumor microenvironment. In addition to an apparent pro-angiogenic role of TH
17 associated cytokines, some reports suggest that they play a role in cancer spread as well. Recently, Li et al. reported that IL-17A can promote hepatocellular carcinoma metastasis through the regulation of metalloprotease expression.24
In addition to promoting cytokines with tumor promoting capacities, HIF-1’s regulation of glycolysis-associated genes in cancer cells is considered a major contribution to the progression of tumors. Specifically, HIF is important for the establishment of the Warburg effect. In this metabolic shift from aerobic respiration, the machinery of glycolysis is upregulated in cancer cells, giving them a metabolic advantage for surviving and thriving in the oxygen poor microenvironment of the tumor. It is likely that HIF-1 inhibitors will rob the tumor cells of needed vascularization, a chance to spread and the metabolic edge imparted by their glycolytic lifestyle.
In all it stands to reason that targeting HIF-1 in the tumor microenvironment should prove an effective, multiple pronged anti-cancer treatment strategy for a variety of cancers. Since as mentioned above, certain effects of the TH
17 response may promote tumor development, growth and spread in some cancer models and given HIF-1’s importance in the cancer cell itself, well characterized HIF-1 inhibitors make tempting potential therapeutic tools. Indeed pharmacological inhibition of HIF-1 in tumor models has yielded promising, yet preliminary findings. Specifically, Semenza and colleagues report that treatment of mice with subcuteanous tumors with digoxin or acriflavine (inhibitors of HIF-1 expression and function, respectively) limits tumor growth.15
In these and other studies, a major effect of general HIF-1 inhibition was a reduction in neovascularization (process of angiogenesis) and the switch to glycolytic metabolism. Neither of these studies addressed how these inhibitors were impacting the T cell response to the tumor and it remains to be seen how much of the tumor growth suppressing effect of these compounds is actually attributable to the presumed inhibition of the TH
While chemical targeting HIF-1 appears to be a highly viable anti-cancer strategy with multiple potential benefits, studies using mice with HIF-1 deficient T cells sound a note of caution when considering HIF-1 inhibition as monotherapy cancer treatment. While HIF-1 inhibitors can interfere with the tumor-promoting processes of angiogenesis and the favoring of glycolytic metabolism, they may, as suggested by the previous work of our group and others, also elevate the frequency of immune suppressive Treg cells. These cells are known to stymie anti-tumor immune response by promoting immune tolerance—a state permissive to cancer persistence and progression. Nevertheless, it is still likely that HIF-1 inhibition may yet prove particularly advantageous in the treatment of cancer. It may be prudent or necessary to evaluate the efficacy of HIF-1 inhibition as a cancer therapy in combination with other agents aimed at counteracting the potential increases in suppressor cell generation such as the drugs used to deplete Treg cells. Such a combinational approach, should in theory, simultaneously neutralize two tumor-promoting T-cell populations. Additionally, since some studies suggest that in the latter stages of tumor development, TH17 cells have anti-tumor effects, restricting the therapeutic window of HIF-1 inhibition to early developing tumors may prove more effective as a treatment strategy.