Hif, Hif, Hif, Hike!

Football season. Except is was 85 degrees here in Massachusetts today and felt more like mid-July. Thankfully there is “fallball” (fall softball season) so we got to enjoy that instead.

We got a good look at the convergence of immune and pathogenic pathways in this week’s issues of Science and Nature. Two papers in Science identify metabolic adjustments made by monocytes and macrophages that may support innate immune memory. The same pathway is hijacked by some tumors to redirect macrophage activity, as described in a very nice Nature paper.

Cheng et al from the Netea lab in The Netherlands used a b-glycan derived from the pathogenic fungus Candida albicans to “educate” monocytes, mimicking an infection event (Cheng et al). C. albicans b-glycan, a carbohydrate moiety, binds the dectin-1 receptor on monocytes, macrophages and other innate immune cells and induces cell activation. This activation response included changes in the epigenetic profile of the cells. The epigenetic signature suggests that monocytes “trained” by exposure to b-glycan alter their metabolic status, in particular by elevating aerobic glycolysis with increased glucose consumption. Key glycolysis enzymes such as hexokinase and pyruvate kinase were epigenetically upregulated, supporting the shift to glycolysis. Aerobic glycolysis produces lactic acid and increased lactate production was also observed: these b-glycan activated monocytes have really committed to this metabolic state.

This metabolic shift was mediated by signaling from dectin-1 to AKT and mTOR. This signaling pathway is responsible for many cellular responses, including induction of HIF-1α (hypoxia-inducible factor–1α). In turn, HIF-1α-dependent signals turn on many genes needed to adapt to the metabolic shift. This is a common tactic in hypoxic conditions for example. Blockade of any steps in the pathway abrogated the metabolic shift and prevented “trained immunity”. The role of epigenetic components in induction of the metabolic shift in monocytes was demonstrated using the epigenetic inhibitors methylthioadenosine, a methyltransferase inhibitor, and givinostat, a class I/II histone deacetylase (HDAC) inhibitor.

A second paper from the same group dives deeper into the monocyte to macrophage differentiation program (Saeed et al). Short-term culture of monocytes with LPS (a TLR4 agonist) or b-glycan yielded distinct macrophage populations. Serum culture (mimicking the homeostatic state) yielded yet a 3rd type. This paper is a technical grind so have at it if you want all the complex details. I was interested in the conclusions. As in the b-glycan study referenced above, LPS and serum culture induced distinct epigenetic signatures. Genome-wide mapping of histone modifications identified epigenetically marked clusters – that is, reactive regions of the genome. Within these clusters we would expect to find transcription regulatory regions, and indeed four such clusters were differentially modulated when monocytes were exposed to LPS or b-glucan. Targets within these clusters include G protein–coupled receptors, protein kinases, and additional epigenetic enzymes. The authors therefore affirm the “trained immunity” state identified in the first paper and now elucidate a macrophage “exhaustion” phenotype induced by short term exposure to LPS. By my reading of the paper it appears both of these induced phenotypes are extensions of the M-CSF/serum induced homeostatic differentiation profile. This makes sense, as monocytes are recruited from circulation so they can differentiate into macrophages at sites of inflammation, a process that optimally requires M-CSF.

In the first paper the production of lactic acid and lactate was noted as a consequence of differentiation to the “trained”, glycolysis-driven phenotype. Turning now to a paper in Nature from Medzhitov and colleagues at Yale, we find ourselves confronting a chicken and egg story (Colegio et al). In this study the crosstalk of tumor-resident macrophages and “client” tumor cells was examined. The premise is that tumor-associated macrophages (TAMs) perform key homeostatic functions that support tumor growth and survival. In this case it appears that the tumor microenvironment subverts macrophage function via production of lactic acid. There are important differences in the study designs – the papers published in Science use short-term culture techniques while the Nature paper relies on in vivo tumor/macrophage development in syngeneic mouse models – but with this caveat in mind the convergence of pathway data is striking. TAMs sorted from implanted lung (LLC) or melanoma (B16-F1) tumors expressed high levels of VEGF and arginase 1 (Arg1) mRNA, accounting for nearly all of the expression of these proteins in tumor samples. Strikingly, tumors induced macrophage expression of VEGF via stabilization of HIF1a in a manner that did not require hypoxia. This is interesting as it identifies a pathway by which tumor cells can stimulate angiogenesis (blood vessel formation) via VEGF and Arg1 prior to a hypoxic challenge. The soluble tumor cell effector capable of turning on this pathway was identified as … lactate. Here it is worth quoting from the paper:

“Warburg observed that cancer cells preferentially perform aerobic glycolysis: that is, they convert most glucose molecules into lactate regardless of the amount of oxygen present. Furthermore, the eponymous Warburg effect is also observed in most cells undergoing rapid proliferation. It has been hypothesized that aerobic glycolysis is conducive to cell proliferation because, despite the consequent reduction in ATP production, aerobic glycolysis produces metabolic precursors, such as lactate, for biosynthetic pathways, and these precursors may be the limiting factor during rapid cell proliferation”

The suggestion here is that tumor cells are going a step further in order to ensure that their supportive microenvironment, which includes TAMs, step in line. Lactate is taken up by TAMs via specific cell surface receptors (the monocarboxylate transporters) and the effect is potentiated by acidic pH (from all the lactic acid) and perhaps requires other mediators such as M-CSF. Once all is said and done the TAMs are surviving and thriving using the same machinery as the tumor cells.

From the drug development perspective it is probably worth asking whether AKT and mTOR inhibitors impact TAM activity in the tumor microenvironment (perhaps someone already has). Conversely, one might speculate on the impact of such inhibitors of macrophage responses to infection. More selectively, I suspect there is a clever way of targeting the epigenetic responses to derail the TAM phenotype and disrupt the tumor-supportive microenvironment while either simultaneously targeting the tumor, as in a combination therapy setting with a therapeutic that targets tumor biology directly. Also, in the era of immune checkpoint therapeutics I wonder if there isn’t some signal to “wake-up” these “trained” macrophages and have them turn on their clients – the tumor cells.

A few other questions:

How is the macrophage glycolysis pathway maintained once initiated by exposure to tumor derived lactate? There must be a feedback mechanism, perhaps similar to the one used by “trained” macrophages?

Do the HIF2-dependent tumors (some renal cell carcinomas for example) also hijack resident TAMs in the same manner, or different?

The tumor microenvironment includes tumor-associated fibroblasts – are these also impacted by exposure to lactic acid?

If there is intimate cross-talk between the macrophage and it’s client (a tumor cell) then disabling that conversation at the level of the macrophage (and other stromal cells) should be therapeutic – or will the tumor (in this case) simply adapt? Remember that in this setting the epigenetic changes are not necessarily addictive (oncogenic).

interesting stuff to consider in this new era of combination therapies….

stay tuned