Targeting Class IIb HDACs

Localized primarily to the cytoplasm, HDAC6 not only regulates the acetylation of multiple proteins, such as a-tubulin and heat shock protein 90 (HSP90), but also has deacetylase-independent functions (Grozinger et al. 1999; Hubbert et al. 2002; Kovacs et al. 2005; Bali et al. 2005; Valenzuela-Fernandez et al. 2008). Unique in the field of HDACi, multiple HDAC6 isoform-selective HDACi (HDAC6i) are reported (Haggarty et al. 2003; Suzuki et al. 2006; Schafer et al. 2008; Chen et al. 2008; Kozikowski et al. 2008; Smil et al. 2009; Butler et al. 2010). These considerations led us to explore the effects of HDAC6 targeting on Tregs and, by extension, whether isoform-selective HDAC6i show utility as anti-inflammatory agents (Hancock et al. 2008). We found that HDAC6 was expressed at several-fold higher levels in Tregs versus conventional T cells, and HDAC6 knockout mice thereby provided a gold-standard as to how effective pharmacologic inhibitors of HDAC6 might be expected to be in modulating immune events. HDAC6—/— mice are known to be immunocompetent and not prone to tumorigenesis or chronic infections (Zhang et al. 2008). However, their Tregs were more suppressive in vitro and in vivo than WT Tregs. While HDAC6—/— Tregs express more Foxp3, CTLA4 and IL-10 than their WT counterparts, the basis for this increased suppressive capability may be multifaceted. HDAC6 gene targeting would likely disrupt both the deacetylase-dependent and -independent functions of normal HDAC6. The latter include roles for HDAC6 in regulation of cell migration and proteasomal degradation.

Evidence of the effects of HDAC6 targeting on deacetylase-dependent functions was readily apparent in our studies, including hyperacetylation of HSP90 and the upregulation of many HSFl-regulated genes in HDAC6—/— Tregs, including multiple HSP family members. While many additional genes of importance to Treg biology, but without known regulation via HSF1, were also differentially expressed in HDAC6—/— Tregs, effects on the HSF1/HSP pathway are likely of major importance both mechanistically and therapeutically. We have recently shown that HSP70 forms a complex with Foxp3 in Tregs, that upregulation of HSP70 promotes Treg survival and suppressive functions under conditions of cell stress, and that inhibition of HSP70 impairs Treg survival and suppressive functions (de Zoeten et al. 2010). The current data point to a major role for intracellular heat shock responses in control of Treg functions.

We found in colitis and transplant models that the presence or absence of HDAC6 just within Tregs is a powerful determinant of Treg-dependent resolution of colitis and resistance to allograft rejection (Hancock et al. 2008). These data underscore the importance of HDAC6 as a therapeutic target for modulation of Treg responses. Analysis of the effects of HSP90i in vitro and in vivo in our studies indicated that at least for the models under consideration, targeting of HDAC6 or HSP90 had broadly comparable effects and did not show obvious additional benefits when used together. Such combination might allow for lower doses of each inhibitor to be used, but the broad message from our work so far is that the benefits of targeting the HSF1/HSP pathway appear to be achieved by pharmacologic modulation of HDAC6 or HSP90. Some 14 HSP90i compounds, including 17-AAG, are currently being evaluated in Phase 1 and Phase 2 clinical trials; while data are preliminary, toxicity was rarely observed (Porter et al. 2010; Pacey et al. 2010). Clinical development of HDAC6i is less developed, but

HDAC6 targeting is being considered as a therapy for neurodegenerative conditions (Butler et al. 2010).

Our finding that selective targeting of an individual HDAC isoform can provide comparable effects on Tregs, and associated suppression of T cell-dependent immune responses, to that seen using broadly acting pan-HDACi provides a powerful rationale for the ongoing evaluation of HDAC6i in the regulation of inflammation. Ultimately, selective HDAC6i may provide an alternate, pharmacologic approach to therapies dependent upon Treg expansion and adoptive transfer for the management of autoimmunity and transplant rejection.

4.3 Targeting Class IV HDAC

In the first evidence as to a physiologic function for the sole class IV HDAC, HDAC11, data from gene targeting and siRNA approaches showed that HDAC11 expression suppressed macrophage production of IL-10 (Villagra et al. 2009). HDAC11 has antiproliferative effects (Glozak and Seto 2009; Wong et al. 2010) and is upregulated in at least some cell types, such as pancreatic beta cells, by cytokine simulation (Lundh et al. 2010). As for class IIa HDACs, no specific inhibitors of HDAC11 are reported, as yet. However, HDAC11 may be present in complexes that also contain HDAC6 (Gao et al. 2002; Toropainen et al. 2010), such that studies of the effects of HDAC6i on the biology of HDAC11— /— mice, including in models of inflammation, may be informative.

5 Summary

HDACi act in cancer models by inhibiting the cell cycle, inducing apoptosis and limiting angiogenesis. While HDACi likely exhibit the same effects in models of inflammation, the relative importance of these actions is likely to be markedly different. HDACi exhibit anti-inflammatory effects in a remarkable variety of models and contexts, although their effects on macrophages and DC are such that Th1-dependent responses are most commonly suppressed than Th2-dependent responses, at least in models reported to date. There are also new mechanisms that involve further cell types than the commonly studied APC and T cells. These include clinically important effects on the acetylation of Foxp3 and potentiation of Foxp3+ Treg-dependent immune suppression. Ongoing studies to further dissect and target individual HDAC isoenzymes are underway and may have important advantages over the predominant one-size-fits-all strategy of using pan-HDACi. In particular, targeting of HDAC6 using selective HDAC6i has considerable therapeutic potential inflammation.

0 0

Post a comment