Effects of histone deacetylase inhibitors on HIF-1

Abstract

Hypoxia inducible factors (HIF) are the master transcriptional regulators of angiogenesis and energy metabolism in mammals. Histone deacetylase inhibitors (HDAIs) are among the promising anti -cancer compounds currently in clinical trials. In addition to inducing hyperacetylation of histones, HDAIs have been found to repress HIF function, which has been construed as an important pharmacological mechanism underlying the HDAI -mediated repression of tumor growth and angiogenesis. While HDAIs are potent inhibitors of HIF function and thus may be useful in the prevention and treatment of cancers, a major dilemma is that they may induce hyperacetylation of nonspecific targets thus causing side effects. A better understanding is now required of the molecular and biochemical mechanisms underlying the anti -HIF effects of these compounds. Here we summarize the recent advances towards a better understanding of these molecular and biochemical mechanisms.

Figure 1

Oxygen-dependent conventional regulatory pathways of HIF-α. In the presence of sufficient amount of oxygen and other substrates or cofactors, PHDs hydroxylate HIF-α at two prolyl sites of ODD (Pro402 and Pro564 in HIF-1α). VHL-containing E3 ubiquitin ligase complex recognizes the hydroxylated prolyl sites and leads to HIF-α degradation through the UPS system. Similarly, FIH catalyzes the hydroxylation of an Asn residue (Asn803 in HIF-1α) in the CAD, which blocks the interaction between HIF-αCAD and p300/CBP.

Figure 2

Schematic outline of the HUPS and AUPS pathways of HIF-1α degradation. In the AUPS pathway, ARD1 is proposed to acetylate Lys532, and such modification leads to VHL recognition. Both HUPS and AUPS are dependent on functional E1, E2 and VHL for ubiquitination.

Figure 3

Additional evidence to support the HDAC6-Hsp90 model. (A) Hep3B cells were cultured at normal condition (Nmx: normoxia), or exposed to a hydroxylase inhibitor (Dfo, desferioxamine, 100 μM). Whole cell lysates were prepared and the effects of TSA (600 nM), sodium butyrate (NaB, 2.5 mM) and suberoylanilide hydroxamic acid (SAHA, 2.5 μM) on HIF-1α stability were examined by Western blot. (B) VHL^−/− RCC4 cells were incubated with DMSO or TSA (600 nM). In the absence of a translation blocker, the HIF-1α levels represent a balance between gradual degradation and de novo synthesis. Addition of TSA broke this balance and caused reduced HIF-1α levels. In the presence of a translation blocker (CHX: Cyclohexamide, 20 μg/ml), however, TSA did not expedite the degradation of HIF-1α synthesized prior to the treatment.

Figure 4

Schematic outline of the HUPS and the UIPS. Hsp90-containing chaperone function is proposed to be required for the folding and maturation of nascent HIF-α. Upon maturation, correctly folded HIF-α is regulated by the VHL-dependent HUPS pathway. When Hsp90 function is inhibited either by HDAIs or Hsp90 inhibitors, nascent HIF-α cannot be folded correctly, and the misfolded HIF-α may be disposed either by the UIPS pathway or the UPS pathway. The ubiquitination of misfolded HIF-α may be independent of hydroxylation or VHL, the HIF-α specific ubiquitin ligase.

Figure 5

Possible molecular basis underlying the HDAI-mediated repression of HIF-αCAD TAP. A deacetylation event is proposed to be essential for the function of HIF-αCAD/p300 complex. The addition of HDAIs blocks the deacetylation event and causes the hyperacetylation of an inhibitory protein factor (IPF) or p300. The eventual consequence would be a change in the formation of HIF-αCAD/p300 complex. Proteasome inhibitors may enhance the levels of IPF, thus repressing HIF function in a similar model. ARNT: aryl hydrocarbon receptor nuclear translocator, the dimerization partner of HIF-α, also known as HIF-1β.