Comparative analysis of novel and conventional Hsp90 inhibitors on HIF activity and angiogenic potential in clear cell renal cell carcinoma: implications for clinical evaluation
BMC Cancer
Comparative analysis of novel and conventional Hsp90 inhibitors on HIF activity and angiogenic potential in clear cell renal cell carcinoma: implications for clinical evaluation
Jessica ES Bohonowych 0
Shuping Peng 2
Udhayakumar Gopal 0
Michael W Hance 0
Shane B Wing 1
Kelley M Argraves 1
Karen Lundgren 3
Jennifer S Isaacs 0
0 Department of Cell and Molecular Pharmacology, Medical University of South Carolina , Charleston, SC , USA
1 Department of Regenerative Medicine and Cell Biology, Medical University of South Carolina , Charleston, SC , USA
2 Cancer Research Institute, Central South University , Changsha , China
3 Biogen Idec , San Diego, CA , USA
Background: Perturbing Hsp90 chaperone function targets hypoxia inducible factor (HIF) function in a von HippelLindau (VHL) independent manner, and represents an approach to combat the contribution of HIF to cell renal carcinoma (CCRCC) progression. However, clinical trials with the prototypic Hsp90 inhibitor 17-AAG have been unsuccessful in halting the progression of advanced CCRCC. Methods: Here we evaluated a novel next generation small molecule Hsp90 inhibitor, EC154, against HIF isoforms and HIF-driven molecular and functional endpoints. The effects of EC154 were compared to those of the prototypic Hsp90 inhibitor 17-AAG and the histone deacetylase (HDAC) inhibitor LBH589. Results: The findings indicate that EC154 is a potent inhibitor of HIF, effective at doses 10-fold lower than 17-AAG. While EC154, 17-AAG and the histone deacetylase (HDAC) inhibitor LBH589 impaired HIF transcriptional activity, CCRCC cell motility, and angiogenesis; these effects did not correlate with their ability to diminish HIF protein expression. Further, our results illustrate the complexity of HIF targeting, in that although these agents suppressed HIF transcripts with differential dynamics, these effects were not predictive of drug efficacy in other relevant assays. Conclusions: We provide evidence for EC154 targeting of HIF in CCRCC and for LBH589 acting as a suppressor of both HIF-1 and HIF-2 activity. We also demonstrate that 17-AAG and EC154, but not LBH589, can restore endothelial barrier function, highlighting a potentially new clinical application for Hsp90 inhibitors. Finally, given the discordance between HIF activity and protein expression, we conclude that HIF expression is not a reliable surrogate for HIF activity. Taken together, our findings emphasize the need to incorporate an integrated approach in evaluating Hsp90 inhibitors within the context of HIF suppression.
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Background
Hypoxia inducible factor (HIF) is a master regulator of
the hypoxic response and plays a critical role in the
development and progression of numerous solid cancers
[1,2]. HIF functions as a heterodimeric transcription
factor composed of an oxygen regulated a-subunit and a
constitutively expressed b-subunit (or ARNT). HIF
activity is tightly regulated by oxygen tension wherein
its activity is restrained under oxygenated conditions via
von-Hippel Lindau (VHL) ubiquitin ligase mediated
degradation of the a subunit [3]. In contrast, tumor
hypoxia facilitates HIF-a stabilization, dimerization, and
transcriptional activation. HIF regulates a multitude of
genes that contribute to pro-tumorigenic processes
including invasion, angiogenesis and therapeutic
resistance [2,4-6]. Importantly, inhibition of HIF function
suppresses tumor formation and progression, and
restores treatment sensitivity, highlighting HIF as a
clinically relevant therapeutic target [1,7].
Clear cell renal cell carcinoma (CCRCC) tumors are
highly vascularized and among the most lethal kidney
tumors [8]. CCRCC, with its defined loss of VHL
function and resulting constitutive HIF expression and
activity, is a useful model to decipher the role of HIF in
cancer progression and to evaluate HIF targeting
strategies. Although the sufficiency of HIF for CCRCC
remains somewhat controversial [9], HIF is a major
participant in CCRCC within the context of VHL loss
[10-13]. Of the two main pro-tumorigenic HIF-a
isoforms, HIF-2a elicits tumor formation in CCRCC
xenograft models [10,14] and appears to be more commonly
upregulated in CCRCC relative to HIF-1a [4]. However,
HIF-1a driven CCRCC xenograft models have also been
documented [15], as well as compensatory mechanisms
between the two isoforms [16]. Therefore, the targeting
of both HIF isoforms may represent the most effective
therapeutic approach. In spite of this, few studies have
addressed the ability of candidate agents to target both
isoforms.
A number of generalized HIF targeted approaches
have been employed, including modulation of HIF
expression, transcription, translation, dimerization,
transactivation, and stability [17-23]. Small molecule
inhibitors of the chaperone heat shock protein 90
(Hsp90) represent a growing class of clinically utilized
anti-tumorigenic agents that have been collectively
exploited as an alternative means of targeting HIF-a,
given their (...truncated)