Jennifer S. Isaacs

Growing evidence indicates that inflammation is a contributing factor leading to cancer development. However, pathways involved in this progression are not well understood. To examine whether HIF-1alpha is a factor linking inflammation and tumorigenesis, we investigated whether the HIF-1 signaling pathway was stimulated by the pro-inflammatory cytokine interleukin-1beta (IL-1beta) in A549 cells. We find that IL-1beta up-regulated HIF-1alpha protein under normoxia and activated the HIF-1-responsive gene vascular endothelial growth factor (VEGF) via a pathway dependent on nuclear factor kappaB (NFkB). Interestingly, although this pathway is stimulated by upstream signaling via AKT and mTOR and requires new transcription, IL-1 mediated HIF-1alpha induction also utilizes a post-transcriptional mechanism that involves antagonism of VHL-dependent HIF-1alpha degradation, which results in increased HIF-1alpha protein stability. IL-1 mediated NFkB-dependent cyclooxygenases-2 (COX-2) expression served as a positive effector for HIF-1alpha induction. Although COX-2 inhibitors attenuated IL-1 mediated HIF-1alpha induction, prostaglandin E2 (PGE2), a physiological product of COX-2, induced HIF-1alpha protein in a dose-dependent manner. Our data, therefore, demonstrate that IL-1beta up-regulates functional HIF-1alpha protein through a classical inflammatory signaling pathway involving NFkB and COX-2, culminating in up-regulation of VEGF, a potent angiogenic factor required for tumor growth and metastasis. Thus, HIF-1 is identified as a pivotal transcription factor linking the inflammatory and oncogenic pathways.

HIF-1 alpha is a normally labile proangiogenic transcription factor that is stabilized and activated in hypoxia. Although the von Hippel Lindau (VHL) gene product, the ubiquitin ligase responsible for regulating HIF-1 alpha protein levels, efficiently targets HIF-1 alpha for rapid proteasome-dependent degradation under normoxia, HIF-1 alpha is resistant to the destabilizing effects of VHL under hypoxia. HIF-1 alpha also associates with the molecular chaperone Hsp90. To examine the role of Hsp90 in HIF-1 alpha function, we used renal carcinoma cell (RCC) lines that lack functional VHL and express stable HIF-1 alpha protein under normoxia. Geldanamycin (GA), an Hsp90 antagonist, promoted efficient ubiquitination and proteasome-mediated degradation of HIF-1 alpha in RCC in both normoxia and hypoxia. Furthermore, HIF-1 alpha point mutations that block VHL association did not protect HIF-1 alpha from GA-induced destabilization. Hsp90 antagonists also inhibited HIF-1 alpha transcriptional activity and dramatically reduced both hypoxia-induced accumulation of VEGF mRNA and hypoxia-dependent angiogenic activity. These findings demonstrate that disruption of Hsp90 function 1) promotes HIF-1 alpha degradation via a novel, oxygen-independent E3 ubiquitin ligase and 2) diminishes HIF-1 alpha transcriptional activity. Existence of an Hsp90-dependent pathway for elimination of HIF-1 alpha predicts that Hsp90 antagonists may be hypoxic cell sensitizers and possess antiangiogenic activity in vivo, thus extending the utility of these drugs as therapeutic anticancer agents.

Lineage plasticity has emerged as an important mechanism of treatment resistance in prostate cancer. Treatment-refractory prostate cancers are increasingly associated with loss of luminal prostate markers, and in many cases induction of developmental programs, stem cell-like phenotypes, and neuroendocrine/neuronal features. Clinically, lineage plasticity may manifest as low PSA progression, resistance to androgen receptor (AR) pathway inhibitors, and sometimes small cell/neuroendocrine pathologic features observed on metastatic biopsy. This mechanism is not restricted to prostate cancer as other malignancies also demonstrate lineage plasticity during resistance to targeted therapies. At present, there is no established therapeutic approach for patients with advanced prostate cancer developing lineage plasticity or small cell neuroendocrine prostate cancer (NEPC) due to knowledge gaps in the underlying biology. Few clinical trials address questions in this space, and the outlook for patients remains poor. To move forward, urgently needed are: (i) a fundamental understanding of how lineage plasticity occurs and how it can best be defined; (ii) the temporal contribution and cooperation of emerging drivers; (iii) preclinical models that recapitulate biology of the disease and the recognized phenotypes; (iv) identification of therapeutic targets; and (v) novel trial designs dedicated to the entity as it is defined. This Perspective represents a consensus arising from the NCI Workshop on Lineage Plasticity and Androgen Receptor-Independent Prostate Cancer. We focus on the critical questions underlying lineage plasticity and AR-independent prostate cancer, outline knowledge and resource gaps, and identify strategies to facilitate future collaborative clinical translational and basic studies in this space.