HIF-1 regulation: not so easy come, easy go.

The hypoxia-inducible factor-1 (HIF-1) is the master regulator of the cellular response to hypoxia and its expression levels are tightly controlled through synthesis and degradation. It is widely accepted that HIF-1alpha protein accumulation during hypoxia results from inhibition of its oxygen-dependent degradation by the von Hippel Lindau protein (pVHL) pathway. However, recent data describe new pVHL- or oxygen-independent mechanisms for HIF-1alpha degradation. Furthermore, the hypoxia-induced increase in HIF-1alpha levels is facilitated by the continued translation of HIF-1alpha during hypoxia despite the global inhibition of protein translation. Recent work has contributed to an increased understanding of the mechanisms that control the translation and degradation of HIF-1alpha under both normoxic and hypoxic conditions.

HIF-1alpha and cancer therapy.

Most solid tumors develop regions of hypoxia as they grow and outstrip their blood supply. In order to survive in the stressful hypoxic environment, tumor cells have developed a coordinated set of responses orchestrating their adaptation to hypoxia. The outcomes of the cellular responses to hypoxia are aggressive disease, resistance to therapy, and decreased patient survival. A critical mediator of the hypoxic response is the transcription factor hypoxia-inducible factor 1 (HIF-1) that upregulates expression of proteins that promote angiogenesis, anaerobic metabolism, and many other survival pathways. Regulation of HIF-1alpha, a component of the HIF-1 heterodimer, occurs at multiple levels including translation, degradation, and transcriptional activation, and serves as a testimony to the central role of HIF-1. Studies demonstrating the importance of HIF-1alpha expression for tumor survival have made HIF-1alpha an attractive target for cancer therapy. The growing l.ist of pharmacological inhibitors of HIF-1 and their varied targets mirrors the complex molecular mechanisms controlling HIF-1. In this chapter, we summarize recent findings regarding the regulation of HIF-1alpha and the progress made in identifying new therapeutic agents that inhibit HIF-1alpha.

Hypoxia triggers hedgehog-mediated tumor-stromal interactions in pancreatic cancer.

Pancreatic cancer is characterized by a desmoplastic reaction that creates a dense fibroinflammatory microenvironment, promoting hypoxia and limiting cancer drug delivery due to decreased blood perfusion. Here, we describe a novel tumor-stroma interaction that may help explain the prevalence of desmoplasia in this cancer. Specifically, we found that activation of hypoxia-inducible factor-1α (HIF-1α) by tumor hypoxia strongly activates secretion of the sonic hedgehog (SHH) ligand by cancer cells, which in turn causes stromal fibroblasts to increase fibrous tissue deposition. In support of this finding, elevated levels of HIF-1α and SHH in pancreatic tumors were determined to be markers of decreased patient survival. Repeated cycles of hypoxia and desmoplasia amplified each other in a feed forward loop that made tumors more aggressive and resistant to therapy. This loop could be blocked by HIF-1α inhibition, which was sufficient to block SHH production and hedgehog signaling. Taken together, our findings suggest that increased HIF-1α produced by hypoxic tumors triggers the desmoplasic reaction in pancreatic cancer, which is then amplified by a feed forward loop involving cycles of decreased blood flow and increased hypoxia. Our findings strengthen the rationale for testing HIF inhibitors and may therefore represent a novel therapeutic option for pancreatic cancer.

Inhibiting the hypoxia response for cancer therapy: the new kid on the block.

The hypoxia-inducible transcription factor (HIF)-1alpha inhibitor KC7F2 described in this issue of Clinical Cancer Research is the newest addition to an emerging class of antitumor agents targeting the hypoxia response. Here, we discuss the proposed mechanism of action of KC7F2 and its potential strengths and limitations in comparison with other promising HIF-1alpha inhibitors.

TRAIL-induced cleavage and inactivation of SPAK sensitizes cells to apoptosis.

Ste20-related proline-alanine-rich kinase (SPAK) has been linked to various cellular processes, including proliferation, differentiation, and ion transport regulation. Recently, we showed that SPAK mediates signaling by the TNF receptor, RELT. The presence of a caspase cleavage site in SPAK prompted us to study its involvement in apoptotic signaling induced by another TNF member, TRAIL. We show that TRAIL stimulated caspase 3-like proteases that cleaved SPAK at two distinct sites. Cleavage had little effect on the activity of SPAK but removed its substrate-binding domain. In addition, TRAIL reduced the activity of SPAK in HeLa cells in a caspase-independent manner. Thus, TRAIL inhibited SPAK by two mechanisms: activation of caspases, which removed its substrate-binding domain, and caspase-independent down-regulation of SPAK activity. Furthermore, reducing the amount of SPAK by siRNA increased the sensitivity of HeLa cells to TRAIL-induced apoptosis. Thus, TRAIL down-regulation of SPAK is an important event that enhances its apoptotic effects.