Proteases are important for multiple processes during malignant progression including tumor angiogenesis, invasion and metastasis. Recent evidence reveals that tumor-promoting proteases function as part of an extensive multidirectional network of proteolytic interactions, in contrast to the unidirectional caspase cascade. These networks involve different constituents of the tumor microenvironment, and key proteases — such as cathepsin B, urokinase-type plasminogen activator and several matrix metalloproteinases — occupy central nodes for amplifying proteolytic signals passing through the network. The proteolytic network interacts with other important signaling pathways in tumor biology, involving chemokines, cytokines, and kinases. Viewing these proteolytic interactions as a system of activating and inhibiting reactions provides insight into tumor biology and reveals relevant pharmaceutical targets. This review will examine recent advances in understanding proteases in cancer, and summarize how their network of activity is co-opted to promote tumor progression.

Metastasis remains the most common cause of death in most cancers, with limited therapies for combating disseminated disease. While the primary tumor microenvironment is an important regulator of cancer progression, it is less well understood how different tissue environments influence metastasis. We analyzed tumor-stroma interactions that modulate organ tropism of brain, bone and lung metastasis in xenograft models. We identified a number of potential modulators of site-specific metastasis, including cathepsin S as a regulator of breast-to-brain metastasis. High cathepsin S expression at the primary site correlated with decreased brain metastasis-free survival in breast cancer patients. Both macrophages and tumor cells produce cathepsin S, and only the combined depletion significantly reduced brain metastasis in vivo. Cathepsin S specifically mediates blood-brain barrier transmigration via proteolytic processing of the junctional adhesion molecule (JAM)-B. Pharmacological inhibition of cathepsin S significantly reduced experimental brain metastasis, supporting its consideration as a therapeutic target for this disease.

The physiological flux of oxygen is extreme in exercising skeletal muscle. Hypoxia is thus a critical parameter in muscle function, influencing production of ATP, utilization of energy-producing substrates, and manufacture of exhaustion-inducing metabolites. Glycolysis is the central source of anaerobic energy in animals, and this metabolic pathway is regulated under low-oxygen conditions by the transcription factor hypoxia-inducible factor 1α (HIF-1α). To determine the role of HIF-1α in regulating skeletal muscle function, we tissue-specifically deleted the gene encoding the factor in skeletal muscle. Significant exercise-induced changes in expression of genes are decreased or absent in the skeletal-muscle HIF-1α knockout mice (HIF-1α KOs); changes in activities of glycolytic enzymes are seen as well. There is an increase in activity of rate-limiting enzymes of the mitochondria in the muscles of HIF-1α KOs, indicating that the citric acid cycle and increased fatty acid oxidation may be compensating for decreased flow through the glycolytic pathway. This is corroborated by a finding of no significant decreases in muscle ATP, but significantly decreased amounts of lactate in the serum of exercising HIF-1α KOs. This metabolic shift away from glycolysis and toward oxidation has the consequence of increasing exercise times in the HIF-1α KOs. However, repeated exercise trials give rise to extensive muscle damage in HIF-1α KOs, ultimately resulting in greatly reduced exercise times relative to wild-type animals. The muscle damage seen is similar to that detected in humans in diseases caused by deficiencies in skeletal muscle glycogenolysis and glycolysis. Thus, these results demonstrate an important role for the HIF-1 pathway in the metabolic control of muscle function.

Hypoxia-inducible transcription factor 1 (HIF-1) and HIF-2␣ regulate the expression of an expansive array of genes associated with cellular responses to hypoxia. Although HIF-regulated genes mediate crucial beneficial short-term biological adaptations, we hypothesized that chronic activation of the HIF pathway in cardiac muscle, as occurs in advanced ischemic heart disease, is detrimental. We generated mice with cardiac myocyte-specific deletion of the von Hippel-Lindau protein (VHL), an essential component of an E3 ubiquitin ligase responsible for suppressing HIF levels during normoxia. These mice were born at expected frequency and thrived until after 3 months postbirth, when they developed severe progressive heart failure and premature death. VHL-null hearts developed lipid accumulation, myofibril rarefaction, altered nuclear morphology, myocyte loss, and fibrosis, features seen for various forms of human heart failure. Further, nearly 50% of VHL ؊/؊ hearts developed malignant cardiac tumors with features of rhabdomyosarcoma and the capacity to metastasize. As compelling evidence for the mechanistic contribution of HIF-1␣, the concomitant deletion of VHL and HIF-1␣ in the heart prevented this phenotype and restored normal longevity. These findings strongly suggest that chronic activation of the HIF pathway in ischemic hearts is maladaptive and contributes to cardiac degeneration and progression to heart failure.In the cardiovascular system, hypoxia is encountered in a number of important clinical settings, including sleep apnea, chronic obstructive pulmonary disease, and, most commonly, ischemic heart disease (IHD). Myocardial hypoxia, as a component of ischemia, may also occur in other common clinical conditions, such as valve disease, pathological cardiac hypertrophy, and severe systemic hypertension. As such, understanding how hypoxia-induced molecular changes affect the heart is of great importance. Altered gene expression mediated by hypoxia-inducible transcription factor 1␣ (HIF-1␣) and HIF-2␣ is one of the most fundamental and ubiquitous mechanisms whereby biological adaptations to hypoxia occur. The HIFs are basic helix-loop-helix transcription factors that regulate the expression of a wide repertoire of genes involved in a myriad of biological functions, including angiogenesis, apoptosis, and cellular metabolism (14,17,51). A third family member, HIF-3␣, lacks a transcriptional activation domain and may act as a competitive inhibitor of HIF-1␣ and -2␣ activity (41).Although HIF-1␣ and -2␣ appear to bind the same hypoxia response elements (HREs) in hypoxia-inducible genes, it has been established that they preferentially regulate different genes in different cell types and therefore are not redundant.The role of HIF-1␣ in the transcriptional control of angiogenesis has led to the ongoing development of HIF-1␣ as a therapeutic stimulator of angiogenesis in IHD and peripheral vascular disease (12,15,48,60), although clinical development of HIF-2␣ for this purpose has also been considered. Others and we ...

During endurance training, exercising skeletal muscle experiences severe and repetitive oxygen stress. The primary transcriptional response factor for acclimation to hypoxic stress is hypoxia-inducible factor-1alpha (HIF-1alpha), which upregulates glycolysis and angiogenesis in response to low levels of tissue oxygenation. To examine the role of HIF-1alpha in endurance training, we have created mice specifically lacking skeletal muscle HIF-1alpha and subjected them to an endurance training protocol. We found that only wild-type mice improve their oxidative capacity, as measured by the respiratory exchange ratio; surprisingly, we found that HIF-1alpha null mice have already upregulated this parameter without training. Furthermore, untrained HIF-1alpha null mice have an increased capillary to fiber ratio and elevated oxidative enzyme activities. These changes correlate with constitutively activated AMP-activated protein kinase in the HIF-1alpha null muscles. Additionally, HIF-1alpha null muscles have decreased expression of pyruvate dehydrogenase kinase I, a HIF-1alpha target that inhibits oxidative metabolism. These data demonstrate that removal of HIF-1alpha causes an adaptive response in skeletal muscle akin to endurance training and provides evidence for the suppression of mitochondrial biogenesis by HIF-1alpha in normal tissue.