Proteases display biased agonism at protease-activated receptors: location matters!

Kallikrein-related peptidase 4: a new activator of the aberrantly expressed protease-activated receptor 1 in colon cancer cells

Certain serine proteases are considered to be signaling molecules that act through protease-activated receptors (PARs). Our recent studies have implicated PAR1 and PAR4 (thrombin receptors) and PAR2 (trypsin receptor) in human colon cancer growth. Here we analyzed the expression of KLK4, a member of the kallikrein-related peptidase (KLK) family of serine proteases and explored whether this member can activate PAR1 and PAR2 in human colon cancer cells. Immunohistochemistry showed KLK4 expression in human colon adenocarcinomas and its absence in normal epithelia. KLK4 (1 micromol/L) initiated loss of PAR1 and PAR2 from the HT29 cell surface as well as increased intracellular calcium transients in HT29 cells. This KLK4-induced Ca2+ flux was abrogated after an initial challenge of the cells with TRAP (SFLLR-NH2; 100 micromol/L), which is known to desensitize PAR1 and PAR2. Interestingly, PAR1 blocking antibody, which inhibits cleavage and activation by thrombin, dramatically reduced KLK4-induced Ca2+ influx, but blocking cleavage of PAR2 failed to attenuate the KLK4-induced Ca2+ flux. Consistently, desensitization with AP1 (TFFLR-NH2), targeting PAR1, attenuated most of the Ca2+ flux induced by KLK4. KLK4 also induced a rapid and significant ERK1/2 phosphorylation in HT29 cells. Our results demonstrate, for the first time, that KLK4 is aberrantly expressed in colon cancer and capable of inducing PAR1 signaling in cancer cells. These data suggest that KLK4 signaling via PAR1 may represent a novel pathway in colon tumorigenesis.

Biased agonism

Seven-transmembrane receptors are commonly coupled to multiple signaling pathways in cells. The simple model describing agonists for these receptors as producing a common active state to induce uniform activation of the pathways linked to the receptor has been shown to be untenable in light of a large body of data that suggest that some agonists produce activation of some but not all available pathways. These agonists are referred to as ‘biased’ in that they select which signaling pathways become activated upon binding to the receptor. The data to support this mechanism as well as ideas on the possible therapeutic application of this effect will be discussed.

Parstatin, a novel protease-activated receptor 1-derived inhibitor of angiogenesis

Angiogenesis, the process of forming new blood vessels, is a well established and clinically relevant feature of a variety of disease states. Whether blood vessels sprout in a given tissue environment depends on the balance between factors that stimulate angiogenesis and those that impede it. Potent pro-angiogenic factors such as vascular endothelial growth factor (VEGF) have been identified, validated, and successfully used in the clinic. Likewise, anti-angiogenic factors are also emerging as biologically relevant and therapeutically useful entities. PAR1 is a G protein-coupled receptor (GPCR) that participates in hemostasis and vascular development and that mediates the angiogenic activity of thrombin. PAR1 is activated through proteolytic cleavage of its first forty-one extracellular residues by a variety of proteases, most notably thrombin. However, little effort has focused on the forty-one-residue peptide fragment liberated during PAR1 activation. Tsopanoglou and colleagues have now demonstrated that this peptide, parstatin, has intriguing antiangiogenic activity, and, in a follow-up study, they demonstrate its potential pharmacological utility using a rat model of ischemic heart disease.

In search of analgesia: emerging roles of GPCRs in pain

Of all clinically marketed drugs, greater than thirty percent are modulators of G protein-coupled receptors (GPCRs). Nearly 400 GPCRs (i.e., excluding odorant and light receptors) are encoded within the human genome, but only a small fraction of these seven-transmembrane proteins have been identified as drug targets. Chronic pain affects more than one-third of the population, representing a substantial societal burden in use of health care resources and lost productivity. Furthermore, currently available treatments are often inadequate, underscoring the significant need for better therapeutic strategies. The expansion of the identified human GPCR repertoire, coupled with recent insights into the function and structure of GPCRs, offers new opportunities for the development of novel analgesic therapeutics.