Proline-rich tyrosine kinase 2 regulates osteoprogenitor cells and bone formation, and offers an anabolic treatment approach for osteoporosis

Off-target decoding of a multitarget kinase inhibitor by chemical proteomics

Unbiased: Chemical proteomics was used to profile compound interactions in an unbiased fashion. We present here the application of different compound-immobilization routes for decoding nonprotein kinase off-targets of the multitarget kinase inhibitor C1, which interacts with distinct compound moieties. Since the approval of the first selective tyrosine kinase inhibitor, imatinib, various drugs have been developed to target protein kinases. However, due to a high degree of structural conservation of the ATP binding site, off-target effects have been reported for several drugs. Here, we report on off-target decoding for a multitarget protein kinase inhibitor by chemical proteomics, by focusing on interactions with nonprotein kinases. We tested two different routes for the immobilization of the inhibitor on a carrier matrix, and thus identified off-targets that interact with distinct compound moieties. Besides several of the kinases known to bind to the compound, the pyridoxal kinase (PDXK), which has been described to interact with the CDK inhibitor (R)-roscovitine, was captured. The PDXK-inhibitor interaction was shown to occur at the substrate binding site rather than at the ATP binding site. In addition, carbonic anhydrase 2 (CA2) binding was demonstrated, and the determination of the IC(50) revealed an enzyme inhibition in the submicromolar range. The data demonstrate that different compound immobilization routes for chemical proteomics approaches are a valuable method to improve the knowledge about the off-target profile of a compound.

B cell receptor-induced phosphorylation of Pyk2 and focal adhesion kinase involves integrins and the Rap GTPases and is required for B cell spreading

Signaling by the B cell receptor (BCR) promotes integrin-mediated adhesion and cytoskeletal reorganization. This results in B cell spreading, which enhances the ability of B cells to bind antigens and become activated. Proline-rich tyrosine kinase (Pyk2) and focal adhesion kinase (FAK) are related cytoplasmic tyrosine kinases that regulate cell adhesion, cell morphology, and cell migration. In this report we show that BCR signaling and integrin signaling collaborate to induce the phosphorylation of Pyk2 and FAK on key tyrosine residues, a modification that increases the kinase activity of Pyk2 and FAK. Activation of the Rap GTPases is critical for BCR-induced integrin activation as well as for BCR- and integrin-induced reorganization of the actin cytoskeleton. We now show that Rap activation is essential for BCR-induced phosphorylation of Pyk2 and for integrin-induced phosphorylation of Pyk2 and FAK. Moreover Rap-dependent phosphorylation of Pyk2 and FAK required an intact actin cytoskeleton as well as actin dynamics, suggesting that Rap regulates Pyk2 and FAK via its effects on the actin cytoskeleton. Importantly B cell spreading induced by BCR/integrin co-stimulation or by integrin engagement was inhibited by short hairpin RNA-mediated knockdown of either Pyk2 or FAK expression and by treatment with PF-431396, a chemical inhibitor that blocks the kinase activities of both Pyk2 and FAK. Thus Pyk2 and FAK are downstream targets of the Rap GTPases that play a key role in regulating B cell morphology.

Mechanobiology of the skeleton

Mechanical loading of the skeleton is essential for the development, growth, and maintenance of strong, weight-bearing bones. Bone strength is plastic and can be modulated in adults, as illustrated by the increased bone mass in the playing arms of athletes as compared with their nonplaying arms. Our studies have shown that mechanical loading improves bone strength by inducing bone formation in regions of high strain energy. Therefore, bone tissue has a mechanosensing apparatus that directs osteogenesis to where it is most needed to increase bone strength. The most likely sensors of mechanical loading are the osteocytes, which are visco-elastically coupled to the bone matrix so that their biological response increases with loading rate; thus, increasing loading frequency improves the responsiveness of bone to loading. The osteocyte-specific protein sclerostin, an inhibitor of the Wnt signaling pathway, appears to be one of the mediators of the mechanical loading response. Mechanical loading suppresses osteocyte sclerostin secretion, which allows Wnt signaling-dependent bone formation to occur. Intracellular calcium signaling, adenosine triphosphate signaling, and signaling through second messengers, such as prostaglandins and nitric oxide, precede sclerostin secretion. Stretch-activated ion channels and focal adhesion proteins may play a role in triggering these pathways upstream of sclerostin. In particular, focal adhesion kinase and proline-rich tyrosine kinase 2 appear to be sensors of mechanical loads in bone cells.

Focal adhesion kinase is important for fluid shear stress-induced mechanotransduction in osteoblasts

Mechanical loading of bone is important for maintenance of bone mass and structural stability of the skeleton. When bone is mechanically loaded, movement of fluid within the spaces surrounding bone cells generates fluid shear stress (FSS) that stimulates osteoblasts, resulting in enhanced anabolic activity. The mechanisms by which osteoblasts convert the external stimulation of FSS into biochemical changes, a process known as mechanotransduction, remain poorly understood. Focal adhesions are prime candidates for transducing external stimuli. Focal adhesion kinase (FAK), a nonreceptor tyrosine kinase found in focal adhesions, may play a key role in mechanotransduction, although its function has not been directly examined in osteoblasts. We examined the role of FAK in osteoblast mechanotransduction using short interfering RNA (siRNA), overexpression of a dominant negative FAK, and FAK(-/-) osteoblasts to disrupt FAK function in calvarial osteoblasts. Osteoblasts were subjected to varying periods oscillatory fluid flow (OFF) from 5 min to 4 h, and several physiologically important readouts of mechanotransduction were analyzed including: extracellular signal-related kinase 1/2 phosphorylation, upregulation of c-fos, cyclooxygenase-2, and osteopontin, and release of prostaglandin E(2). Osteoblasts with disrupted FAK signaling exhibited severely impaired mechanical responses in all endpoints examined. These data indicate the importance of FAK for both short and long periods of FSS-induced mechanotransduction in osteoblasts.

Structural characterization of proline-rich tyrosine kinase 2 (PYK2) reveals a unique (DFG-out) conformation and enables inhibitor design

Proline-rich tyrosine kinase 2 (PYK2) is a cytoplasmic, non-receptor tyrosine kinase implicated in multiple signaling pathways. It is a negative regulator of osteogenesis and considered a viable drug target for osteoporosis treatment. The high-resolution structures of the human PYK2 kinase domain with different inhibitor complexes establish the conventional bilobal kinase architecture and show the conformational variability of the DFG loop. The basis for the lack of selectivity for the classical kinase inhibitor, PF-431396, within the FAK family is explained by our structural analyses. Importantly, the novel DFG-out conformation with two diarylurea inhibitors (BIRB796, PF-4618433) reveals a distinct subclass of non-receptor tyrosine kinases identifiable by the gatekeeper Met-502 and the unique hinge loop conformation of Leu-504. This is the first example of a leucine residue in the hinge loop that blocks the ATP binding site in the DFG-out conformation. Our structural, biophysical, and pharmacological studies suggest that the unique features of the DFG motif, including Leu-504 hinge-loop variability, can be exploited for the development of selective protein kinase inhibitors.

Mechanical signaling for bone modeling and remodeling

Proper development of the skeleton in utero and during growth requires mechanical stimulation. Loading results in adaptive changes in bone that strengthen bone structure. Bone's adaptive response is regulated by the ability of resident bone cells to perceive and translate mechanical energy into a cascade of structural and biochemical changes within the cells a process known as mechanotransduction. Mechanotransduction pathways are among the most anabolic in bone, and consequently, there is great interest in elucidating how mechanical loading produces its observed effects, including increased bone formation, reduced bone loss, changes in bone cell differentiation and lifespan, among others. A molecular understanding of these processes is developing, and with it comes a profound new insight into the biology of bone. In this article, we review the nature of the physical stimulus to which bone cells mount an adaptive response, including the identity of the sensor cells, their attributes and physical environment, and putative mechanoreceptors they express. Particular attention is allotted to the focal adhesion and Wnt signaling, in light of their emerging role in bone mechanotransduction. Te cellular mechanisms for increased bone loss during disuse, and reduced bone loss during loading are considered. Finally, we summarize the published data on bone cell accommodation, whereby bone cells stop responding to mechanical signaling events. Collectively, these data highlight the complex yet finely orchestrated process of mechanically regulated bone homeostasis.

Sevelamer restores bone volume and improves bone microarchitecture and strength in aged ovariectomized rats

Sevelamer hydrochloride, a noncalcium phosphate binder, has been shown to reduce coronary artery and aortic calcification, and to improve trabecular bone mineral density in hemodialysis patients with chronic kidney disease. Here, we examined whether sevelamer given orally for 12 wk with normal food could restore bone volume (BV) and strength in aged ovariectomized (OVX) rats starting at 4 wk after OVX. Dual-energy x-ray absorptiometry, microcomputerized tomography, and bone histomorphometry analyses showed that OVX animals receiving sevelamer had increased trabecular BV (51%), trabecular number (43%), trabecular thickness (9%), cortical thickness (16%), mineral apposition rate (103%), bone formation rate (25%), and enhanced cortical and trabecular bone mechanical strength as compared with OVX rats. Sevelamer decreased collagen C telopeptide, increased osteocalcin levels, and decreased phosphate and magnesium levels without affecting calcium levels in the blood. Although sevelamer was not absorbed systemically, it stimulated osteoblast differentiation in BM-derived mesenchymal stem cell cultures, as evaluated by alkaline phosphatase positive colony-forming units, and inhibited recombinant human soluble receptor activator of nuclear factor-kappaB ligand-induced osteoclast differentiation, as evaluated by tartrate-resistant acid phosphatase positive cells in bone mineral-hematopoietic stem cell cultures. Surface enhanced laser desorption/ionization time-of-flight mass spectrometry analysis revealed that 69 proteins were differently expressed after OVX, of which 30% (20 of 69) were reversed to sham activity after sevelamer intake. PTH, fibroblast growth factor-23, and cytokine profile in serum were not significantly changed. Together, these results suggest that sevelamer in food increases the BV and improves biomechanical properties of bone in OVX rats.

alpha- and gamma-Protocadherins negatively regulate PYK2

Genetic studies demonstrate that gamma-protocadherins (PCDH-gamma) are required for the survival and synaptic connectivity in neuronal subpopulations of the central nervous system. However, the intracellular signaling mechanisms for PCDH-gamma are poorly understood. Here, we show that PCDH-gamma binds two tyrosine kinases, PYK2 and focal adhesion kinase (FAK), and interaction with PCDH-gamma inhibits kinase activity. Consistent with this, PYK2 activity is abnormally up-regulated in the Pcdh-gamma-deficient neurons. Overexpression of PYK2 induces apoptosis in the chicken spinal cord. Thus, negative regulation of PYK2 activity by PCDH could contribute to the survival of subsets of neurons. Surprisingly, we found that PCDH-alpha interacts similarly with PYK2 and FAK despite containing a distinct cytoplasmic domain. In neural tissue, PCDH-gamma, together with PCDH-alpha, forms functional complexes with PYK2 and/or FAK. Therefore, the identification of common intracellular effectors for PCDH-gamma and PCDH-alpha suggests that dozens of protocadherins generated by Pcdh-alpha and Pcdh-gamma gene clusters can converge different extracellular signals into common intracellular pathways.

Dual focal adhesion kinase/Pyk2 inhibitor has positive effects on bone tumors: implications for bone metastases

BACKGROUND

Lytic bone metastases occur frequently in cancer patients and present major clinical issues including lack of effective therapies. The mechanism of lytic bone metastases involves interactions between tumor cells, bone matrix, and bone cells. Both focal adhesion kinase (FAK) and Pyk2 are implicated in the biology and physiology of bone and cancer.

METHODS

The efficacy of PF-562,271 was evaluated using MDA-MB-231 cells implanted in the tibia of nude rats. The drug was administered orally at a dose of 5 mg/kg, 7 days per week for 28 days. Serum and urine biomarkers, imaging, and histologic techniques were deployed to monitor tumor take rate, disease progression, and response to therapy.

RESULTS

The compound was well tolerated. Both compound-treated groups demonstrated significant and similar increases in osteocalcin and cancellous bone parameters. Radiographic evaluation of tumor-bearing tibiae revealed tumor expansion in nontreated rats compared with a decrease in tumor growth and signs of bone healing in rats treated with PF-562,271. Tartrate-resistant acid phosphatase and fluorescent in situ hybridization analysis revealed that the majority of bone resorption at the tumor site was performed by osteoclasts of rat origin.

CONCLUSIONS

The oral administration of PF-562,271 at a dose of 5 mg/kg suppressed the growth and local spread of intratibial tumors and restored tumor-induced bone loss. The unique ability of PF-562,271 to both curb tumor growth and safely increase bone formation may be an effective therapy for many cancer patients with bone metastases and cancer-associated osteoporosis.

High-throughput kinase profiling as a platform for drug discovery

To fully exploit the potential of kinases as drug targets, novel strategies for the efficient discovery of inhibitors are required. In contrast to the traditional, linear process of inhibitor discovery, high-throughput kinase profiling enables a parallel approach by interrogating compounds against hundreds of targets in a single screen. Compound potency and selectivity are determined simultaneously, providing a choice of targets to pursue that is guided by the quality of lead compounds available, rather than by target biology alone.

Antitumor activity and pharmacology of a selective focal adhesion kinase inhibitor, PF-562,271

Cancer cells are characterized by the ability to grow in an anchorage-independent manner. The activity of the nonreceptor tyrosine kinase, focal adhesion kinase (FAK), is thought to contribute to this phenotype. FAK localizes in focal adhesion plaques and has a role as a scaffolding and signaling protein for other adhesion molecules. Recent studies show a strong correlation between increased FAK expression and phosphorylation status and the invasive phenotype of aggressive human tumors. PF-562,271 is a potent, ATP-competitive, reversible inhibitor of FAK and Pyk2 catalytic activity with a IC(50) of 1.5 and 14 nmol/L, respectively. Additionally, PF-562,271 displayed robust inhibition in an inducible cell-based assay measuring phospho-FAK with an IC(50) of 5 nmol/L. PF-562,271 was evaluated against multiple kinases and displays >100x selectivity against a long list of nontarget kinases. PF-562,271 inhibits FAK phosphorylation in vivo in a dose-dependent fashion (calculated EC(50) of 93 ng/mL, total) after p.o. administration to tumor-bearing mice. In vivo inhibition of FAK phosphorylation (>50%) was sustained for >4 hours with a single p.o. dose of 33 mg/kg. Antitumor efficacy and regressions were observed in multiple human s.c. xenograft models. No weight loss, morbidity, or mortality were observed in any in vivo experiment. Tumor growth inhibition was dose and drug exposure dependent. Taken together, these data show that kinase inhibition with an ATP-competitive small molecule inhibitor of FAK decreases the phospho-status in vivo, resulting in robust antitumor activity.

Protein tyrosine phosphatases in osteoclast differentiation, adhesion, and bone resorption

Osteoclasts are large cells derived from the monocyte-macrophage hematopoietic cell lineage. Their primary function is to degrade bone in various physiological contexts. Osteoclasts adhere to bone via podosomes, specialized adhesion structures whose structure and subcellular organization are affected by mechanical contact of the cell with bone matrix. Ample evidence indicates that reversible tyrosine phosphorylation of podosomal proteins plays a major role in determining the organization and dynamics of podosomes. Although roles of several tyrosine kinases are known in detail in this respect, little is known concerning the roles of protein tyrosine phosphatases (PTPs) in regulating osteoclast adhesion. Here we summarize available information concerning the known and hypothesized roles of the best-researched PTPs in osteoclasts - PTPRO, PTP epsilon, SHP-1, and PTP-PEST. Of these, PTPRO, PTP epsilon, and PTP-PEST appear to support osteoclast activity while SHP-1 inhibits it. Additional studies are required to provide full molecular details of the roles of these PTPs in regulating osteoclast adhesion, and to uncover additional PTPs that participate in this process.

Glucocorticoids induce osteocyte apoptosis by blocking focal adhesion kinase-mediated survival. Evidence for inside-out signaling leading to anoikis

Bone fragility induced by chronic glucocorticoid excess is due, at least in part, to induction of osteocyte apoptosis through direct actions on these cells. However, the molecular mechanism by which glucocorticoids shorten osteocyte life span has remained heretofore unknown. We report that apoptosis of osteocytic MLO-Y4 cells induced by the synthetic glucocorticoid dexamethasone is abolished by the glucocorticoid receptor antagonist RU486, but not by inhibition of protein or RNA synthesis. Dexamethasone-induced apoptosis is preceded by a decrease in the number of cytoplasmic processes, an indicator of cell detachment. In addition, the focal adhesion kinase FAK prevents dexamethasone-induced apoptosis, whereas the FAK-related kinase Pyk2 increases the basal levels of apoptosis. Dexamethasone-induced apoptosis is also prevented in cells expressing kinase-deficient or phosphorylation-defective (Y402F) dominant negative mutants of Pyk2. Consistent with the requirement of tyrosine 402, dexamethasone induces rapid Pyk2 phosphorylation in this residue. Moreover, knocking down Pyk2 expression abolishes apoptosis and cell detachment induced by dexamethasone, and transfection with human Pyk2 rescues both responses. Furthermore, induction of apoptosis as well as cell detachment by dexamethasone is abolished by inhibiting the activity of JNK, a recognized downstream target of Pyk2 activation. These results demonstrate that glucocorticoids promote osteocyte apoptosis via a receptor-mediated mechanism that does not require gene transcription and that is mediated by rapid activation of Pyk2 and JNK, followed by inside-out signaling that leads to cell detachment-induced apoptosis or anoikis.