Protein kinase C delta null mice exhibit structural alterations in articular surface, intra-articular and subchondral compartments

Loss of PKCδ/Prkcd prevents cartilage degeneration in joints but exacerbates hyperalgesia in an experimental osteoarthritis mouse model

Pain is the prime symptom of osteoarthritis (OA) that directly affects the quality of life. Protein kinase Cδ (PKCδ/Prkcd) plays a critical role in OA pathogenesis; however, its significance in OA-related pain is not entirely understood. The present study investigated the functional role of PKCδ in OA pain sensation. OA was surgically induced in control (Prkcdfl/fl), global- (Prkcdfl/fl; ROSACreERT2), and sensory neuron-specific conditional knockout (cKO) mice (Prkcdfl/fl; NaV1.8/Scn10aCreERT2) followed by comprehensive analysis of longitudinal behavioral pain, histopathology and immunofluorescence studies. GlobalPrkcd cKO mice prevented cartilage deterioration by inhibiting matrix metalloproteinase-13 (MMP13) in joint tissues but significantly increased OA pain. Sensory neuron-specificdeletion of Prkcd in mice did not protect cartilage from degeneration but worsened OA-associated pain. Exacerbated pain sensitivity observed in global- and sensory neuron-specific cKO of Prkcd was corroborated with markedly increased specific pain mediators in knee synovium and dorsal root ganglia (DRG). These specific pain markers include nerve growth factor (NGF) and vascular endothelial growth factor (VEGF), and their cognate receptors, including tropomyosin receptor kinase A (TrkA) and vascular endothelial growth factor receptor-1 (VEGFR1). The increased levels of NGF/TrkA and VEGF/VEGFR1 were comparable in both global- and sensory neuron-specific cKO groups. These data suggest that the absence of Prkcd gene expression in the sensory neurons is strongly associated with OA hyperalgesia independent of cartilage protection. Thus, inhibition of PKCδ may be beneficial for cartilage homeostasis but could aggravate OA-related pain symptoms.

Role of Apoptosis in the Pathogenesis of Osteoarthritis: An Explicative Review

Apoptosis is a complex regulatory, active cell death process that plays a role in cell development, homeostasis, and ageing. Cancer, developmental defects, and degenerative diseases are all pathogenic disorders caused by apoptosis dysregulation. Osteoarthritis (OA) is by far the most frequently diagnosed joint disease in the aged, and it is characterized by the ongoing breakdown of articular cartilage, which causes severe disability. Multiple variables regulate the anabolic and catabolic pathways of the cartilage matrix, which either directly or indirectly contribute to cartilage degeneration in osteoarthritis. Articular cartilage is a highly specialized tissue made up of an extracellular matrix of cells that are tightly packed together. As a result, chondrocyte survival is crucial for the preservation of an optimal cartilage matrix, and chondrocyte characteristics and survival compromise may result in articular cartilage failure. Inflammatory cytokines can either promote or inhibit apoptosis, the process of programmed cell death. Pro-apoptotic cytokines like TNF-α can induce cell death, while anti-apoptotic cytokines like IL-4 and IL-10 protect against apoptosis. The balance between these cytokines plays a critical role in determining cell fate and has implications for tissue damage and disease progression. Similarly, they contribute to the progression of OA by disrupting the metabolic balance in joint tissues by promoting catabolic and anabolic pathways. Their impact on cell joints, as well as the impacts of cell signalling pathways on cytokines and inflammatory substances, determines their function in osteoarthritis development. Apoptosis is evident in osteoarthritic cartilage; however, determining the relative role of chondrocyte apoptosis in the aetiology of OA is difficult, and the rate of apoptotic chondrocytes in osteoarthritic cartilage is inconsistent. The current study summarises the role of apoptosis in the development of osteoarthritis, the mediators, and signalling pathways that trigger the cascade of events, and the other inflammatory features involved.

Lentivirus-based shRNA of Caspase-3 gene silencing inhibits chondrocyte apoptosis and delays the progression of surgically induced osteoarthritis

Chondrocyte apoptosis is an important pathological feature of osteoarthritis (OA). Excessive apoptosis of chondrocytes disrupts the dynamic balance of cell proliferation and apoptosis, with a marked reduction in chondrocytes and cartilage matrix disintegration, which represents the main pathology of OA. Caspases, especially Caspase-3, play a central role in cell apoptosis. In this study, a lentiviral vector was used to transduce caspase-3 short hairpin RNA (shRNA) into rat chondrocytes (RCs), and the apoptotic and phenotypic genes of RCs were analyzed using real-time PCR and western blotting in vitro. In addition, in vivo intra-articular injection of Caspase-3 shRNA lentivirus was performed in a surgically induced OA rat model. Our results showed that Caspase-3 gene silencing could down-regulate the TNF-α-mediated inflammatory gene expression of TNFR1, FADD, and IL-1β, apoptotic gene expression of APAF1, Caspase-3, and Caspase-9, thereby attenuating the apoptotic pathway in vitro. Caspase-3 gene silencing also attenuated TNF-α-mediated decreased gene expression of ACAN, Col1-a1, and Col2-a1. Furthermore, Caspase-3 gene silencing could effectively reduce the OARSI score, and gene expression of Caspase-3, Caspase-9, MMP13, and TNF-α in a surgically induced OA rat model. Caspase-3 gene silencing may serve as a novel therapeutic strategy for cartilage injury and OA.

Off-target pharmacological activity at various kinases: Potential functional and pathological side effects

In drug discovery, during the lead optimization and candidate characterization stages, novel small molecules are frequently evaluated in a battery of in vitro pharmacology assays to identify potential unintended, off-target interactions with various receptors, transporters, ion channels, and enzymes, including kinases. Furthermore, these screening panels may also provide utility at later stages of development to provide a mechanistic understanding of unexpected safety findings. Here, we present a compendium of the most likely functional and pathological outcomes associated with interaction(s) to a panel of 95 kinases based on an extensive curation of the scientific literature. This panel of kinases was designed by AbbVie based on safety-related data extracted from the literature, as well as from over 20 years of institutional knowledge generated from discovery efforts. For each kinase, the scientific literature was reviewed using online databases and the most often reported functional and pathological effects were summarized. This work should serve as a practical guide for small molecule drug discovery scientists and clinical investigators to predict and/or interpret adverse effects related to pharmacological interactions with these kinases.

The Keap1-Nrf2 System: A Mediator between Oxidative Stress and Aging

Oxidative stress, a term that describes the imbalance between oxidants and antioxidants, leads to the disruption of redox signals and causes molecular damage. Increased oxidative stress from diverse sources has been implicated in most senescence-related diseases and in aging itself. The Kelch-like ECH-associated protein 1- (Keap1-) nuclear factor-erythroid 2-related factor 2 (Nrf2) system can be used to monitor oxidative stress; Keap1-Nrf2 is closely associated with aging and controls the transcription of multiple antioxidant enzymes. Simultaneously, Keap1-Nrf2 signaling is also modulated by a more complex regulatory network, including phosphoinositide 3-kinase (PI3K)/protein kinase B (Akt), protein kinase C, and mitogen-activated protein kinase. This review presents more information on aging-related molecular mechanisms involving Keap1-Nrf2. Furthermore, we highlight several major signals involved in Nrf2 unbinding from Keap1, including cysteine modification of Keap1 and phosphorylation of Nrf2, PI3K/Akt/glycogen synthase kinase 3β, sequestosome 1, Bach1, and c-Myc. Additionally, we discuss the direct interaction between Keap1-Nrf2 and the mammalian target of rapamycin pathway. In summary, we focus on recent progress in research on the Keap1-Nrf2 system involving oxidative stress and aging, providing an empirical basis for the development of antiaging drugs.

PKC-δ deficiency in B cells displays osteopenia accompanied with upregulation of RANKL expression and osteoclast-osteoblast uncoupling

PKC-δ is an important molecule for B-cell proliferation and tolerance. B cells have long been recognized to play a part in osteoimmunology and pathological bone loss. However, the role of B cells with PKC-δ deficiency in bone homeostasis and the underlying mechanisms are unknown. We generated mice with PKC-δ deletion selectively in B cells by crossing PKC-δ-loxP mice with CD19-Cre mice. We studied their bone phenotype using micro-CT and histology. Next, immune organs were obtained and analyzed. Western blotting was used to determine the RANKL/OPG ratio in vitro in B-cell cultures, ELISA assay and immunohistochemistry were used to analyze in vivo RANKL/OPG balance in serum and bone sections respectively. Finally, we utilized osteoclastogenesis to study osteoclast function via hydroxyapatite resorption assay, and isolated primary calvaria osteoblasts to investigate osteoblast proliferation and differentiation. We also investigated osteoclast and osteoblast biology in co-culture with B-cell supernatants. We found that mice with PKC-δ deficiency in B cells displayed an osteopenia phenotype in the trabecular and cortical compartment of long bones. In addition, PKC-δ deletion resulted in changes of trabecular bone structure in association with activation of osteoclast bone resorption and decrease in osteoblast parameters. As expected, inactivation of PKC-δ in B cells resulted in changes in spleen B-cell number, function, and distribution. Consistently, the RANKL/OPG ratio was elevated remarkably in B-cell culture, in the serum and in bone specimens after loss of PKC-δ in B cells. Finally, in vitro analysis revealed that PKC-δ ablation suppressed osteoclast differentiation and function but co-culture with B-cell supernatant reversed the suppression effect, as well as impaired osteoblast proliferation and function, indicative of osteoclast–osteoblast uncoupling. In conclusion, PKC-δ plays an important role in the interplay between B cells in the immune system and bone cells in the pathogenesis of bone lytic diseases.

Sotrastaurin, a PKC inhibitor, attenuates RANKL-induced bone resorption and attenuates osteochondral pathologies associated with the development of OA

Osteoarthritis (OA) is a common degenerative disease that affects the musculoskeletal structure of the whole joint, which is characterized by progressive destruction of both articular cartilage and subchondral bone. Treatment of the bone pathologies, particularly osteoclast‐mediated subchondral bone loss in the early stages of OA, could prevent subsequent cartilage degeneration and progression of OA. In the present study, the PKC inhibitor, Sotrastaurin, was found to inhibit RANKL‐induced osteoclast formation in vitro in a dose‐ and time‐dependent manner. In particular, SO exerted its anti‐osteoclastic effect predominantly at the early stages of RANKL stimulation, suggesting inhibitory effects on precursor cell fusion. Using mature osteoclasts cultured on bovine bone discs, we showed that SO also exerts anti‐resorptive effects on mature osteoclasts bone resorptive function. Mechanistically, SO attenuates the early activation of the p38, ERK and JNK signalling pathways, leeding to impaired induction of crucial osteoclast transcription factors c‐Jun, c‐Fos and NFATc1. We also showed that SO treatment significantly inhibited the phosphorylation of PKCδ and MARCKS, an upstream regulator of cathepsin K secretion. Finally, in animal studies, SO significantly alleviates the osteochondral pathologies of subchondral bone destruction as well as articular cartilage degeneration following DMM‐induced OA, markedly improving OARSI scores. The reduced subchondral bone loss was associated with marked reductions in TRAP(+) osteoclasts in the subchondral bone tissue. Collectively, our data provide evidence for the protective effects of SO against OA by preventing aberrant subchondral bone and articular cartilage changes. Thus, SO demonstrates potential for further development as an alternative therapeutic option against OA.

Conditional Knockout of PKC-δ in Osteoclasts Favors Bone Mass Accrual in Males Due to Decreased Osteoclast Function

Protein kinase C delta (PKC-δ) functions as an important regulator in bone metabolism. However, the precise involvement of PKC-δ in the regulation of osteoclasts remains elusive. We generated an osteoclast specific PKC-δ knockout mouse strain to investigate the function of PKC-δ in osteoclast biology. Bone phenotype was investigated using microcomputed tomography. Osteoclast and osteoblast parameters were assessed using bone histomorphometry, and analysis of osteoclast formation and function with osteoclastogensis and hydroxyapatite resorption assays. The molecular mechanisms by which PKC-δ regulated osteoclast function were dissected by Western Blotting, TUNEL assay, transfection and transcriptome sequencing. We found that ablation of PKC-δ in osteoclasts resulted in an increase in trabecular and cortical bone volume in male mice, however, the bone mass phenotype was not observed in female mice. This was accompanied by decreased osteoclast number and surface, and Cathepsin-K protein levels in vivo, as well as decreased osteoclast formation and resorption in vitro in a male-specific manner. PKC-δ regulated androgen receptor transcription by binding to its promoter, moreover, PKC-δ conditional knockout did not increase osteoclast apoptosis but increased MAPK signaling and enhanced androgen receptor transcription and expression, finally leding to significant alterations in gene expression and signaling changes related to extracellular matrix proteins specifically in male mice. In conclusion, PKC-δ plays an important role in osteoclast formation and function in a male-specific manner. Our work reveals a previously unknown target for treatment of gender-related bone diseases.

Protein Kinase C as Regulator of Vascular Smooth Muscle Function and Potential Target in Vascular Disorders

Vascular smooth muscle (VSM) plays an important role in maintaining vascular tone. In addition to Ca2+-dependent myosin light chain (MLC) phosphorylation, protein kinase C (PKC) is a major regulator of VSM function. PKC is a family of conventional Ca2+-dependent α, β, and γ, novel Ca2+-independent δ, ɛ, θ, and η, and atypical ξ, and ι/λ isoforms. Inactive PKC is mainly cytosolic, and upon activation it undergoes phosphorylation, maturation, and translocation to the surface membrane, the nucleus, endoplasmic reticulum, and other cell organelles; a process facilitated by scaffold proteins such as RACKs. Activated PKC phosphorylates different substrates including ion channels, pumps, and nuclear proteins. PKC also phosphorylates CPI-17 leading to inhibition of MLC phosphatase, increased MLC phosphorylation, and enhanced VSM contraction. PKC could also initiate a cascade of protein kinases leading to phosphorylation of the actin-binding proteins calponin and caldesmon, increased actin-myosin interaction, and VSM contraction. Increased PKC activity has been associated with vascular disorders including ischemia-reperfusion injury, coronary artery disease, hypertension, and diabetic vasculopathy. PKC inhibitors could test the role of PKC in different systems and could reduce PKC hyperactivity in vascular disorders. First-generation PKC inhibitors such as staurosporine and chelerythrine are not very specific. Isoform-specific PKC inhibitors such as ruboxistaurin have been tested in clinical trials. Target delivery of PKC pseudosubstrate inhibitory peptides and PKC siRNA may be useful in localized vascular disease. Further studies of PKC and its role in VSM should help design isoform-specific PKC modulators that are experimentally potent and clinically safe to target PKC in vascular disease.

Chondrocyte Apoptosis in the Pathogenesis of Osteoarthritis

Apoptosis is a highly-regulated, active process of cell death involved in development, homeostasis and aging. Dysregulation of apoptosis leads to pathological states, such as cancer, developmental anomalies and degenerative diseases. Osteoarthritis (OA), the most common chronic joint disease in the elderly population, is characterized by progressive destruction of articular cartilage, resulting in significant disability. Because articular cartilage depends solely on its resident cells, the chondrocytes, for the maintenance of extracellular matrix, the compromising of chondrocyte function and survival would lead to the failure of the articular cartilage. The role of subchondral bone in the maintenance of proper cartilage matrix has been suggested as well, and it has been proposed that both articular cartilage and subchondral bone interact with each other in the maintenance of articular integrity and physiology. Some investigators include both articular cartilage and subchondral bone as targets for repairing joint degeneration. In late-stage OA, the cartilage becomes hypocellular, often accompanied by lacunar emptying, which has been considered as evidence that chondrocyte death is a central feature in OA progression. Apoptosis clearly occurs in osteoarthritic cartilage; however, the relative contribution of chondrocyte apoptosis in the pathogenesis of OA is difficult to evaluate, and contradictory reports exist on the rate of apoptotic chondrocytes in osteoarthritic cartilage. It is not clear whether chondrocyte apoptosis is the inducer of cartilage degeneration or a byproduct of cartilage destruction. Chondrocyte death and matrix loss may form a vicious cycle, with the progression of one aggravating the other, and the literature reveals that there is a definite correlation between the degree of cartilage damage and chondrocyte apoptosis. Because current treatments for OA act only on symptoms and do not prevent or cure OA, chondrocyte apoptosis would be a valid target to modulate cartilage degeneration.