Jin Mo Park

The p38 MAPK pathway is essential for skeletogenesis and bone homeostasis in mice

Nearly every extracellular ligand that has been found to play a role in regulating bone biology acts, at least in part, through MAPK pathways. Nevertheless, much remains to be learned about the contribution of MAPKs to osteoblast biology in vivo. Here we report that the p38 MAPK pathway is required for normal skeletogenesis in mice, as mice with deletion of any of the MAPK pathway member-encoding genes MAPK kinase 3 (Mkk3), Mkk6, p38a, or p38b displayed profoundly reduced bone mass secondary to defective osteoblast differentiation. Among the MAPK kinase kinase (MAP3K) family, we identified TGF-beta-activated kinase 1 (TAK1; also known as MAP3K7) as the critical activator upstream of p38 in osteoblasts. Osteoblast-specific deletion of Tak1 resulted in clavicular hypoplasia and delayed fontanelle fusion, a phenotype similar to the cleidocranial dysplasia observed in humans haploinsufficient for the transcription factor runt-related transcription factor 2 (Runx2). Mechanistic analysis revealed that the TAK1-MKK3/6-p38 MAPK axis phosphorylated Runx2, promoting its association with the coactivator CREB-binding protein (CBP), which was required to regulate osteoblast genetic programs. These findings reveal an in vivo function for p38beta and establish that MAPK signaling is essential for bone formation in vivo. These results also suggest that selective p38beta agonists may represent attractive therapeutic agents to prevent bone loss associated with osteoporosis and aging.

The kinase p38|[alpha]| serves cell type|[ndash]|specific inflammatory functions in skin injury and coordinates pro- and anti-inflammatory gene expression

The mitogen-activated protein kinase p38 mediates cellular responses to injurious stress and immune signaling. Among the many p38 isoforms, p38α is the most widely expressed in adult tissues and can be targeted by various pharmacological inhibitors. Here we investigated how p38α activation is linked to cell type–specific outputs in mouse models of cutaneous inflammation. We found that both myeloid and epithelial p38α elicit inflammatory responses, yet p38α signaling in each cell type served distinct inflammatory functions and varied depending on the mode of skin irritation. In addition, myeloid p38α limited acute inflammation via activation of anti-inflammatory gene expression dependent on mitogen- and stress-activated kinases. Our results suggest a dual function for p38α in the regulation of inflammation and show mixed potential for its inhibition as a therapeutic strategy.

Selenoproteins Mediate T Cell Immunity through an Antioxidant Mechanism

Selenium is an essential dietary element with antioxidant roles in immune regulation, but there is little understanding of how this element acts at the molecular level in host defense and inflammatory disease. Selenium is incorporated into the amino acid selenocysteine (Sec), which in turn is inserted into selenoproteins in a manner dependent on Sec tRNA[Ser]Sec. To investigate the molecular mechanism that links selenium to T cell immunity, we generated mice with selenoprotein-less T cells by cell type-specific ablation of the Sec tRNA[Ser]Sec gene (trsp). Herein, we show that these mutant mice exhibit decreased pools of mature T cells and a defect in T cell-dependent antibody responses. We also demonstrate that selenoprotein deficiency leads to oxidant hyperproduction in T cells and thereby suppresses T cell proliferation in response to T cell receptor stimulation. These findings offer novel insights into immune function of selenium and physiological antioxidants.

The Drosophila inhibitor of apoptosis (IAP) DIAP2 is dispensable for cell survival, required for the innate immune response to gram-negative bacterial infection, and can be negatively regulated by the reaper/hid/grim family of IAP-binding apoptosis inducers.

Many inhibitor of apoptosis (IAP) family proteins inhibit apoptosis. IAPs contain N-terminal baculovirus IAP repeat domains and a C-terminal RING ubiquitin ligase domain. Drosophila IAP DIAP1 is essential for the survival of many cells, protecting them from apoptosis by inhibiting active caspases. Apoptosis initiates when proteins such as Reaper, Hid, and Grim bind a surface groove in DIAP1 baculovirus IAP repeat domains via an N-terminal IAP-binding motif. This evolutionarily conserved interaction disrupts DIAP1-caspase interactions, unleashing apoptosis-inducing caspase activity. A second Drosophila IAP, DIAP2, also binds Rpr and Hid and inhibits apoptosis in multiple contexts when overexpressed. However, due to a lack of mutants, little is known about the normal functions of DIAP2. We report the generation of diap2 null mutants. These flies are viable and show no defects in developmental or stress-induced apoptosis. Instead, DIAP2 is required for the innate immune response to Gram-negative bacterial infection. DIAP2 promotes cytoplasmic cleavage and nuclear translocation of the NF-κB homolog Relish, and this requires the DIAP2 RING domain. Increasing the genetic dose of diap2 results in an increased immune response, whereas expression of Rpr or Hid results in down-regulation of DIAP2 protein levels. Together these observations suggest that DIAP2 can regulate immune signaling in a dose-dependent manner, and this can be regulated by IBM-containing proteins. Therefore, diap2 may identify a point of convergence between apoptosis and immune signaling pathways.

Selective roles of MAPKs during the macrophage response to IFN-gamma.

Macrophages perform essential functions in the infection and resolution of inflammation. IFN-γ is the main endogenous macrophage Th1 type activator. The classical IFN-γ signaling pathway involves activation of Stat-1. However, IFN-γ has also the capability to activate members of the MAPK family. In primary bone marrow-derived macrophages, we have observed strong activation of p38 at early time points of IFN-γ stimulation, whereas weak activation of ERK-1/2 and JNK-1 was detected at a more delayed stage. In parallel, IFN-γ exerted repressive effects on the expression of a number of MAPK phosphatases. By using selective inhibitors and knockout models, we have explored the contributions of MAPK activation to the macrophage response to IFN-γ. Our findings indicate that these kinases regulate IFN-γ-mediated gene expression in a rather selective way: p38 participates mainly in the regulation of the expression of genes required for the innate immune response, including chemokines such as CCL5, CXCL9, and CXCL10; cytokines such as TNF-α; and inducible NO synthase, whereas JNK-1 acts on genes involved in Ag presentation, including CIITA and genes encoding MHC class II molecules. Modest effects were observed for ERK-1/2 in these studies. Interestingly, some of the MAPK-dependent changes in gene expression observed in these studies are based on posttranscriptional regulation of mRNA stability.

Role of selenium-containing proteins in T-cell and macrophage function

Selenium (Se) has been known for many years to have played a role in boosting the immune function, but the manner in which this element acts at the molecular level in host defence and inflammatory diseases is poorly understood. To elucidate the role of Se-containing proteins in the immune function, we knocked out the expression of this protein class in T-cells or macrophages of mice by targeting the removal of the selenocysteine tRNA gene using loxP-Cre technology. Mice with selenoprotein-less T-cells manifested reduced pools of mature and functional T-cells in lymphoid tissues and an impairment in T-cell-dependent antibody responses. Furthermore, selenoprotein deficiency in T-cells led to an inability of these cells to suppress reactive oxygen species production, which in turn affected their ability to proliferate in response to T-cell receptor stimulation. Selenoprotein-less macrophages, on the other hand, manifested mostly normal inflammatory responses, but this deficiency resulted in an altered regulation in extracellular matrix-related gene expression and a diminished migration of macrophages in a protein gel matrix. These observations provided novel insights into the role of selenoproteins in the immune function and tissue homeostasis.

p38 MAPK Activation Is Downstream of the Loss of Intercellular Adhesion in Pemphigus Vulgaris

Pemphigus vulgaris (PV) is a potentially fatal blistering disease characterized by autoantibodies against the desmosomal adhesion protein desmoglein (Dsg) 3. Whether autoantibody steric hindrance or signaling through pathways such as p38 MAPK is primary in disease pathogenesis is controversial. PV mAbs that cause endocytosis of Dsg3 but do not dissociate keratinocytes because of compensatory adhesion by Dsg1 do not activate p38. The same mAbs plus exfoliative toxin to inactivate Dsg1 but not exfoliative toxin alone activate p38, suggesting that p38 activation is secondary to loss of adhesion. Mice with epidermal p38α deficiency blister after passive transfer of PV mAbs; however, acantholytic cells retain cell surface Dsg3 compared with wild-type mice. In cultured keratinocytes, p38 knockdown prevents loss of desmosomal Dsg3 by PV mAbs, and exogenous p38 activation causes internalization of Dsg3, desmocollin 3, and desmoplakin. p38α MAPK is therefore not required for the loss of intercellular adhesion in PV, but may function downstream to augment blistering via Dsg3 endocytosis. Treatments aimed at increasing keratinocyte adhesion could be used in conjunction with immunosuppressive agents, potentially leading to safer and more effective combination therapy regimens.

MAPKAP kinase 2 (MK2)-dependent and independent models of blister formation in pemphigus vulgaris

Pemphigus vulgaris (PV) is an autoimmune blistering disease characterized by autoantibodies to the keratinocyte adhesion protein desmoglein (Dsg) 3. Previous studies suggest that PV pathogenesis involves p38 mitogen activated protein kinase-dependent and -independent pathways. However, p38 is a difficult protein to study and therapeutically target because it has four isoforms and multiple downstream effectors. In the current study, we identify MAPKAP kinase 2 (MK2) as a downstream effector of p38 signaling in PV and describe MK2-dependent and -independent mechanisms of blister formation using passive transfer of human anti-Dsg IgG4 mAbs to neonatal mice. In human keratinocytes, PV mAbs activate MK2 in a dose-dependent manner. MK2 is also activated in human pemphigus skin blisters, causing translocation of MK2 from the nucleus to the cytosol. Small molecule inhibition of MK2 and silencing of MK2 expression block PV mAb-induced Dsg3 endocytosis in human keratinocytes. Additionally, small molecule inhibition and genetic deletion of p38α and MK2 inhibit spontaneous, but not induced, suprabasal blisters by PV mAbs in mouse passive transfer models. Collectively, these data suggest that MK2 is a key downstream effector of p38 that can modulate PV autoantibody pathogenicity. MK2 inhibition may be a valuable adjunctive therapy for control of pemphigus blistering.

Molecular cloning of cDNAs encoding precursors of frog skin antimicrobial peptides from Rana rugosa

Gaegurins, a family of peptide antibiotics with sizes ranging from 24 to 37 amino acids, have recently been purified from Rana rugosa skin (Park, J.M., Jung, J.-E. and Lee, B.J. (1994) Biochem. Biophys. Res. Commun. 205, 948-954). Two complete cDNAs encoding gaegurins 4 and 5 were isolated from a library constructed with the frog skin mRNAs. Each clone contained a single open reading frame that encodes a gaegurin precursor polypeptide. The deduced amino acid sequences revealed that the precursors have a unique tripartite structure: a putative signal sequence at the NH2-terminus followed by an acidic spacer region rich in glutamic and aspartic acids, and a mature gaegurin peptide at the COOH-terminus. Similar modes of organization were also found in antimicrobial or opioid peptide precursors of other frog species, although their mature peptides show little sequence homology. The family of peptides with this characteristic now expands. Northern analysis revealed that gaegurins are extensively expressed in the skin tissue, but not in liver and muscle.

Cell-Selective Inhibition of NF-κB Signaling Improves Therapeutic Index in a Melanoma Chemotherapy Model

UNLABELLED The transcription factor NF-κB promotes survival of cancer cells exposed to doxorubicin and other chemotherapeutic agents. IκB kinase is essential for chemotherapy-induced NF-κB activation and considered a prime target for anticancer treatment. An IκB kinase inhibitor sensitized human melanoma xenografts in mice to killing by doxorubicin, yet also exacerbated treatment toxicity in the host animals. Using mouse models that simulate cell-selective targeting, we found that impaired NF-κB activation in melanoma and host myeloid cells accounts for the therapeutic and the adverse effects, respectively. Ablation of tumor-intrinsic NF-κB activity resulted in apoptosis-driven tumor regression following doxorubicin treatment. By contrast, chemotherapy in mice with myeloid-specific loss of NF-κB activation led to a massive intratumoral recruitment of interleukin-1β-producing neutrophils and necrotic tumor lesions, a condition associated with increased host mortality but not accompanied by tumor regression. Therefore, a molecular target-based therapy may be steered toward different clinical outcomes depending on the drugs cell-specific effects. SIGNIFICANCE Our findings show that the IκB kinase–NF-κB signaling pathway is important for both promoting treatment resistance and preventing host toxicity in cancer chemotherapy; however, the two functions are exerted by distinct cell type–specific mechanisms and can therefore be selectively targeted to achieve an improved therapeutic outcome.