Seyoung Lim

Sphingosine-1-phosphate promotes lymphangiogenesis by stimulating S1P1/Gi/PLC/Ca2+ signaling pathways

The lymphatic system plays pivotal roles in mediating tissue fluid homeostasis and immunity, and excessive lymphatic vessel formation is implicated in many pathological conditions, which include inflammation and tumor metastasis. However, the molecular mechanisms that regulate lymphatic vessel formation remain poorly characterized. Sphingosine-1-phosphate (S1P) is a potent bioactive lipid that is implicated in a variety of biologic processes such as inflammatory responses and angiogenesis. Here, we first report that S1P acts as a lymphangiogenic mediator. S1P induced migration, capillary-like tube formation, and intracellular Ca(2+) mobilization, but not proliferation, in human lymphatic endothelial cells (HLECs) in vitro. Moreover, a Matrigel plug assay demonstrated that S1P promoted the outgrowth of new lymphatic vessels in vivo. HLECs expressed S1P1 and S1P3, and both RNA interference-mediated down-regulation of S1P1 and an S1P1 antagonist significantly blocked S1P-mediated lymphangiogenesis. Furthermore, pertussis toxin, U73122, and BAPTA-AM efficiently blocked S1P-induced in vitro lymphangiogenesis and intracellular Ca(2+) mobilization of HLECs, indicating that S1P promotes lymphangiogenesis by stimulating S1P1/G(i)/phospholipase C/Ca(2+) signaling pathways. Our results suggest that S1P is the first lymphangiogenic bioactive lipid to be identified, and that S1P and its receptors might serve as new therapeutic targets against inflammatory diseases and lymphatic metastasis in tumors.

Human mesenchymal stem cell differentiation to the osteogenic or adipogenic lineage is regulated by AMP-activated protein kinase

AMP‐activated protein kinase (AMPK) is an energy‐sensing kinase that has recently been shown to regulate the differentiation of preadipocytes and osteoblasts. However, the role of AMPK in stem cell differentiation is largely unknown. Using in vitro culture models, the present study demonstrates that AMPK is a critical regulatory factor for osteogenic differentiation. We observed that expression and phosphorylation of AMPK were increased during osteogenesis in human adipose tissue‐derived mesenchymal stem cells (hAMSC). To elucidate the role of AMPK in osteogenic differentiation, we investigated the effect of AMPK inhibition or knockdown on mineralization of hAMSC. Compound C, an AMPK inhibitor, reduced mineralized matrix deposition and suppressed the expression of osteoblast‐specific genes, including alkaline phosphatase (ALP), runt‐related transcription factor 2 (RUNX2), and osteocalcin (OCN). Knockdown of AMPK by shRNA‐lentivirus infection also reduced osteogenesis. In addition, inhibition or knockdown of AMPK during osteogenesis inhibited ERK phosphorylation, which is required for osteogenesis. Interestingly, inhibition of AMPK induced adipogenic differentiation of hAMSC, even in osteogenic induction medium (OIM). These results provide a potential mechanism involving AMPK activation in osteogenic differentiation of hAMSC and suggest that commitment of hAMSC to osteogenic or adipogenic lineage is governed by activation or inhibition of AMPK, respectively. J. Cell. Physiol. 227: 1680–1687, 2012.

Comparative analysis of the secretory proteome of human adipose stromal vascular fraction cells during adipogenesis.

Adipogenesis is a complex process that is accompanied by a number of molecular events. In this study, a proteomic approach was adopted to identify secretory factors associated with adipogenesis. A label‐free shotgun proteomic strategy was implemented to analyze proteins secreted by human adipose stromal vascular fraction cells and differentiated adipocytes. A total of 474 proteins were finally identified and classified according to quantitative changes and statistical significances. Briefly, 177 proteins were significantly upregulated during adipogenesis (Class I), whereas 60 proteins were significantly downregulated (Class II). Changes in the expressions of several proteins were confirmed by quantitative RT‐PCR and immunoblotting. One obvious finding based on proteomic data was that the amounts of several extracellular modulators of Wnt and transforming growth factor‐β (TGF‐β) signaling changed during adipogenesis. The expressions of secreted frizzled‐related proteins, dickkopf‐related proteins, and latent TGF‐β‐binding proteins were found to be altered during adipogenesis, which suggests that they participate in the fine regulation of Wnt and TGF‐β signaling. This study provides useful tools and important clues regarding the roles of secretory factors during adipogenic differentiation, and provides information related to obesity and obesity‐related metabolic diseases.

Activation of AMP-activated Protein Kinase Is Essential for Lysophosphatidic Acid-induced Cell Migration in Ovarian Cancer Cells

Lysophosphatidic acid (LPA) is a bioactive phospholipid that affects various biological functions, such as cell proliferation, migration, and survival, through LPA receptors. Among them, the motility of cancer cells is an especially important activity for invasion and metastasis. Recently, AMP-activated protein kinase (AMPK), an energy-sensing kinase, was shown to regulate cell migration. However, the specific role of AMPK in cancer cell migration is unknown. The present study investigated whether LPA could induce AMPK activation and whether this process was associated with cell migration in ovarian cancer cells. We found that LPA led to a striking increase in AMPK phosphorylation in pathways involving the phospholipase C-β3 (PLC-β3) and calcium/calmodulin-dependent protein kinase kinase β (CaMKKβ) in SKOV3 ovarian cancer cells. siRNA-mediated knockdown of AMPKα1, PLC-β3, or (CaMKKβ) impaired the stimulatory effects of LPA on cell migration. Furthermore, we found that knockdown of AMPKα1 abrogated LPA-induced activation of the small GTPase RhoA and ezrin/radixin/moesin proteins regulating membrane dynamics as membrane-cytoskeleton linkers. In ovarian cancer xenograft models, knockdown of AMPK significantly decreased peritoneal dissemination and lung metastasis. Taken together, our results suggest that activation of AMPK by LPA induces cell migration through the signaling pathway to cytoskeletal dynamics and increases tumor metastasis in ovarian cancer.

Lysophosphatidic acid regulates blood glucose by stimulating myotube and adipocyte glucose uptake

Lysophosphatidic acid (LPA) is known to have diverse cellular effects, but although LPA is present in many biological fluids, including blood, its effects on glucose metabolism have not been elucidated. In this study, we investigated whether LPA stimulation is related to glucose regulation. LPA was found to enhance glucose uptake in a dose-dependent manner both in L6 GLUT4myc myotubes and 3T3-L1 adipocytes by triggering GLUT4 translocation to the plasma membrane. Moreover, the effect of LPA on glucose uptake was completely inhibited by pretreating both cells with LPA receptor antagonist Ki16425 and Gi inhibitor pertussis toxin. In addition, LPA increased the phosphorylation of AKT-1 with no effects on IRS-1, and LPA-induced glucose uptake was abrogated by pretreatment with the PI 3-kinase inhibitor LY294002. When low concentration of insulin and LPA were treated simultaneously, an additive effect on glucose uptake was observed in both cell types. In line with its cellular functions, LPA significantly lowered blood glucose levels in normal mice but did not affect insulin secretion. LPA also had a glucose-lowering effect in streptozotocin-treated type 1 diabetic mice. In combination, these results suggest that LPA is involved in the regulation of glucose homeostasis in muscle and adipose tissues.

DJ-1 promotes angiogenesis and osteogenesis by activating FGF receptor-1 signaling

Communication between osteoblasts and endothelial cells is essential for bone fracture repair, but the molecular identities of such communicating factors are not well defined. Here we identify DJ-1 as a novel mediator of the cross-talk between osteoblasts and endothelial cells through an unbiased screening of molecules secreted from human mesenchymal stem cells during osteogenesis. We show that DJ-1 stimulates the differentiation of human mesenchymal stem cells to osteoblasts and that DJ-1 induces angiogenesis in endothelial cells through activation of fibroblast growth factor receptor-1 signalling. In a rodent model of bone fracture repair, extracellular application of DJ-1 enhances bone regeneration in vivo by stimulating the formation of blood vessels and new bones. Both these effects are blocked by antagonizing fibroblast growth factor receptor-1 signalling. These findings uncover previously undefined extracellular roles of DJ-1 to promote angiogenesis and osteogenesis, suggesting DJ-1 may have therapeutic potential to stimulate bone regeneration.

Wedelolactone inhibits adipogenesis through the ERK pathway in human adipose tissue‐derived mesenchymal stem cells

Wedelolactone is an herbal medicine that is used to treat septic shock, hepatitis and venom poisoning. Although in differentiated and cancer cells, wedelolactone has been identified as anti‐inflammatory, growth inhibitory, and pro‐apoptotic, the effects of wedelolactone on stem cell differentiation remain largely unknown. Here, we report that wedelolactone inhibits the adipogenic differentiation of human adipose tissue‐derived mesenchymal stem cells (hAMSCs). Wedelolactone reduced the formation of lipid droplets and the expression of adipogenesis‐related proteins, such as CCAAT enhancer‐binding protein‐α (C/EBP‐α), peroxisome proliferator‐activated receptor‐γ (PPAR‐γ), lipoprotein lipase (LPL), and adipocyte fatty acid‐binding protein aP2 (aP2). Wedelolactone mediated this process by sustaining ERK activity. In addition, inhibition of ERK activity with PD98059 resulted in reversion of the wedelolactone‐mediated inhibition of adipogenic differentiation. Taken together, these results indicate that wedelolactone inhibits adipogenic differentiation through ERK pathway and suggest a novel inhibitory effect of wedelolactone on adipogenic differentiation in hAMSCs. J. Cell. Biochem. 113: 3436–3445, 2012.

Phospholipase C-η1 is activated by intracellular Ca2+ mobilization and enhances GPCRs/PLC/Ca2+ signaling

Phospholipase C-η1 (PLC-η1) is the most recently identified PLC isotype and is primarily expressed in nerve tissue. However, its functional role is unclear. In the present study, we report for the first time that PLC-η1 acts as a signal amplifier in G protein-coupled receptor (GPCR)-mediated PLC and Ca(2+) signaling. Short-hairpin RNA (shRNA)-mediated knockdown of endogenous PLC-η1 reduced lysophosphatidic acid (LPA)-, bradykinin (BK)-, and PACAP-induced PLC activity in mouse neuroblastoma Neuro2A (N2A) cells, indicating that PLC-η1 participates in GPCR-mediated PLC activation. Interestingly, ionomycin-induced PLC activity was significantly decreased by PLC-η1, but not PLC-η2, knockdown. In addition, we found that intracellular Ca(2+) source is enough for PLC-η1 activation. Furthermore, the IP(3) receptor inhibitor, 2-APB, inhibited LPA-induced PLC activity in control N2A cells, whereas this effect was not observed in PLC-η1 knockdown N2A cells, suggesting a pivotal role of intracellular Ca(2+) mobilization in PLC-η1 activation. Finally, we found that LPA-induced ERK1/2 phosphorylation and expression of the downstream target gene, krox-24, were significantly decreased by PLC-η1 knockdown, and these knockdown effects were abolished by 2-APB. Taken together, our results strongly suggest that PLC-η1 is activated via intracellular Ca(2+) mobilization from the ER, and therefore amplifies GPCR-mediated signaling.

Subtype-specific role of phospholipase C-β in bradykinin and LPA signaling through differential binding of different PDZ scaffold proteins

Among phospholipase C (PLC) isozymes (beta, gamma, delta, epsilon, zeta and eta), PLC-beta plays a key role in G-protein coupled receptor (GPCR)-mediated signaling. PLC-beta subtypes are often overlapped in their distribution, but have unique knock-out phenotypes in organism, suggesting that each subtype may have the different role even within the same type of cells. In this study, we examined the possibility of the differential coupling of each PLC-beta subtype to GPCRs, and explored the molecular mechanism underlying the specificity. Firstly, we found that PLC-beta1 and PLC-beta 3 are activated by bradykinin (BK) or lysophosphatidic acid (LPA), respectively. BK-triggered phosphoinositides hydrolysis and subsequent Ca(2+) mobilization were abolished specifically by PLC-beta1 silencing, whereas LPA-triggered events were by PLC-beta 3 silencing. Secondly, we showed the evidence that PDZ scaffold proteins is a key mediator for the selective coupling between PLC-beta subtype and GPCR. We found PAR-3 mediates physical interaction between PLC-beta1 and BK receptor, while NHERF2 does between PLC-beta 3 and LPA(2) receptor. Consistently, the silencing of PAR-3 or NHERF2 blunted PLC signaling induced by BK or LPA respectively. Taken together, these data suggest that each subtype of PLC-beta is selectively coupled to GPCR via PDZ scaffold proteins in given cell types and plays differential role in the signaling of various GPCRs.

An activator of the cAMP/PKA/CREB pathway promotes osteogenesis from human mesenchymal stem cells†

Mesenchymal stem cells (MSCs) are multipotent adult stem cells capable of differentiating along the osteoblast, adipocyte, and chondrocyte lineages. Regulation of MSCs differentiation may be a useful tool for regenerative medicine and cell‐based therapy. The discovery of small molecule that activates the osteogenic differentiation of MSCs could aid in the development of a new anabolic drug for osteoporosis treatment. We identified CW008, a derivative of pyrazole–pyridine, that stimulates osteoblast differentiation of human MSCs and increases bone formation in ovariectomized mice. CW008 promotes osteogenesis by activating cAMP/PKA/CREB signaling pathway and inhibiting leptin secretion. These results suggest that CW008 is an agonist of cAMP/PKA/CREB pathway in osteogenic differentiation and that application of CW008 may be useful for the treatment of bone‐related diseases and for the study of bone biology. J. Cell. Physiol. 228: 617–626, 2013.