Bone acidic glycoprotein-75 delineates the extracellular sites of future bone sialoprotein accumulation and apatite nucleation in osteoblastic cultures

Addition of an organophosphate source to UMR osteoblastic cultures activates a mineralization program in which BSP localizes to extracellular matrix sites where hydroxyapatite crystals are subsequently nucleated. This study identifies for the first time novel extracellular spherical structures, termed biomineralization foci (BMF), containing bone acidic glycoprotein-75 (BAG-75), bone sialoprotein (BSP), and alkaline phosphatase that are the exclusive sites of initial nucleation of hydroxyapatite crystals in the UMR model. Importantly, in the absence of added phosphate, UMR cultures after reaching confluency contain two size populations of morphologically identifiable BMF precursors enriched in BAG-75 (15-25 and 150-250 microm in diameter). The shape and size of the smaller population are similar to structures assembled in vitro through self-association of purified BAG-75 protein. After organophosphate addition, BSP accumulates within these BAG-75-containing BMF precursors, with hydroxyapatite crystal nucleation occurring subsequently. In summary, BAG-75 is the earliest detectable biomarker that accurately predicts the extracellular sites of de novo biomineralization in UMR cultures. We hypothesize that BAG-75 may perform a key structural role in the assembly of BMF precursors and the recruitment of other proteins such as alkaline phosphatase and BSP. Furthermore, we propose a hypothetical mechanism in which BAG-75 and BSP function actively in nucleation of apatite within BMF.     

hPTH-fragments (53-84) and (28-48) antagonize the stimulation of calcium release and repression of alkaline phosphatase activity by hPTH-(1-34) in vitro

Different C-terminal fragments of parathyroid hormone (PTH)-(1-84) in blood participate in the regulation of calcium homeostasis by PTH-(1-84), and an antagonizing effect for the large carboxyl-terminal parathyroid hormone (C-PTH)-fragment (7-84) on calcium release has been described in vivo and in vitro. In this study the smaller C-PTH-fragment (53-84) and mid-regional PTH fragment (28-48), which represent discrete areas of activity in the PTH-(7-84) molecule, were assayed for their effects on calcium release and alkaline phosphatase (ALP) activity in a chick bone organ culture system. Neither PTH-(28-48) nor PTH-(53-84) had any effect on calcium release into the medium and both fragments stimulated ALP activity in the bone tissue, suggesting that the cAMP/PKA signalling pathway was not affected by these fragments. However they suppressed the calcium release induced by PTH-(1-34) and attenuated the down regulation of ALP activity caused by PTH-(1-34), suggesting that the effect on the cAMP/PKA signalling pathway may be indirectly. In conclusion, the study shows that the PTH-fragments (53-84) and (28-48) antagonize the PTH-(1-34) induced effects on calcium release and inhibition of ALP activity in a chick bone organ culture system.     

The direct involvement of cAMP-dependent protein kinase in the regulation of collagen synthesis by parathyroid hormone (PTH) and PTH-related peptide in osteoblast-like osteosarcoma cells (UMR-106).

The present study was performed to characterize the direct involvement of cAMP-dependent protein kinase (PKA) in the regulation of collagen synthesis by parathyroid hormone (PTH) and PTH-related peptide (PTHrP) in osteoblastic osteosarcoma cells, UMR-106. Sp-cAMPS (10(-4)M), a direct activator of PKA, as well as dibutyryl cAMP (dbcAMP, 10(-4)M) significantly inhibited collagen synthesis. Human (h) PTH-(1-34) (10(-7)M) and hPTHrP (10(-7) M) inhibited collagen synthesis to the same degree. Although Rp-cAMPS, which acted directly as an antagonist in the activation of PKA, did not affect collagen synthesis by itself, it significantly antagonized dbcAMP- and Sp-cAMPS-induced inhibition of collagen synthesis. Moreover, Rp-cAMPS antagonized PTH- and PTHrP-induced inhibition of collagen synthesis to the same degree. The present study first indicated that the activation of PKA was directly linked to the regulation of collagen synthesis by PTH in osteoblast and that PTHrP had the same effect on collagen synthesis presumably through the same mechanism as PTH.     

Analysis of the requirements for parathyroid hormone action in renal membranes with the use of inhibiting analogues.

Two synthetic analogues of bovine parathyroid hormone (PTH) with NH2-terminal modifications, PTH-(3-34) and [desamino-Ala-1]PTH-(1-34), were found to lack agonist activity but to demonstrate antagonist properties when tested in the rat renal cortical adenylyl cyclase assay in vitro against the native hormone or against PTH-(1-34), the active synthetic NH2-terminal tetratriacontapeptide. The inhibition exhibited by these analogues was proportional in degree to the dose of inhibitor, abolished by oxidation of the analogue, reversible by addition of an excess of active hormone, and specific for parathyroid hormone-stimulated renal adenylyl cyclase. No inhibition of basal or sodium fluoride-stimulated renal adenylyl cyclase could be demonstrated. Two other synthetic bovine analogues, PTH-(13-34) and PTH-(1-26), were devoid of agonist and antagonist properties. The over-all results suggest that the requirements for receptor binding of parathyroid hormone are rather broad. Conformational factors or binding interactions involving specific residues, or both seem to require the entire sequence from residue 3 to residue 27 for receptor binding to occur. A dichotomy between receptor binding and adenylyl cyclase activation was demonstrated only by alterations or deletions involving the first 2 NH2-terminal residues of the hormone and emphasizes the importance of these residues in eliciting the biological activity of parathyroid hormone. The two antagonists, [desamino-Ala-1]PTH-(1-34) and PTH-(3-34), should be useful in further analysis of the initial steps in hormone action.     

The role of protein kinase C-delta in PTH stimulation of IGF-binding protein-5 mRNA in UMR-106-01 cells

We have investigated the role of protein kinase C (PKC) signal transduction pathways in parathyroid hormone (PTH) regulation of insulin-like growth factor-binding protein-5 (IGFBP-5) gene expression in the rat osteoblast-like cell line UMR-106-01. Involvement of the PKC pathway was determined by the findings that bisindolylmaleimide I inhibited 40% of the PTH effect, and 1 microM bovine PTH-(3-34) stimulated a 10-fold induction of IGFBP-5 mRNA. PTH-(1-34) and PTH-(3-34) (100 nM) both stimulated PKC-delta translocation from the membrane to the nuclear fraction. Rottlerin, a PKC-delta-specific inhibitor, and a dominant negative mutant of PKC-delta were both able to significantly inhibit PTH-(1-34) and PTH-(3-34) induction of IGFBP-5 mRNA, suggesting a stimulatory role for PKC-delta in the effects of PTH. Phorbol 12-myristate 13-acetate (PMA) stimulated PKC-alpha translocation from the cytosol to the membrane and inhibited approximately 50% of the PTH-(1-34), forskolin, and 8-bromoadenosine 3',5'-cyclic monophosphate-stimulated IGFBP-5 mRNA levels, suggesting that PKC-alpha negatively regulates protein kinase A (PKA)-mediated induction of IGFBP-5 mRNA. These results suggest that the induction of IGFBP-5 by PTH is both PKA and PKC dependent and PKC-delta is the primary mediator of the effects of PTH via the PKC pathway.     

Rho-ROCK signaling differentially regulates chondrocyte spreading on fibronectin and bone sialoprotein

The mammalian growth plate is a dynamic structure rich in extracellular matrix (ECM). Interactions of growth plate chondrocytes with ECM proteins regulate cell behavior. In this study, we compared chondrocyte adhesion and spreading dynamics on fibronectin (FN) and bone sialoprotein (BSP). Chondrocyte adhesion and spreading were also compared with fibroblasts to analyze potential cell-type-specific effects. Chondrocyte adhesion to BSP is independent of posttranslational modifications but is dependent on the RGD sequence in BSP. Whereas chondrocytes and fibroblasts adhered at similar levels on FN and BSP, cells displayed more actin-dependent spread on FN despite a 16x molar excess of BSP adsorbed to plastic. To identify intracellular mediators responsible for this difference in spreading, we investigated focal adhesion kinase (FAK)-Src and Rho-Rho kinase (ROCK) signaling. Although activated FAK localized to the vertices of adhered chondrocytes, levels of FAK activation did not correlate with the extent of spreading. Furthermore, Src inhibition reduced chondrocyte spreading on both FN and BSP, suggesting that FAK-Src signaling is not responsible for less cell spreading on BSP. In contrast, inhibition of Rho and ROCK in chondrocytes increased cell spreading on BSP and membrane protrusiveness on FN but did not affect cell adhesion. In fibroblasts, Rho inhibition increased fibroblast spreading on BSP while ROCK inhibition changed membrane protrusiveness of FN and BSP. In summary, we identify a novel role for Rho-ROCK signaling in regulating chondrocyte spreading and demonstrate both cell- and matrix molecule-specific mechanisms controlling cell spreading.     

Plasminogen-dependent activation of latent transforming growth factor beta (TGFβ) by growing cultures of osteoblast-like cells

Osteoblasts secrete transforming growth factor beta (TGFβ) as a biologically inert, latent complex that must be dissociated before the growth factor can exert its effects. We have examined the production and proteolytic activation of latent TGFβ (LTGFβ) by clonal UMR 106-01 rat osteosarcoma cells and neonatal mouse calvarial (MC) osteoblast-like cells in vitro. Synthetic bPTH-(1–34) increased the activity of tissue-type (tPA) and urokinase-type (uPA) plasminogen activators (PA) in cell lysates (CL) of UMR 106-01 cells. The concentration of active TGFβ in serum-free CM from cultures treated with bPTH-(1–34) and plasminogen was significantly greater than in CM from untreated controls and cultures treated with either bPTH-(1–34) or plasminogen alone. This effect occurred at concentrations of PTH-(1–34) that increased PA activity and was prevented by aprotinin, an inhibitor of plasmin activity. Treatment with bPTH-(1–34) had no effect on the concentration of TGFβ in acid-activated samples of CM. Functional consequences of proteolytically activated TGFβ was examined in primary cultures of neonatal MC osteoblast-like cells. Human platelet TGFβ1 caused a dose-dependent increase in the migration of these cells in an in vitro wound healing assay. Cell migration was also stimulated in cultures treated with bPTH-(1–34) and plasminogen together. This effect was blocked by an anti-TGFβ1 antibody. The results of these studies demonstrate that (1) LTGFβ secreted by osteoblasts in vitro is activated under conditions where the plasmin activity in the cultures is increased, and (2) the TGFβ generated by plasmin-mediated proteolysis is biologically active. We suggest that the local concentration of TGFβ in bone may be controlled by the osteoblast-associated plasminogen activator/plasmin system. Furthermore, since several calciotropic factors influence osteoblast PA activity, this system may have an important role in mediating their anabolic and/or catabolic effects. © 1993 Wiley-Liss, Inc.     

Second messenger signaling in the regulation of cytosolic pH and DNA synthesis by parathyroid hormone (PTH) and PTH-related peptide in osteoblastic osteosarcoma cells: role of Na+/H+ exchange.

The present study was performed to investigate the regulation of cytosolic pH (pHi) and DNA synthesis by parathyroid hormone(PTH) and PTH-related peptide (PTHrP) in osteoblasts, using osteoblastic osteosarcoma cells, UMR-106 which possessed PTH-responsive dual signal transduction systems (cAMP-dependent protein kinase (PKA) and calcium/protein kinase C [Ca/PKC]) and amiloride-inhibitable Na+/H+ exchange system. Both human (h)PTH-(1-34) and hPTHrP-(1-34) caused a progressive decrease in pHi and the inhibition of [3H]thymidine incorporation (TdR) to the same degree in a dose-dependent manner with a minimal effective dose of 10(-10) M. Dibutyryl cAMP (10(-4) M and Sp-cAMPS (10(-4) M), a direct stimulator of PKA also caused a progressive decrease in pHi, and calcium ionophores (A23187 and ionomycin, 10(-6) M) caused a transient decrease in pHi. Pretreatment with amiloride (0.3 mM) mostly blocked dbcAMP- and Sp-cAMPS-induced decrease in pHi but did not affect calcium ionophore-induced decrease in pHi. In the presence of amiloride, PTH and PTHrP caused a transient decrease in pHi, which was similar to the pattern of calcium ionophore-induced change in pHi. Amiloride did not affect the inhibition of TdR by PTH or PTHrP as well as that by cAMP analogues or calcium ionophores. The present study indicated that PTH and PTHrP caused cytosolic acidification through PKA-inhibited Na+/H+ exchange and increased cytosolic calcium-induced pathway and that the regulation of DNA synthesis by PTH and PTHrP was not via Na+/H+ exchange system.     

Parathyroid hormone-induced lipolysis in human adipose tissue

Relative lipolytic activity of human parathyroid hormone-(1-34) (hPTH-(1-34], hPTH-(3-34), desamino-Ser1-hPTH-(1-34), and rat PTH-(1-34) was compared in human subcutaneous adipose tissues in vitro. Human PTH-(1-34), rat PTH-(1-34), and desamino-Ser1-hPTH-(1-34) stimulated in vitro lipolysis significantly above basal level at the concentration of 10(-6) M. Average increments of lipolytic rate were 2.39, 1.82, and 0.87 mumol/g per 2 hr, respectively, being significantly different among the three groups. On the other hand, hPTH-(3-34)-induced lipolytic rate was 0.83 +/- 0.18 mumol/g per 2 hr, not significantly different from the basal level (0.71 +/- 0.20 mumol/g per 2 hr). The effect of hPTH-(3-34) on glycerol release stimulated by hPTH-(1-34), isoproterenol, or forskolin was subsequently investigated. Human PTH-(3-34) produced a dose-dependent inhibition of hPTH-(1-34)-stimulated lipolysis. In contrast, isoproterenol- and forskolin-induced lipolytic rates were not influenced by hPTH-(3-34). The effect of propranolol on hPTH-(1-34)- or isoproterenol-induced lipolysis was also studied. Propranolol dose-dependently inhibited isoproterenol-induced lipolysis but had no effect on lipolysis stimulated by hPTH-(1-34). These results suggest that the amino acids at positions 1 (serine) and 2 (valine) of PTH are critical for the stimulation of lipolysis in human adipose tissue. Human PTH-(1-34) causes lipolysis after binding to receptors distinct from beta-adrenergic receptors of fat cells and possibly hPTH-(3-34) inhibits hPTH-(1-34)-stimulated lipolysis by competing at the level of PTH receptor.     

Expression of adenylate cyclase-coupled osseous parathyroid hormone and parathyroid hormone-like peptide receptors in Xenopus oocytes.

Functional parathyroid hormone (PTH) and PTH-like peptide receptors were expressed in Xenopus laevis oocytes after injection of poly(A)+ RNA isolated from the rat osteogenic sarcoma cell line, UMR 106. Increases in cAMP were seen in individual oocytes in response to added bovine (b) PTH-(1-34) (10(-6) M), human (h) PLP-(1-34) (hPLP-(1-34), 10(-6) M), isoproterenol (10(-4) M), and forskolin (10(-4) M). Although both intracellular and extracellular cAMP levels were stimulated approximately 1.5-2-fold by these agonists, intracellular concentrations of cAMP were substantially higher than extracellular concentrations. Peak increases with bPTH-(1-34) occurred after a 30-min incubation with the hormone 48 h after oocyte injection. bPTH-(1-34) caused a concentration-dependent augmentation of cAMP in injected oocytes, and the in vitro antagonist hPLP-(3-34) produced dose-dependent inhibition of both bPTH-(1-34)- and hPLP-(1-34)-stimulated cAMP accumulation. Specific binding of PTH to oocyte membranes was also demonstrated 48 h after oocyte injection with UMR 106 cell mRNA. Following size fractionation of isolated UMR 106 poly(A)+ RNA by sucrose density gradients, mRNA directing the expression of both PTH- and PLP-stimulated cAMP in oocytes appeared in the 3.5-4.9-kilobase fraction. These results demonstrate that adenylate cyclase-coupled osseous PTH and PLP receptors can be expressed after injection of naturally occurring mRNA into Xenopus oocytes, that PTH- and PLP-stimulated increases in cAMP concentrations can be detected in individual oocytes injected with bone cell-derived mRNA, that PTH and PLP appear to cross-react at a common receptor after injection of UMR 106 cell mRNA into oocytes, and that size selection of mRNA encoding the PTH and PLP receptors can be achieved by density gradient centrifugation. These studies, therefore, indicate the potential usefulness of the Xenopus oocyte system in expression cloning of PTH and PLP receptor cDNAs and illustrate the feasibility of employing this system to examine the biology of PTH and PLP receptors.     

Association of specific proteolytic processing of bone sialoprotein and bone acidic glycoprotein-75 with mineralization within biomineralization foci

Mineral crystal nucleation in UMR 106-01 osteoblastic cultures occurs within 15-25-microm extracellular vesicle-containing biomineralization foci (BMF) structures. We show here that BAG-75 and BSP, biomarkers for these foci, are specifically enriched in laser capture microscope-isolated mineralized BMF as compared with the total cell layer. Unexpectedly, fragments of each protein (45-50 kDa in apparent size) were also enriched within captured BMF. When a series of inhibitors against different protease classes were screened, serine protease inhibitor 4-(2-aminoethyl)benzenesulfonylfluoride HCl (AEBSF) was the only one that completely blocked mineral nucleation within BMF in UMR cultures. AEBSF appeared to act on an osteoblast-derived protease at a late differentiation stage in this culture model just prior to mineral deposition. Similarly, mineralization of bone nodules in primary mouse calvarial osteoblastic cultures was completely blocked by AEBSF. Cleavage of BAG-75 and BSP was also inhibited at the minimum dosage of AEBSF sufficient to completely block mineralization of BMF. Two-dimensional SDS-PAGE comparisons of AEBSF-treated and untreated UMR cultures showed that fragmentation/activation of a limited number of other mineralization-related proteins was also blocked. Taken together, our results indicate for the first time that cleavage of BAG-75 and BSP by an AEBSF-sensitive, osteoblast-derived serine protease is associated with mineral crystal nucleation in BMF and suggest that such proteolytic events are a permissive step for mineralization to proceed.     

Inhibition of fibronectin matrix assembly by the heparin-binding domain of vitronectin.

The deposition of fibronectin into the extracellular matrix is an integrin-dependent, multistep process that is tightly regulated in order to ensure controlled matrix deposition. Reduced fibronectin deposition has been associated with altered embryonic development, tumor cell invasion, and abnormal wound repair. In one of the initial steps of fibronectin matrix assembly, the amino-terminal region of fibronectin binds to cell surface receptors, termed matrix assembly sites. The present study was undertaken to investigate the role of extracellular signals in the regulation of fibronectin deposition. Our data indicate that the interaction of cells with the extracellular glycoprotein, vitronectin, specifically inhibits matrix assembly site expression and fibronectin deposition. The region of vitronectin responsible for the inhibition of fibronectin deposition was localized to the heparin-binding domain. Vitronectin's heparin-binding domain inhibited both beta(1) and non-beta(1) integrin-dependent matrix assembly site expression and could be overcome by treatment of cells with lysophosphatidic acid, an agent that promotes actin polymerization. The interaction of cells with the heparin-binding domain of vitronectin resulted in changes in actin microfilament organization and the subcellular distribution of the actin-associated proteins alpha-actinin and talin. These data suggest a mechanism whereby the heparin-binding domain of vitronectin regulates the deposition of fibronectin into the extracellular matrix through alterations in the organization of the actin cytoskeleton.     

The (1-14) fragment of parathyroid hormone (PTH) activates intact and amino-terminally truncated PTH-1 receptors.

Recent mutagenesis and cross-linking studies suggest that residues in the carboxyl-terminal portion of PTH(1-34) interact with the amino-terminal extracellular domain of the receptor and thereby contribute strongly to binding energy; and that residues in the amino-terminal portion of the ligand interact with the receptor region containing the transmembrane helices and extracellular loops and thereby induce second messenger signaling. We investigated the latter component of this hypothesis using the short amino-terminal fragment PTH(1-14) and a truncated rat PTH-1 receptor (r delta Nt) that lacks most of the amino-terminal extracellular domain. The binding of PTH(1-14) to LLC-PK1 or COS-7 cells transfected with the intact PTH-1 receptor was too weak to detect; however, PTH(1-14) dose-dependently stimulated cAMP formation in these cells over the dose range of 1-100 microM. PTH(1-14) also stimulated cAMP formation in COS-7 cells transiently transfected with r delta Nt, and its potency with this receptor was nearly equal to that seen with the intact receptor. In contrast, PTH(1-34) was approximately 100-fold weaker in potency with r delta Nt than it was with the intact receptor. Alanine scanning of PTH(1-14) revealed that for both the intact and truncated receptors, the 1-9 segment of PTH forms a critical receptor activation domain. Taken together, these results demonstrate that the amino-terminal portion of PTH(1-34) interacts with the juxtamembrane regions of the PTH-1 receptor and that these interactions are sufficient for initiating signal transduction.     

Parathyroid hormone receptor trafficking contributes to the activation of extracellular signal-regulated kinases but is not required for regulation of cAMP signaling

Agonist-mediated activation of the type 1 parathyroid hormone receptor (PTH1R) results in several signaling events and receptor endocytosis. It is well documented that arrestins contribute to desensitization of both G(s)- and G(q)-mediated signaling and mediate PTH1R internalization. However, whether PTH1R trafficking directly contributes to signaling remains unclear. To address this question, we investigated the role of PTH1R trafficking in cAMP signaling and activation of extracellular signal-regulated kinases ERK1/2 in HEK-293 cells. Dominant negative forms of dynamin (K44A-dynamin) and beta-arrestin1 (beta-arrestin1-(319-418)) abrogated PTH1R internalization but had no effect on cAMP signaling; neither acute cAMP production by PTH nor desensitization and resensitization of cAMP signaling were affected. Therefore, PTH1R trafficking is not necessary for regulation of cAMP signaling. PTH-(1-34) induced rapid and robust activation of ERK1/2. A PTHrP-based analog ([p-benzoylphenylalanine1, Ile5,Arg(11,13),Tyr36]PTHrP-(1-36)NH2), which selectively activates the G(s)/cAMP pathway without inducing PTH1R endocytosis, failed to stimulate ERK1/2 activity. Inhibition of PTH1R endocytosis by K44A-dynamin dampened ERK1/2 activation in response to PTH-(1-34) by 69%. Incubation with the epidermal growth factor receptor inhibitor AG1478 reduced ERK1/2 phosphorylation further. In addition, ERK1/2 phosphorylation occurred following internalization of a PTH1R mutant induced by PTH-(7-34) in the absence of G protein signaling. Collectively, these data indicate that PTH1R trafficking and G(q) (but not G(s)) signaling independently contribute to ERK1/2 activation, predominantly via transactivation of the epidermal growth factor receptor.     

The hydrophobic residues phenylalanine 184 and leucine 187 in the type-1 parathyroid hormone (PTH) receptor functionally interact with the amino-terminal portion of PTH-(1-34).

Recent mutagenesis and cross-linking studies suggest that three regions of the PTH-1 receptor play important roles in ligand interaction: (i) the extreme NH(2)-terminal region, (ii) the juxtamembrane base of the amino-terminal extracellular domain, and (iii) the third extracellular loop. In this report, we analyzed the second of these segments in the rat PTH-1 receptor (residues 182-190) and its role in functional interaction with short PTH fragment analogs. Twenty-eight singly substituted PTH-1 receptors were transiently transfected into COS-7 cells and shown to be fully expressed by surface antibody binding analysis. Alanine-scanning analysis identified Phe(184), Arg(186), Leu(187), and Ile(190) as important determinants of maximum binding of (125)I-labeled bovine PTH-(1-34) and (125)I-labeled bovine PTH-(3-34) and determinants of responsiveness to the NH(2)-terminal analog, PTH-(1-14) in cAMP stimulation assays. Alanine mutations at these four sites augmented the ability of the COOH-terminal peptide [Glu(22), Trp(23)]PTHrP-(15-36) to inhibit the cAMP response induced by PTH-(1-34). At Phe(184) and Leu(187), hydrophobic substitutions (e.g. Ile, Met, or Leu) preserved PTH-(1-34)-mediated cAMP signaling potency, whereas hydrophilic substitutions (e.g. Asp, Glu, Lys, or Arg) weakened this response by 20-fold or more, as compared with the unsubstituted receptor's response. The results suggest that hydrophobicity at positions occupied by Phe(184) and Leu(187) in the PTH-1 receptor plays an important role in determining functional interaction with the 3-14 portion of PTH.     

Deposition of tropoelastin into the extracellular matrix requires a competent elastic fiber scaffold but not live cells.

The initial steps of elastic fiber assembly were investigated using an in vitro assembly model in which purified recombinant tropoelastin (rbTE) was added to cultures of live or dead cells. The ability of tropoelastin to associate with preexisting elastic fibers or microfibrils in the extracellular matrix was then assessed by immunofluorescence microscopy using species-specific tropoelastin antibodies. Results show that rbTE can associate with elastic fiber components in the absence of live cells through a process that does not depend on crosslink formation. Time course studies show a transformation of the deposited protein from an initial globular appearance early in culture to a more fibrous structure as the matrix matures. Deposition required the C-terminal region of tropoelastin and correlated with the presence of preexisting elastic fibers or microfibrils. Association of exogenously added tropoelastin to the cellular extracellular matrix was inhibited by the addition of heparan sulfate but not chondroitin sulfate sugars. Together, these results suggest that the matrix elaborated by the cell is sufficient for the initial deposition of tropoelastin in the extracellular space and that elastin assembly may be influenced by the composition of sulfated proteoglycans in the matrix.     

Protein kinase A activity and anchoring are required for ovarian cancer cell migration and invasion

Epithelial ovarian cancer (EOC) is the deadliest of the gynecological malignancies, due in part to its clinically occult metastasis. Therefore, understanding the mechanisms governing EOC dissemination and invasion may provide new targets for antimetastatic therapies or new methods for detection of metastatic disease. The cAMP-dependent protein kinase (PKA) is often dysregulated in EOC. Furthermore, PKA activity and subcellular localization by A-kinase anchoring proteins (AKAPs) are important regulators of cytoskeletal dynamics and cell migration. Thus, we sought to study the role of PKA and AKAP function in both EOC cell migration and invasion. Using the plasma membrane-directed PKA biosensor, pmAKAR3, and an improved migration/invasion assay, we show that PKA is activated at the leading edge of migrating SKOV-3 EOC cells, and that inhibition of PKA activity blocks SKOV-3 cell migration. Furthermore, we show that while the PKA activity within the leading edge of these cells is mediated by anchoring of type-II regulatory PKA subunits (RII), inhibition of anchoring of either RI or RII PKA subunits blocks cell migration. Importantly, we also show--for the first time--that PKA activity is up-regulated at the leading edge of SKOV-3 cells during invasion of a three-dimensional extracellular matrix and, as seen for migration, inhibition of either PKA activity or AKAP-mediated PKA anchoring blocks matrix invasion. These data are the first to demonstrate that the invasion of extracellular matrix by cancer cells elicits activation of PKA within the invasive leading edge and that both PKA activity and anchoring are required for matrix invasion. These observations suggest a role for PKA and AKAP activity in EOC metastasis.