Rho and Rho kinase are involved in parathyroid hormone-stimulated protein kinase C alpha translocation and IL-6 promoter activity in osteoblastic cells

The role of small G-proteins in PTH-stimulated PKC translocation and IL-6 promoter expression in UMR-106 cells was determined. The effects of PTH(1-34) and PTH(3-34) in stimulating PKCalpha translocation and IL-6 were inhibited by agents that interfere with the activity of small G-proteins of the Rho family and with the downstream kinase Rho kinase.

INTRODUCTION

Activation of protein kinase C (PKC) is a signaling mechanism by which parathyroid hormone (PTH) modulates interleukin-6 (IL-6) in osteoblasts, leading to osteoclastogenesis and bone resorption. PKCalpha and PKCbetaI are translocated after treatment with PTH in UMR-106 osteoblastic cells; however, the pathway leading to PKC isozyme translocation is not established. Diacylglycerol (DAG) generation from phospholipase D (PLD) is one pathway of PKC activation, and PTH-mediated PLD activity is dependent on small G-proteins of the Rho family. This study investigated whether Rho proteins modulate the PKCalpha translocation and IL-6 promoter activity stimulated by PTH in UMR-106 cells.

MATERIALS AND METHODS

UMR-106 cells were treated with PTH(1-34) or PTH(3-34). PKC translocation was determined by immunofluorescence, Rho A activation by Rhotekin assay and by translocation assessed by Western blotting in membrane and cytosol fractions, and IL-6 promoter expression by luciferase assay.

RESULTS AND CONCLUSIONS

Inhibition of Rho proteins with Clostridium difficile toxin B or inhibition of Rho prenylation with GGTI attenuated PTH(1-34)- and PTH(3-34)-stimulated translocation of endogenous PKCalpha and IL-6 promoter activity. Expression of a constitutively active RhoA (RhoA63L) mimicked the effect of PTH(1-34) or PTH(3-34) to promote membrane localization of PKCalpha, whereas cells expressing a dominant negative RhoA (RhoA19N) did not respond to PTH(1-34) or PTH(3-34). The Rho kinase inhibitor Y27632 attenuated PTH(1-34)- and PTH(3-34)-stimulated PKCalpha translocation and IL-6 promoter activation. Rho seemed to be acting at a step before production of diacylglycerol (DAG), because the stimulation of PKCalpha translocation by the DAG mimetic phorbol 12,13 dibutyrate (PDBu) was unaffected by C. difficile toxin B or Y27632. These results indicate that Rho proteins are an important component of PTH signaling in osteoblastic cells and provide further demonstration of convergence between PKC and small G-protein signaling pathways.

Role of protein kinase A, phospholipase C and phospholipase D in parathyroid hormone receptor regulation of protein kinase Cα and interleukin-6 in UMR-106 osteoblastic cells

Parathyroid hormone (PTH) stimulates both bone formation and resorption by activating diverse osteoblast signalling pathways. Upstream signalling for PTH stimulation of protein kinase C-alpha (PKCalpha) membrane translocation and subsequent expression of the pro-resorptive cytokine interleukin-6 (IL-6) was investigated in UMR-106 osteoblastic cells. PTH 1-34, PTH 3-34, PTHrP and PTH 1-31 stimulated PKCalpha translocation and IL-6 promoter activity. Pharmacologic intervention at the adenylyl cyclase (AC) pathway (forskolin, IBMX, PKI) failed to alter PTH 1-34- or PTH 3-34-stimulated PKCalpha translocation. The phosphoinositol-phospholipase C (PI-PLC) antagonist U73122 slightly decreased PTH 1-34-stimulated PKCalpha translocation; however, the control analogue U73343 acted similarly. Propranolol, an inhibitor of phosphatidic acid (PA) phosphohydrolase, decreased diacylglycerol (DAG) formation and attenuated PTH 1-34- and PTH 3-34-stimulated PKCalpha translocation and IL-6 promoter activity, suggesting a phospholipase D (PLD)-dependent mechanism. This is the first demonstration that PLD-mediated signalling leads to both PKC-alpha translocation and IL-6 promoter activation in osteoblastic cells.

Regulation of parathyroid hormone-stimulated phospholipase D in UMR-106 cells by calcium, MAP kinase, and small G proteins

Signaling intermediates for PTH and phorbol activation of PLD in UMR-106 cells were determined. Calcium was required, and the effects of PTH, phorbol, and calcium were dependent on p42/44 MAP kinase and small G proteins, specifically RhoA, acting through Rho kinase.

INTRODUCTION

Phospholipase D (PLD) plays a key signaling role in numerous cellular processes. PLD-stimulated hydrolysis of phosphatidylcholine (PC) generates phosphatidic acid, a source of diacylglycerol (DAG). We previously reported that parathyroid hormone (PTH) stimulates PLD activity in UMR-106 osteoblastic cells by a protein kinase C (PKC)-independent mechanism. The current study investigated the roles of calcium, MAP kinase, and small G proteins in PTH- and phorbol-12,13-dibutyrate (PDBu)-stimulated transphosphatidylation of ethanol, a reaction catalyzed by PLD.

METHODS

UMR-106 cells were labeled with 3H-palmitic and treated in the presence of ethanol. Phosphatidylethanol was separated by thin-layer chromatography and detected by autoradiography, and the bands were scraped and counted. Statistical significance of the responses from three to nine replicates was determined by ANOVA and Tukey's post-test.

RESULTS AND CONCLUSIONS

PTH and PDBu effects were attenuated by EGTA, BAPTA, nifedipine, and dantrolene, whereas ionomycin or 2X calcium increased basal PLD activity. PTH activated p42/p44 MAP kinase, and the effects of PTH, PDBu, and ionomycin on PLD, but not on calcium influx, were prevented by the MEK inhibitors PD98059 and U0126. Small G proteins were shown to be involved in the effects of PTH, PDBu, and ionomycin on PLD. Inhibition of ARF by brefeldin prevented the PLD activation by all three agonists. A nonselective Rho/Rac/cdc-42 inhibitor, Clostridium difficile toxin B, also inhibited the effects of all three agonists on PLD. More selective inhibition of RhoA with a dominant negative RhoA construct or by inhibiting geranylgeranyltransferase I antagonized the effects of PTH, PDBu, and ionomycin, as did inhibiting the downstream kinase, Rho kinase. The current results reveal the importance of calcium, MAP kinase, and small G proteins in PTH and PDBu stimulation of PLD activity in UMR-106 cells.

Parathyroid hormone stimulates translocation of protein kinase C isozymes in UMR-106 osteoblastic osteosarcoma cells

Studies with antagonists have provided evidence that protein kinase C (PKC) is involved in several of the actions of parathyroid hormone (PTH) on bone. PTH increases total PKC activity in bone and bone cells. The current studies investigated whether PTH can activate specific PKC isozymes, as demonstrated by translocation of these isozymes from cytosolic to membrane fractions. The isozymes selected for study, alpha, betaI, delta, epsilon, and zeta, were shown previously by us to be present in normal osteoblasts and several osteosarcoma-derived osteoblastic cells. UMR-106 cells, a widely used osteoblastic cell line, were selected for the current study. PKC isozymes in whole cell lysates and cell fractions were visualized by western blotting; isozyme distribution was also visualized by immunofluorescence. The total amounts of the isozymes and their relative distribution between membrane and cytosolic fractions in untreated cells were stable over a range of passages (5-20 from initial plating). In untreated cells, the concentrations of PKC alpha, betaI, and zeta were higher in the cytosol, and PKC delta and epsilon were higher in the membrane fraction. Treatment with 1 or 10 nmol/L PTH for 1 or 5 min stimulated translocation of PKC alpha and betaI, with variable effects on the other isozymes. Treatment with phorbol-12,13-dibutyrate (PDBu), 1 micromol/L for 5 min, elicited similar effects to those of PTH on PKC alpha and betaI. Treatment with PDBu for 48 h resulted in a downregulation of PKC alpha, whereas a 48 h treatment with PTH did not cause downregulation. The results indicate that PTH can affect specific PKC isozymes, providing a mechanism for differential regulation of cellular actions through this pathway.

Involvement of PKC-beta in PTH, TNF-alpha, and IL-1 beta effects on IL-6 promoter in osteoblastic cells and on PTH-stimulated bone resorption

Protein kinase C (PKC) has been shown to be activated by parathyroid hormone (PTH) in osteoblasts. Prior evidence suggests that this activation mediates responses leading to bone resorption, including production of the osteoclastogenic cytokine interleukin-6 (IL-6). However, the importance of specific PKC isozymes in this process has not been investigated. A selective antagonist of PKC-beta, LY379196, was used to determine the role of the PKC-beta isozyme in the expression of IL-6 in UMR-106 rat osteoblastic cells and in bone resorption in fetal rat limb bone organ cultures. PTH, tumor necrosis factor-alpha (TNF-alpha), and interleukin-1 beta (IL-1 beta) induced translocation of PKC-alpha and -beta(I) to the plasma membrane in UMR-106 cells within 5 min. The stimulation of PKC-beta(I) translocation by PTH, TNF-alpha or IL-1 beta was inhibited by LY379196. In contrast, LY379196 did not affect PTH, TNF-alpha-, or IL-1 beta-stimulated translocation of PKC-alpha. PTH, TNF-alpha, and IL-1 beta increased luciferase expression in UMR-106 cells transiently transfected with a -224/+11 bp IL-6 promoter-driven reporter construct. The IL-6 responses were also attenuated by treatment with LY379196. Furthermore, LY379196 inhibited bone resorption elicited by PTH in fetal rat bone organ cultures. These results indicate that PKC-beta(I) is a component of the signaling pathway that mediates PTH-, TNF-alpha-, and IL-1 beta-stimulated IL-6 expression and PTH-stimulated bone resorption.

Phosphatidylcholine-specific phospholipase C inhibitor, tricyclodecan-9-yl xanthogenate (D609), increases phospholipase D-mediated phosphatidylcholine hydrolysis in UMR-106 osteoblastic osteosarcoma cells

Our previous studies have shown that parathyroid hormone (PTH) stimulates phosphatidylcholine (PC) hydrolysis by phospholipase D (PLD) and transphosphatidylation in UMR-106 osteoblastic cells. To determine whether phospholipase C (PLC) is also involved in the PTH-mediated PC hydrolysis, we used the inhibitor, tricyclodecan-9-yl xanthogenate (D609), a putatively selective antagonist of this pathway. Consistent with this proposed mechanism, D609 decreased (3)H-phosphocholine in extracts from UMR-106 cells prelabeled with (3)H-choline. Unexpectedly, D609 enhanced PC hydrolysis and transphosphatidylation, suggesting that either there was a compensatory increase in PLD activity when PLC was inhibited, or that D609 directly increased PLD activity. The D609-stimulated increase in PC hydrolysis was rapid, being seen as early as 2 min. The effect of D609 was temperature-sensitive, consistent with an enzymatic mechanism. The D609-stimulated increase in PC hydrolysis was PKC-independent, based upon the lack of effect of down-regulation of PKC by phorbol 12,13-dibutyrate on the response. The studies reveal a novel action of this inhibitor on signaling in osteoblastic cells which might influence downstream responses.

Modulation of [3H]Dopamine release from rat nucleus accumbens by neuropeptide Y may involve a sigma1-like receptor

Sigma receptors are located in limbic areas, including the nucleus accumbens, where increased dopamine levels have been linked to psychosis and reinforcement. Neuropeptide Y (NPY) has been named as a possible endogenous ligand for a subpopulation of sigma receptors on the basis of its ability to compete for sigma receptor binding. Using a superfusion system, we found that NPY enhanced N-methyl-D-asparate-stimulated [3H]dopamine release in rat nucleus accumbens, whereas the prototypical sigma agonist (+)pentazocine inhibited release. However, four sigma antagonists, one of which is sigma1 selective, as well as a Y receptor antagonist, all reversed the enhancement by NPY and the inhibition by (+)pentazocine. A sigma2-selective antagonist had no effect on either NPY-mediated enhancement or (+)pentazocine-mediated inhibition. [Leu31,Pro34]NPY and NPY13-36 also enhanced release, but the effects were not reversed by sigma antagonists. Peptide YY did not mimic the effect of NPY. Our findings are consistent with the potential role of NPY as an endogenous ligand for a subtype of sigma receptor with characteristics different from Y1, Y2 and Y3 receptors but sensitive to Ac-[3-(2,6-dichlorobenzyl)Tyr27,D-Thr32NPY-(27-36)amide. Our findings suggest a role for NPY, via sigma receptors, in the regulation of dopamine levels in areas of brain critical to psychosis and reinforcement.