Inhibition of bone collagen synthesis by the tumor promoter phorbol 12-myristate 13-acetate

Parathyroid hormone inhibits collagen synthesis and the activity of rat col1a1 transgenes mainly by a cAMP-mediated pathway in mouse calvariae

We examined the effect of parathyroid hormone and various signaling molecules on collagen synthesis and chloramphenicol acetyltransferase activity in cultured transgenic mouse calvariae carrying fusion genes of the rat Col1a1 promoter and the chloramphenicol acetyltransferase reporter. After 48 h of culture, parathyroid hormone, forskolin, dibutyryl cAMP, 8-bromo cAMP, and phorbol myristate acetate inhibited transgene activity, while the calcium ionophore ionomycin had no effect. Pretreatment of calvariae with the phosphodiesterase inhibitor isobutylmethylxanthine potentiated the inhibitory effect of 1 nM parathyroid hormone on transgene activity and collagen synthesis. Parathyroid hormone further inhibited transgene activity and collagen synthesis in the presence of phorbol myristate acetate. Parathyroid hormone inhibition of transgene activity and collagen synthesis was not affected by indomethacin or interleukin-6. After 48 h of culture, parathyroid hormone inhibited chloramphenicol acetyltransferase activity by 50-85% in cultured calvariae carrying transgenes having progressive 5' upstream deletions of promoter DNA down to -1683 bp. These data show that the inhibitory effect of parathyroid hormone on Col1a1 expression in mouse calvariae is mediated mainly by the cAMP signaling pathway. Prostaglandins and IL-6 are not local mediators of the parathyroid hormone response in this model. Finally, regions of the Col1a1 promoter downstream of -1683 bp are sufficient for parathyroid hormone inhibition of the Col1a1 promoter.

Estradiol suppresses type I collagen synthesis in mesangial cells via activation of activator protein-1

BACKGROUND

Estradiol suppresses the synthesis of type I collagen by murine mesangial cells. However, neither the alpha 1(I) nor the alpha 2(I) collagen gene contains an estrogen-response element. Because estradiol modulates the transcription of several genes that lack an estrogen-response element but contain a regulatory activator protein-1 (AP-1) binding motif, we hypothesized that AP-1 may mediate estradiol-induced suppression of type I collagen synthesis.

METHODS

We measured type I collagen synthesis in murine mesangial cells exposed to estradiol, phorbol 12-myristate 13-acetate (an activator of AP-1), or curcumin (an inhibitor of AP-1). We also assessed the effects of estradiol on the steady-state level of c-fos and c-jun mRNA and on the binding of mesangial cell nuclear extracts to an AP-1 consensus binding site oligonucleotide.

RESULTS

Estradiol (10(-10) M to 10(-7) M) suppressed type I collagen synthesis by murine mesangial cells in a dose-dependent manner (10(-7) M, 43.7 +/- 8.2% of control values, P < 0.001). Phorbol 12-myristate 13-acetate (10 microM, four-hr exposure) also decreased type I collagen in the media. In contrast, curcumin (1 microM) increased type I collagen. Estradiol increased the steady-state level of c-fos mRNA twofold at 30 minutes, with a return to basal levels at one hour. This was associated with a greater than threefold increase in the binding of nuclear extracts from estradiol-treated mesangial cells to an AP-1 consensus binding site oligonucleotide. Estradiol-enhanced binding of nuclear extracts to the AP-1 oligonucleotide was reversed by cycloheximide.

CONCLUSIONS

These data suggest that estradiol suppresses collagen I synthesis by murine mesangial cells via enhanced AP-1 activity.

1,25-Dihydroxyvitamin D3 inhibition of col1a1 promoter expression in calvariae from neonatal transgenic mice

We studied the effect of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) on organ cultures of transgenic mouse calvariae containing segments of the Col1a1 promoter extending to -3518, -2297, -1997, -1794, -1763, and -1719 bp upstream of the transcription start site fused to the chloramphenicol acetyltransferase (CAT) reporter gene. 1,25(OH)2D3 had a dose-dependent inhibitory effect on the expression of the -3518 bp promoter construct (ColCAT3.6), with maximal inhibition of about 50% at 10 nM. This level of inhibition was consistent with the previously observed effect on the endogenous Col1a1 gene in bone cell models. All of the shorter constructs were also inhibited by 10 nM 1,25(OH)2D3, suggesting that the sequences required for 1, 25(OH)2D3 inhibition are downstream of -1719 bp. The inhibitory effect of 1,25(OH)2D3 on transgene mRNA was maintained in the presence of the protein synthesis inhibitor cycloheximide, suggesting that the inhibitory effect on Col1a1 gene transcription does not require de novo protein synthesis. We also examined the in vivo effect of 1,25(OH)2D3 treatment of transgenic mice on ColCAT activity, and found that 48 h treatment caused a dose-dependent inhibition of CAT activity in calvariae comparable to that observed in organ cultures. In conclusion, we demonstrated that 1,25(OH)2D3 inhibits Col1A1 promoter activity in transgenic mouse calvariae, both in vivo and in vitro. The results indicate that there is a 1, 25(OH)2D3 responsive element downstream of -1719 bp. The inhibitory effect does not require new protein synthesis.

The effect of cadmium on cytosolic free calcium, protein kinase C, and collagen synthesis in rat osteosarcoma (ROS 17/2.8) cells

Cadmium affects normal bone growth but the mechanisms of Cd2+ toxicity are not fully understood. Calcium is an integral component of bone growth and a second messenger necessary for the actions of calciotropic hormones. Ca2+ activates protein kinase C (PKC), and PKC is a mediator of [Ca2+]1 and mediator of collagen synthesis in osteoblastic cells. Therefore, PKC is a possible loci of Cd2+ effects on Ca2+ metabolism and Ca(2+)-regulated processes. This work was conducted to determine the effect of Cd2+ on cytosolic free Ca2+ ([Ca2+]i) levels, characterize the activation and/or inhibition of PKC by Cd2+ and Ca2+, and measure the effect of Cd2+ on collagen synthesis in ROS 17/2.8 cells. Cells were treated for 120 min with Cd2+ (0 to 30 microM) and [Ca2+]i was measured. Basal [Ca2+]i was 132 nM and the maximal increase to 268 nM occurred in the presence of 5 microM Cd2+. Treatment with 1 or 5 microM Cd2+ caused an increase in [Ca2+]i at 40 min with return to basal levels at 120 min of treatment. Pretreatment (24 hr) with 0.1 microM calphostin C (CC), a PKC inhibitor, produced no change in [Ca2+]i and prevented any rise in [Ca2+]i in response to Cd2+. Free Cd2+ activates PKC with an activation constant of 7.5 X 10(-11) M, while Ca2+ activates PKC with an activation constant of 3.6 X 10(-7) M. Cd2+ also caused a dose-dependent decrease in collagen synthesis, a PKC-mediated process. These data suggest that Cd2+ affects Ca2+ metabolism and Ca(2+)-mediated processes via unwarranted PKC activation as demonstrated by Cd2+ perturbation of collagen synthesis.

Prostaglandin E2/parathyroid hormone-induced suppression of alkaline phosphatase activity is mediated by protein kinase C

1. Bone resorptive factors, prostaglandin E2 and parathyroid hormone are shown to suppress alkaline phosphatase activity in a rat osteoblastic cell line. 2. Phorbol myristate acetate, but not dibutyryl cAMP or calcium ionophore can suppress alkaline phosphatase activity. 3. The protein kinase C inhibitors (H89, staurosporine) are able to block the suppression of alkaline phosphatase activity induced by prostaglandin E2 and parathyroid hormone. 4. These data suggest that protein kinase C is involved in the inhibition of alkaline phosphatase activity induced by prostaglandin E2 and parathyroid hormone.

Lead perturbs epidermal growth factor (EGF) modulation of intracellular calcium metabolism and collagen synthesis in clonal rat osteoblastic (ROS 17/2.8) cells

EGF, a single-chain polypeptide growth factor important for many cellular functions including glycolysis and protein phosphorylation, is known to modulate calcium metabolism in several cell systems. EGF causes an increase in Ca2+ influx and accumulation of inositol triphosphate and probably exhibits many, if not all, of its effects via the calcium messenger system. Lead is known to interact with and perturb normal calcium signaling pathways; therefore, the purpose of this work was to determine if lead perturbs EGF modulation of calcium metabolism in ROS 17/2.8 cells and if lead impairs collagen synthesis, which is controlled by EGF. To characterize 45Ca kinetics, cells were labelled with 45Ca (1.87 mM Ca) for 20 hr in the presence of 5 microM Pb, 50 ng/ml EGF, or 5 microM Pb and 50 ng/ml EGF. Kinetic parameters were determined from 45Ca efflux curves. Three kinetic compartments described the intracellular metabolism of 45Ca; 5 microM Pb significantly altered the effect of EGF on intracellular calcium metabolism. Calcium distribution was shifted from the fast-exchanging, quantitatively small calcium pools S1 and S2 to the slow-exchanging, quantitatively large S3. There was also a 50% increase in total cell calcium in cells treated with 5 microM Pb and 50 ng/ml EGF over calcium in cells treated with 50 ng/ml EGF alone. Because EGF and phorbol 12-myristate 13-acetate (PMA) have similar effects on protein kinase C (PKC) and collagen metabolism, the transient effects of EGF and PMA on 45Ca and 210Pb were also characterized. EGF caused a rapid transient increase in efflux of both isotopes, which was further increased by the addition of PMA. In contrast, PMA pretreatment, which depletes PKC, significantly attenuated the latter effect of EGF, suggesting that downregulation by PKC of EGF-induced increases in 45Ca and 210Pb efflux. Moreover, collagen synthesis was decreased by lead, EGF, and PMA in a similar manner, further suggesting PKC as the common modulator of these effects. These data show that Pb impairs the normal modulation of intracellular calcium homeostasis and collagen synthesis by EGF. Furthermore, these results provide additional support to the postulate that an early and discrete effect of lead involves perturbation of the calcium messenger system at one or several loci.

Protein kinase C activity in UMR-106-01 cells: effects of parathyroid hormone and insulin

The calcium and phospholipid-dependent protein kinase C (PKC) system appears to play an important role in mediating hormonal effects in various tissues including bone. Accordingly, we characterized PKC activity in the UMR-106-01 rat osteosarcoma osteoblastlike cell line and examined its hormonal regulation. UMR-106-01 cells were found to possess a classic, phorbol ester-activated PKC system, which was highly calcium and phospholipid dependent. A 30 s exposure to 10 nM bovine parathyroid hormone (PTH) (1-34) increased cytosolic and membrane-bound PKC activity by 12 and 157%, respectively, resulting in a 2.2-fold increase in the membrane-bound to cytosolic (MB/C) activity ratio (all p less than 0.01). The MB/C activity ratio was highest at 20 min, exhibiting a 2.8-fold increase over the control values (p less than 0.01). In contrast, 10 nM insulin increased cytosolic PKC activity but decreased membrane-bound activity, resulting in a 61% decrease in the MB/C activity ratio at 20 min (p less than 0.02). Moreover, insulin reduced PTH stimulation of the PKC activity ratio by 42 and 62% at 30 s and 20 min, respectively (p less than 0.02). Thus, PTH and insulin have opposing effects on the PKC activity ratio in UMR-106-01 cells.