cAMP stimulation of Dictyostelium discoideum destabilizes the mRNA for 117 antigen

Efeito da atividade física no osso normal e na prevenção e tratamento da osteoporose

A osteoporose é uma doença cada vez mais diagnosticada em mulheres e homens de todo o mundo. Embora os esteróides sexuais sejam importantes na gênese da osteoporose, a inatividade física constitui um fator de risco. O exercício físico atua no osso por efeito direto, via força mecânica, ou indireto, mediado por fatores hormonais. Mas os mecanismos pelos quais a atividade física melhora a massa óssea ainda não são totalmente conhecidos. Baseando-se nos resultados que demonstram os efeitos benéficos da atividade física no tecido ósseo, a prática de esportes vem sendo cada vez mais indicada na prevenção e até mesmo no tratamento da osteoporose. O objetivo desta revisão é descrever os efeitos da atividade física no tecido ósseo normal e na prevenção e tratamento da osteoporose.

Effects of mechanical strain on the function of Gap junctions in osteocytes are mediated through the prostaglandin EP2 receptor

Osteocytes embedded in the matrix of bone are thought to be mechanosensory cells that translate mechanical strain into biochemical signals that regulate bone modeling and remodeling. We have shown previously that fluid flow shear stress dramatically induces prostaglandin release and COX-2 mRNA expression in osteocyte-like MLO-Y4 cells, and that prostaglandin E2 (PGE2) released by these cells functions in an autocrine manner to regulate gap junction function and connexin 43 (Cx43) expression. Here we show that fluid flow regulates gap junctions through the PGE2 receptor EP2 activation of cAMP-dependent protein kinase A (PKA) signaling. The expression of the EP2 receptor, but not the subtypes EP1,EP3, and EP4, increased in response to fluid flow. Application of PGE2 or conditioned medium from fluid flow-treated cells to non-stressed MLO-Y4 cells increased expression of the EP2 receptor. The EP2 receptor antagonist, AH6809, suppressed the stimulatory effects of PGE2 and fluid flow-conditioned medium on the expression of the EP2 receptor, on Cx43 protein expression, and on gap junction-mediated intercellular coupling. In contrast, the EP2 receptor agonist butaprost, not the E1/E3 receptor agonist sulprostone, stimulated the expression of Cx43 and gap junction function. Fluid flow conditioned medium and PGE2 stimulated cAMP production and PKA activity suggesting that PGE2 released by mechanically stimulated cells is responsible for the activation of cAMP and PKA. The adenylate cyclase activators, forskolin and 8-bromo-cAMP, enhanced intercellular connectivity, the number of functional gap junctions, and Cx43 protein expression, whereas the PKA inhibitor, H89, inhibited the stimulatory effect of PGE2 on gap junctions. These studies suggest that the EP2 receptor mediates the effects of autocrine PGE2 on the osteocyte gap junction in response to fluid flow-induced shear stress. These data support the hypothesis that the EP2 receptor, cAMP, and PKA are critical components of the signaling cascade between mechanical strain and gap junction-mediated communication between osteocytes.

Testin induction: the role of cyclic 3',5'-adenosine monophosphate/protein kinase A signaling in the regulation of basal and lonidamine-induced testin expression by rat sertoli cells

Results of previous in vitro and in vivo studies have illustrated that the expression of testin by Sertoli cells is tightly associated with the disruption of Sertoli-germ cell junctions. In the present study, treatment of rats with cadmium chloride (CdCl(2)), which disrupted the inter-Sertoli tight junctions, failed to induce any changes in testicular testin expression. In contrast, lonidamine, an antispermatogenic drug that rearranges the Sertoli cell membrane microfilament structure causing a disruption of Sertoli-germ cell adhesion junctions, induced a drastic increase in testicular testin expression when administered orally. Lonidamine-induced Sertoli cell testin expression involved both ongoing RNA and de novo protein synthesis. Basal testin expression remained stable during the 27-h incubation with actinomycin D but required de novo protein synthesis in vitro. An inhibitor of protein kinase A, Rp-cAMPS, caused a 50% inhibition of Sertoli cell testin expression at 10 microM within 24 h. A biphasic response was noted in testin expression when forskolin was included in the Sertoli cell culture, and high concentrations of cAMP analogues (1 mM) rapidly reduced testin expression. However, lonidamine can abolish the inhibitory effect of cAMP analogues on Sertoli cell testin expression. These results illustrate that the induction of testin expression may involve several signal transduction pathways.

Thyroid hormones stabilize acetylcholinesterase mRNA in neuro-2A cells that overexpress the beta 1 thyroid receptor

We investigated the intracellular events involved in the 3,3',5-triiodo-L-thyronine (T3)-induced accumulation in acetylcholinesterase (AChE) activity in neuroblastoma cells (neuro-2a) that overexpress the human thyroid receptor beta 1 (hTR beta 1). Treatment of these cells with T3 increased AChE activity and its mRNAs after a lag period of 24-48 h, and these levels increased through stabilization of the transcripts by T3. T3 had no effect on the transcriptional rate or processing of AChE transcripts. The protein kinase inhibitor H7 inhibited T3-induced accumulation in AChE activity and its mRNAs, whereas okadaic acid (a potent inhibitor of phosphatases 1 and 2A) potentiated the effect of T3. Okadaic acid and H7 have no effect on the binding of hTR beta 1 to T3 or the transcriptional rate of the AChE gene. Finally, treatment of cells with T3 stimulated cytosolic serine/threonine, but not tyrosine kinase, activities. The time course analysis reveals that the increase in serine/threonine activity precedes the effect of T3 on AChE mRNAs. These results suggest that activation of a serine/threonine protein kinase pathway might be a link between nuclear thyroid hormone receptor activation and stabilization of AChE mRNA.

Identification of two novel Dictyostelium discoideum cysteine proteinases that carry N-acetylglucosamine-1-P-modification

Dictyostelium discoideum makes multiple developmentally regulated lysosomal cysteine proteinases. One of these, a lysosomal enzyme called proteinase I, contains a cluster of GlcNAc-alpha-1-P-Ser residues. We call this phosphoglycosylation. To study its function, a cDNA library from vegetative cells was screened, and two novel cysteine proteinase clones were characterized (cprD and cprE). Each of them has highly conserved regions expected for cysteine proteinases, but unlike any other, each has a serine-rich domain containing three distinct motifs, poly-S, SGSQ, and SGSG. cprD and cprE cDNAs were overexpressed in Dictyostelium and the active enzymes identified. cprD codes for a protein of approximately 36 kDa (CP4), which is recognized by monoclonal antibodies against GlcNAc-1-P and fucose. cprE corresponds to a 29-kDa protein, which is recognized by antibodies against GlcNAc-1-P. mRNA for both enzymes is present in the vegetative phase and increases during growth on bacteria but decreases throughout development. When the formation of the fruiting body is complete the mRNA for both messages is detected again but in very low levels. Having cloned cDNAs for proteins that carry GlcNAc-1-P should allow us to probe the function of the carbohydrate in these putative lysosomal enzymes.

Calcium uptake and gp80 messenger RNA destabilization follows cAMP receptor down regulation in Dictyostelium discoideum

The mechanism by which high concentrations of cAMP selectively destabilize the gp80 mRNA in Dictyostelium discoideum was investigated. This treatment which leads to down-regulation of the cAMP receptor was also found to cause an increase in calcium uptake. Given this observation, we sought a role for calcium as a second messenger in the degradation of the gp80 mRNA. Changes in the mRNA levels were examined after treating cells with compounds known to alter their intracellular Ca2+ concentrations. This included the use of A23187, Ca2+, 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate HC1 (TMB-8), LiCl and 8-p-chlorophenylthioadenosine 3',5'-cyclic monophosphate (ClPhS-Ado-3':5'-P). The sum of the data suggest that it is the cAMP-induced influx of Ca2+ across the plasma membrane, as apposed to a cAMP-mediated release of Ca2+ from intracellular stores, that initiates gp80 mRNA degradation. Treatment of cells with Concanavalin A (ConA) to induce cAMP receptor down-regulation, also causes a reduction in gp80 mRNA levels and an increase in calcium uptake.

Reduced mRNA levels for the multidrug-resistance genes in cAMP-dependent protein kinase mutant cell lines

We have previously shown that in Chinese hamster ovary (CHO) cells, a mutant cell line with a defective regulatory subunit (RI) for the cAMP-dependent protein kinase (Abraham et al: Mol. Cell. Biol., 7:3098-3106, 1987), and a transfectant cell line expressing the same mutant kinase, showed increased sensitivity to a number of drugs that are known to be substrates for the multidrug transporter (P-glycoprotein). In the current study we have investigated the mechanism by which cAMP-dependent protein kinase controls drug resistance. We report here that the sensitivity of the kinase defective CHO cell lines to multiple drugs results from decreased RNA levels for the multidrug-resistance gene. Similar results were obtained with mouse Y1 adrenal cells. Wild-type Y1 cells had high levels of P-glycoprotein due to expression of both the mdr1b and mdr2 genes, whereas the cAMP-dependent protein kinase mutant Kin 8 cells had decreased RNA levels for these genes. A Kin 8 transfectant with restored cAMP-dependent protein kinase activity recovered mdr expression, indicating a cause and effect relationship between the protein kinase mutations and mdr expression. No changes in nuclear run-off assays could be detected, suggesting a non-transcriptional mechanism of regulation. Wild-type Y1 cells are more drug sensitive despite having higher levels of P-glycoprotein than the mutant cells. This paradoxical result may be explained by the higher rate of synthesis of steroids by the wild-type Y1 cells, which appear to be inhibitors of P-glycoprotein transport activity.

Two distinct pathways in the down-regulation of type-1 angiotension II receptor gene in rat glomerular mesangial cells

The mRNA level of the type-1 angiotensin II receptor (AT1) was down-regulated by angiotensin II in cultured rat glomerular mesangial cells. The effect was maximum with 1 microM AII at 6 h, sensitive to cycloheximide, and specific to AT1 since this phenomenon was blocked by DuP753, an AT1 antagonist, but not by type-2 antagonist PD123319. Dibutyryl cAMP, forskolin, and cholera toxin also caused AT1 down-regulation. These effects were not altered by either the protein kinase A inhibitor H-8 or cycloheximide. Calcium ionophore A23187, pertussis toxin, protein kinase C inhibitor staurosporine, or prolonged incubation with phorbol ester were without effect. These results suggest that there are at least two pathways to down-regulate AT1 mRNA; one way is an angiotensin II-induced, protein kinase C-independent, and cycloheximide-sensitive pathway and the other is an angiotensin II-independent, cAMP-induced, and cycloheximide-insensitive pathway.

Regulation of activin βA mRNA level by cAMP

We demonstrated the presence of five species of the activin beta A mRNA in human placenta and one major RNA associated with two minor RNAs of the activin in the fetal membrane. We investigated the effect of 8-bromo-cAMP (8-Br-cAMP) on accumulation of activin beta A subunit mRNA in human fibrosarcoma HT1080 cells. Although low levels of the activin mRNA were detectable in the untreated cells, the one main RNA species was predominantly accumulated by 8-Br-cAMP. We propose that generation of multiple activin mRNAs in the fetal membrane and cAMP-treated HT1080 cells is presumably due to a cell-specific alternative polyadenylation.