PTH stimulates PLCbeta and PLCgamma isoenzymes in rat enterocytes: influence of ageing

ACTH(6-9)PGP Peptide Protects SH-SY5Y Cells from H2O2, tert-Butyl Hydroperoxide, and Cyanide Cytotoxicity via Stimulation of Proliferation and Induction of Prosurvival-Related Genes

Stabilized melanocortin analog peptide ACTH(6-9)PGP (HFRWPGP) possesses a wide range of neuroprotective activities. However, its mechanism of action remains poorly understood. In this paper, we present a study of the proproliferative and cytoprotective activity of the adrenocorticotropic hormone fragment 6-9 (HFRW) linked with the peptide prolyine-glycyl-proline on the SH-SY5Y cells in the model of oxidative stress-related toxicity. The peptide dose-dependently protected cells from H2O2, tert-butyl hydroperoxide, and KCN and demonstrated proproliferative activity. The mechanism of its action was the modulation of proliferation-related NF-κB genes and stimulation of prosurvival NRF2-gene-related pathway, as well as a decrease in apoptosis.

Ubiquitination-deubiquitination balance dictates ligand-stimulated PTHR sorting

Parathyroid hormone receptors (PTHR) are promptly internalized upon stimulation by activating (PTH[1-84], PTH[1-34]) and non-activating (PTH[7-84], PTH[7-34]) ligands. Here, we characterized the mechanism regulating the sorting of internalized receptors between recycling and degradative pathways. PTHR recycles faster after challenge with PTH(1-34) than with PTH(7-34). PTHR recycling is complete by 2 h after PTH(1-34) stimulation, but incomplete at this time in cells treated with PTH(7-34). The slower and incomplete recycling induced by PTH(7-34) is due to proteasomal degradation. Both PTH(1-34) and PTH(7-34) induced PTHR polyubiquitination. Ubiquitination by PTH(1-34) was transient, whereas receptor ubiquitination after PTH(7-34) was sustained. PTH(1-34), but not PTH(7-34), induced expression of the PTHR-specific deubiquitinating enzyme USP2. Overexpression of USP2 prevented PTH(7-34)-induced PTHR degradation. We conclude that PTH(1-34) promotes coupled PTHR ubiquitination and deubiquitination, whereas PTH(7-34) activates only ubiquitination, thereby leading to PTHR downregulation. These findings may explain PTH resistance in diseases associated with elevated PTH(7-84) levels.

PTH inactivates the AKT survival pathway in the colonic cell line Caco-2

In previous works, we found that PTH promotes the apoptosis of human Caco-2 intestinal cells, through the mitochondrial pathway. This study was conducted to investigate the modulation of different players implicated in the AKT survival pathway in PTH-induced intestinal cell apoptosis. We demonstrate, for the first time, that PTH modulates AKT phosphorylation in response to apoptosis via the serine/threonine phosphatase PP2A. PTH treatment induces an association of AKT with the catalytic subunit of PP2A and increases its phosphatase activity. PTH also promotes the translocation of PP2Ac from the cytosol to the mitochondria. Furthermore, our results suggest that PP2A plays a role in hormone-dependent Caco-2 cells viability and in the cleavage of caspase-3 and its substrate PARP. The cAMP pathway also contributes to PTH-mediated AKT dephosphorylation while PKC and p38 MAPK do not participate in this event. Finally, we show that PTH induces the dissociation between 14-3-3 and AKT, but the significance of this response remains unknown. In correlation with PTH-induced Bad dephosphorylation, the hormone also decreases the basal association of 14-3-3 and Bad. Overall, our data suggest that in Caco-2 cells, PP2A and the cAMP pathway act in concert to inactivate the AKT survival pathway in PTH-induced intestinal cell apoptosis.

Pro-apoptotic effects of parathyroid hormone in intestinal cells

Apoptosis, a form of programmed cell death, is a process fundamental to normal growth and development, immune response, tissue remodeling after injury or insult, and homeostasis of the intestinal epithelium. Recently, it has become apparent that apoptosis is a crucial process in skeletal development and homeostasis, and that signaling by the parathyroid hormone (PTH) receptor can either promote or suppress apoptosis depending on the cellular context. In this study, we evaluated the role of PTH in intestinal apoptosis using human colonic Caco-2 cells. To that end, Caco-2 cells were exposed to PTH (10−8 mol/L) for 1–5 days. Evaluation of cell survival by use of resazurin staining, 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS) staining, and crystal violet staining revealed that PTH treatment diminishes the number of viable cells. Assessment of cells after PTH treatment by use of propidium iodide showed that the hormone increased the number of red-stained (dead) cells (178%, 5 days). Moreover, we found that the hormone induced disruption of actin filaments with changes to cellular shape, alteration of cell-to-cell junctions, externalization of membrane phosphatidylserine, chromatin condensation, and DNA fragmentation of the nucleus, which are morphological features consistent with apoptosis. In addition, PTH treatment revealed a cytosolic staining pattern of 14-3-3. However, the significance of such differential localization for 14-3-3 function remains unknown. Taken together, the present study suggests that PTH promotes apoptosis in Caco-2 intestinal cells.

Alterations in the protein kinase C signaling activated by a parthenogenetic agent in oocytes from reproductively old mice

To investigate the effect of female age on oocyte developmental competence, we focused on protein kinase C (PKC), a major component of the signalling pathway involved in oocyte activation, and put forward the hypothesis that, as it occurs in many organs and tissues, aging affects PKC function in mouse oocytes. Biochemical activity of PKC along with the expression and subcellular distribution of some PKC isoforms were monitored in young and old mouse oocytes parthenogenetically activated by SrCl(2). We found that PKC activity increased reaching a level that was lower in old compared to young oocytes in association with an incomplete translocation of PKCbetaI to the plasma membrane. Moreover, old oocytes exhibited a reduced expression of PKCbeta1 and PKCalpha at the protein level, without significant effects on the expression of the Ca(2+)-independent PKCdelta. Detectable amounts of PKCbeta1 mRNA were observed in young and old oocytes at GV stage with no difference between the two groups of age. When meiotic progression to anaphase II up to first cleavage were analyzed, a delay in meiosis resumption and significantly lower rates of pronuclei formation and first cleavage were observed in old compared to young oocytes. Moreover, we found that, in contrast to SrCl(2), PMA (12-O-tetradecanoyl phorbol-13-acetate), a PKC agonist, was ineffective in activating old oocytes. Present findings provide evidence that aging affects the correct storage and activation of some PKCs, functional components of the machinery involved in oocyte activation, and suggest that these changes may negatively influence the activation competence of old oocytes.

PTH-mediated regulation of Na+-K+-ATPase requires Src kinase-dependent ERK phosphorylation

Parathyroid hormone (PTH) inhibits Na+-K+-ATPase activity by serine phosphorylation of the alpha1-subunit through ERK-dependent phosphorylation and translocation of protein kinase Calpha (PKCalpha). On the basis of previous studies, we postulated that PTH regulates sodium pump activity through Src kinase, PLC, and calcium-dependent ERK phosphorylation. In the present work utilizing opossum kidney cells, a model of renal proximal tubule, PTH-stimulated ERK phosphorylation and membrane translocation of PKCalpha were prevented by inhibition of Src kinase, PLC, and calcium entry. Pharmacological inhibition of PLA2 did not prevent PTH-stimulated ERK phosphorylation but completely prevented PKCalpha translocation. Silencing the expression of cytosolic or calcium-independent PLA2 also prevented PTH-mediated phosphorylation of Na+-K+-ATPase alpha1-subunit and PKCalpha without blocking ERK phosphorylation. Inhibition of Na+-K+-ATPase activity by the PLA2 metabolites arachidonic acid and 20-hydroxyeicosatetraenoic acid was prevented by specific inhibition of PKCalpha but not by U0126, a MEK-1 inhibitor. Transient transfection of constitutively active MEK-1 cDNA induced phosphorylation of Na+-K+-ATPase alpha1-subunit and PKCalpha, which was prevented by PLA2 inhibition. We conclude that PTH stimulates Na+-K+-ATPase phosphorylation and decreases the activity of Na+-K+-ATPase by a sequential activation of a signaling pathway involving Src kinase, PLC, ERK, PLA2, and PKCalpha.

Tyrosine kinase and mitogen-activated protein kinase/extracellularly regulated kinase differentially regulate intracellular calcium concentration responses to angiotensin II/III and bradykinin in rat cortical thick ascending limb

The cortical thick ascending limb (CTAL) coexpresses angiotensin (Ang) II/Ang III receptor type 1A (AT(1A)-R) and bradykinin (BK) receptor type 2 (B2-R). In several cell types, these two receptors share the same signaling pathways, although their physiological functions are often opposite. In CTAL, little is known about the intracellular transduction events leading to the final physiological response induced by these two peptides. We investigated and compared in this segment the action of Ang II/III and BK on intracellular calcium concentration ([Ca2+]i) response and metabolic CO2 production, an index of Na+ transport, by using inhibitors of protein kinase C (bisindolylmaleimide), Src tyrosine kinase (herbimycin A and PP2), and MAPK/ERK (PD98059 and UO126). Ang II/III and BK (10(-7) mol/liter) released Ca2+ from the same intracellular pools but activated different Ca2+ entry pathways. Ang II/III- or BK-induced [Ca2+]i increases were similarly potentiated by bisindolylmaleimide. Herbimycin A and PP2 decreased similarly the [Ca2+]i responses induced by Ang II/III and BK. In contrast, PD98059 and UO126 affected the effects of BK to a larger extent than those of Ang II/III. Especially, the Ca2+ influx induced by BK was more strongly inhibited than that induced by Ang II/III in the presence of both compounds. The Na+ transport was inhibited by BK and stimulated by Ang II/III. The inhibitory action of BK on Na+ transport was blocked by UO126, whereas the stimulatory response of Ang II/III was potentiated by UO126 but blocked by bisindolylmaleimide. These data suggest that the inhibitory effect of BK on Na+ transport seems to be directly mediated by an increase in Ca2+ influx dependent on MAPK/ERK pathway activation. In contrast, the stimulatory effect of Ang II/III on Na+ transport is more complex and involves PKC and MAPK/ERK pathways.

PTH and phospholipase A2 in the aging process of intestinal cells

In this study we analyzed, for the first time, alterations in phospholipase A2 (PLA2) activity and response to parathyroid hormone (PTH) in rat enterocytes with aging. We found that PTH, rapidly stimulate arachidonic acid (AA) release in rat duodenal cells (+1- to 2-fold), an effect that is greatly potentiated by aging (+4-fold). We also found that hormone-induced AA release in young animals is Ca2+-dependent via cPLA2, while AA released by PTH in cells from aged rats is due to the activation of cPLA2 and the Ca2+-independent PLA2 (iPLA2). In enterocytes from 3 months old rats, PTH induced, in a time and dose-dependent fashion, the phosphorylation of cPLA2 on serine 505, with a maximum at 10 min (+7-fold). Basal levels of cPLA2 serine-phosphorylation were higher in old enterocytes, affecting the hormone response which was greatly diminished (+2-fold at 10 min). cPLA2 phosphorylation impairment in old animals was not related to changes of cPLA2 protein expression and did not explain the substantial increase on PTH-induced AA release with aging, further suggesting the involvement of a different PLA2 isoform. Intracellular Ca2+ chelation (BAPTA-AM, 5 microM) suppressed the serine phosphorylation of cPLA2 in both, young and aged rats, demonstrating that intracellular Ca2+ is required for full activation of cPLA2 in enterocytes stimulated with PTH. Hormone effect on cPLA2 was suppressed to a great extent by the MAP kinases ERK 1 and ERK2 inhibitor, PD 98059 (20 microM), the cAMP antagonist, Rp-cAMP, and the PKC inhibitor Ro31820 both, in young and aged animals. Enterocytes exposure to PTH also resulted in phospho-cPLA2 translocation from cytosol to nuclei and membrane fractions, where phospholipase substrates reside. Hormone-induced enzyme translocation is also modified by aging where, in contrast to young animals, part of phospho-cPLA2 remained cytosolic. Collectively, these data suggest that PTH activates in duodenal cells, a Ca2+-dependent cytosolic PLA2 and attendant AA release and that this activation requires prior stimulation of intracellular ERK1/2, PKA, and PKC. cPLA2 is the major enzyme responsible for AA release in young enterocytes while cPLA2 and the Ca2+-independent iPLA2, potentiate PTH-induced AA release in aged cells. Impairment of PTH activation of PLA2 isoforms upon aging may result in abnormal hormone regulation of membrane fluidity and permeability and thereby affecting intestinal cell membrane function.

Age-related changes in the response of intestinal cells to parathyroid hormone

The concept of the role(s) of parathyroid hormone (PTH), has expanded from that on acting on the classical target tissues, bone and kidney, to the intestine where its actions are of regulatory and developmental importance: regulation of intracellular calcium through modulation of second messengers and, activation of mitogenic cascades leading to cell proliferation. Several causes have been postulated to modify the hormone response in intestinal cells with ageing, among them, alterations of PTH receptor (PTHR1) binding sites, reduced expression of G proteins and hormone signal transduction changes. The current review summarizes the actual knowledge regarding the molecular and biochemical basis of age-impaired PTH receptor-mediated signaling in intestinal cells. A fundamental understanding of why PTH functions are impaired with age will enhance our understanding of its importance in intestinal cell physiology.

Tyrosine phosphorylation signalling dependent on 1α,25(OH)2-vitamin D3 in rat intestinal cells: effect of ageing

In intestinal cells, as in other target cells, 1alpha,25(OH)(2)D(3) elicits long-term and short-term responses which involve genomic and non-genomic mode of actions, respectively. There is evidence indicating that activation of tyrosine phosphorylation pathways may participate in the responses induced by 1alpha,25(OH)(2)D(3) through its non-genomic mechanism. In this study we have evaluated the involvement of 1alpha,25(OH)(2)D(3) in the tyrosine phosphorylation of PLCgamma and MAPK (ERK1/2) in enterocytes from young (3 months) and aged (24 months) rats. Immunochemical analysis revealed that the hormone stimulates PLCgamma tyrosine phosphorylation in young rat enterocytes. Hormone effect on PLCgamma is rapid, peaking at 2 min (+100%), is dose-dependent (10(-10) to 10(-8)M) and decreases with ageing. 1alpha,25(OH)(2)D(3) also induces the phosphorylation and activation of the mitogen-activated-protein kinases ERK1 and ERK2, effect which was evident at 1 min (three-fold) and reached a maximum at 2 min (six-fold). Hormone-dependent ERK1 and ERK2 phosphorylation and activity is greatly reduced in enterocytes from old rats. In both, young and aged animals, 1alpha,25(OH)(2)D(3)-induced PLCgamma and ERK1/2 phosphorylation was effectively suppressed by the tyrosine kinase inhibitor genistein (100 uM) and suppressed to a great extent by PP1, an inhibitor of c-Src kinases. LY294002, a specific inhibitor of PI3 kinase (PI3K), enzyme with an important role in mitogenesis, did not affect hormone-dependent ERK1/2 phosphorylation, indicating that PI3K is not involved in 1alpha,25(OH)(2)D(3)-induced MAPK activation. In agreement with this data, enzyme activity assays and tyrosine phosphorylation of the regulatory subunit (p85) of PI3K showed that the hormone has no effect on the enzyme activity in rat enterocytes. Taken together, the present study suggest that in intestinal cells, tyrosine phosphorylation is an important mechanism of 1alpha,25(OH)(2)D(3) involved in PLCgamma and MAPK regulation and that this mechanism is impair with ageing.

MEK/ERK and signal transducer and activator of transcription signaling pathways modulate oncostatin M-stimulated CCL2 expression in human osteoblasts through a common transcription factor

OBJECTIVE

To analyze the effects of oncostatin M (OSM), a gp130-type cytokine, on CCL2 expression in MG-63 cells, a human osteosarcoma cell line with a characteristic osteoblastic phenotype, and to investigate the signaling pathway involved.

METHODS

The expression of messenger RNA (mRNA) for CCL2 and c-Fos was analyzed by Northern blotting. Amounts of CCL2 released into the supernatant were measured by enzyme-linked immunosorbent assay. Western blotting was used to examine the activation of MAPK signaling pathways. Interactions between activator protein 1 (AP-1) and DNA were evaluated by electrophoretic mobility shift assay.

RESULTS

OSM stimulated CCL2 expression at both the mRNA and the protein levels. Cyclooxygenase 2 (COX-2) was also induced by OSM. However, the up-regulation of CCL2 mRNA was COX-2-independent but required tyrosine kinase and protein kinase C (PKC). OSM stimulated the phosphorylation of MEK-1/2 and ERK-1/2 but not p38 and JNK. A transient elevation of c-Fos mRNA was induced by OSM, but PD 98059 (MEK inhibitor), fludarabine (signal transducer and activator of transcription 1 [STAT-1] inhibitor), and piceatannol (STAT-3 and STAT-5 inhibitor) abolished this effect. Electrophoretic mobility shift assay revealed that OSM stimulated AP-1-DNA binding, which was also abolished by PD 98059, fludarabine, and piceatannol. Supershift study further confirmed the role of c-Fos in the above interaction. PD 98059, fludarabine, piceatannol, and curcumin (AP-1 inhibitor) inhibited the OSM-induced expression of CCL2.

CONCLUSION

OSM induces CCL-2 expression in osteoblasts. Activation of the MEK/ERK and STAT pathways, which leads to c-Fos expression and AP-1-DNA binding, is involved in the process. The signaling requires tyrosine kinase and PKC but not COX-2.

Activation-independent parathyroid hormone receptor internalization is regulated by NHERF1 (EBP50)

Parathyroid hormone (PTH) regulates extracellular calcium homeostasis through the type 1 PTH receptor (PTH1R) expressed in kidney and bone. The PTH1R undergoes beta-arrestin/dynamin-mediated endocytosis in response to the biologically active forms of PTH, PTH-(1-34), and PTH-(1-84). We now show that amino-truncated forms of PTH that do not activate the PTH1R nonetheless induce PTH1R internalization in a cell-specific pattern. Activation-independent PTH1R endocytosis proceeds through a distinct arrestin-independent mechanism that is operative in cells lacking the adaptor protein Na/H exchange regulatory factor 1 (NHERF1) (ezrin-binding protein 50). Using a combination of radioligand binding experiments and quantitative, live cell confocal microscopy of fluorescently tagged PTH1Rs, we show that in kidney distal tubule cells and rat osteosarcoma cells, which lack NHERF1, the synthetic antagonist PTH-(7-34) and naturally circulating PTH-(7-84) induce internalization of PTH1R in a beta-arrestin-independent but dynamin-dependent manner. Expression of NHERF1 in these cells inhibited antagonist-induced endocytosis. Conversely, expression of dominant-negative forms of NHERF1 conferred internalization sensitivity to PTH-(7-34) in cells expressing NHERF1. Mutation of the PTH1R PDZ-binding motif abrogated interaction of the receptor with NHERF1. These mutated receptors were fully functional but were now internalized in response to PTH-(7-34) even in NHERF1-expressing cells. Removing the NHERF1 ERM domain or inhibiting actin polymerization allowed otherwise inactive ligands to internalize the PTH1R. These results demonstrate that NHERF1 acts as a molecular switch that legislates the conditional efficacy of PTH fragments. Distinct endocytic pathways are determined by NHERF1 that are operative for the PTH1R in kidney and bone cells.

Characterization of PTH/PTHrP receptor in rat duodenum: effects of ageing

In rat enterocytes, signaling through the parathyroid hormone (PTH)/PTH-related peptide receptor type 1(PTHR1) includes stimulation of adenylyl cyclase, increases of intracellular calcium, activation of phospholipase C, and the MAP kinase pathway, mechanisms that suffer alterations with ageing. The purpose of this study was to evaluate whether an alteration at the level of the PTH receptor (PTHR1) is the basis for impaired PTH signaling in aged rat enterocytes. Western Blot analysis with a specific monoclonal anti-PTHR1 antibody revealed that a 85 kDa PTH binding component, the size expected for the mature PTH/PTHrP receptor, localizes in the basolateral (BLM) and brush border (BBM) membranes of the enterocyte, being the protein expression about 7-fold higher in the BLM. Two other bands of 105 kDa (corresponding to highly glycosylated, incompletely processed receptor form) and 65 kDa (proteolytic fragment) were also seen. BLM PTHR1 protein expression significantly decreases with ageing, while no substantial decrease was observed in the BBM from old rats. PTHR1 immunoreactivity was also present in the nucleus where PTHR1 protein levels were similar in enterocytes from young and aged rats. Immunohistochemical analysis of rat duodenal sections showed localization of PTHR1 in epithelial cells all along the villus with intense staining of BBM, BLM, and cytoplasm. The nuclei of these cells were reactive to the PTHR1 antiserum, but not all cells showed the same nuclear staining. The receptor was also detected in the mucosae lamina propria cells, but was absent in globets cells from epithelia. In aged rats, PTHR1 immunoreactivity was diffused in both membranes and cytoplasm and again, PTH receptor expression was lower than in young animals, while the cell nuclei showed a similar staining pattern than in young rats. Ligand binding to PTHR1 was performed in purified BLM. rPTH(1-34) displaced [I(125)]PTH(1-34) binding to PTHR1 in a concentration-dependent fashion. In both, aged (24 months) and young (3 months) rats, binding of [I(125)]PTH was characterized by a single class of high-affinity binding sites. The affinity of the receptor for PTH was not affected by age. The maximum number of specific PTHR1 binding sites was decreased by 30% in old animals. The results of this study suggest that age-related declines in PTH regulation of signal transduction pathways in rat enterocytes may be due, in part, to the loss of hormone receptors.

Involvement of PI3-kinase and its association with c-Src in PTH-stimulated rat enterocytes

Phosphoinositide-3-kinase (PI3K) is a lipid kinase, which phosphorylates the D3 position of phosphoinositides, and is known to be activated by a host of protein tyrosine kinases. PI3K plays an important role in mitogenesis in several cell systems. However, whether parathyroid hormone (PTH) affects the activity and functional roles of PI3K in intestinal cells remain to be determined. The objective of this study was to identify and characterize the PI3K pathway, and its relation to other non-receptor tyrosine kinases in mediating PTH signal transduction in rat enterocytes. PTH dose- and time-dependently increased PI3K activity with a peak occurring at 2 min. The tyrosine kinase inhibitor genistein, c-Src inhibitor PP1 and two structurally different inhibitors of PI3K, LY294002 and wortmannin, suppressed PI3K activity dependent on PTH. Co-immunoprecipitation analysis showed a constitutive association between c-Src and PI3K, which was enhanced by PTH treatment, suggesting that the cytosolic tyrosine kinase forms an immunocomplex with PI3K probably via the N-SH2 domain of the p85alpha regulatory subunit. In response to PTH, tyrosine phosphorylation of p85alpha was enhanced, effect that was abolished by PP1, the inhibitor of c-Src kinase. PTH causes a rapid (0.5-5 min) phosphorylation of Akt/PKB, effect that was abrogated by PI3K inhibitors, indicating that in rat enterocytes, PI3K is an upstream mediator of Akt/PKB activation by PTH. We report here that PI3K is also required for PTH activation of the mitogen-activated protein kinases ERK1 and ERK2. Taken together, the present study demonstrate, for the first time, that PTH rapidly and transiently stimulates PI3K activity and its down effector Akt/PKB in rat enterocytes playing c-Src kinase a central role in PTH-dependent PI3K activation and that PI3K signaling pathway contributes to PTH-mediated MAPK activation.

Effect of ageing in the early biochemical signals elicited by PTH in intestinal cells

In previous work, we have demonstrated that rPTH(1-34) increases cytoplasmic calcium concentration ([Ca(2+)](i)) in isolated rat enterocytes. In the present study, we have identified the sources of PTH-mediated increase in [Ca(2+)](I) and the implication of Ca(2+) on hormone early signals in enterocytes isolated from young (3-month-old) and aged (24-month-old) rats. In young enterocytes, PTH raised [Ca(2+)](i) in a dose-dependent manner (1 pM-100 nM). In cells from aged rats, hormone concentrations higher than physiological (>/=1 nM) were required to observe significant increases in [Ca(2+)](i). Phospholipase C (PLC) inhibitors blocked the initial acute elevation of the [Ca(2+)](i) biphasic response to PTH of young enterocytes while in old cells, no effects were observed. The voltage-dependent calcium-channel blocker (VDCC), nitrendipine, suppressed PTH-dependent changes of the sustained [Ca(2+)](i) phase in young and aged animals. In this study, we analysed, for the first time, alterations in phosphatidylinositol 3-kinase (PI3K) activity and response to PTH in rat enterocytes with ageing. Basal PI3K activity was significantly modified by ageing. Acute treatment with 10(-8) M PTH increased enzyme activity, with a maximun at 2 min (+3-fold) in young rats and only elevated by less than 1-fold basal PI3K activity in aged animals. Hormone-induced tyrosine phosphorylation of p85alpha, the regulatory subunit of PI3K, as well as the phosphorylation on Thr(308) of its downstream effector Akt/PKB was evident in enterocytes from 3-month-old rats, whereas it was greatly reduced in the cells from 24-month-old animals. Intracellular Ca(2+) chelation (BAPTA-AM, 5 microM) affected the tyrosine phosphorylation of p85alpha and inhibited PTH-dependent PI3K activation by 75% in young rats and completely abolished the enzyme activity in aged animals, demonstrating that Ca(2+) is required for full activation of PI3K in enterocytes stimulated with PTH. The Thr phosphorylation of PI3K downeffector, Akt/PKB, was also fully dependent on Ca(2+). Taken together, these results suggest that PTH regulation of enterocyte [Ca(2+)](i) involves Ca(2+) mobilization from IP(3)-sensitive stores and the influx of the cation from the extracellular milieu, the former pathway being blunted during ageing. The data also indicates a positive role for intracellular calcium in one of the early signals of PTH in rat enterocytes, the activation of PI3K, and that hormone regulation of PI3K activity and Akt/PKB phosphorylation on Thr(308) is impaired with ageing.