Pertussis toxin sensitive G-proteins are not involved in the mitogenic signaling pathway of insulin-like growth factor-I in normal rat kidney epithelial (NRKE) cells

Calcium receptor signaling and citrate transport

The calcium sensing receptor (CaSR) in the distal nephron decreases the propensity for calcium stones. Here we investigate if the apical CaSR in the proximal tubule also prevents stone formation acting via regulation of apical dicarboxylate and citrate transport. Urinary citrate, partially reabsorbed as a dicarboxylate in the proximal tubule lumen, inhibits stone formation by complexing calcium. We previously demonstrated a novel apical calcium-sensitive dicarboxylate transport system in OK proximal tubule cells. This calcium-sensitive process has the potential to modulate the amount of citrate available to complex increased urinary calcium. Using isotope labeled succinate uptake in OK cells along with various pharmacologic tools we examined whether the CaSR alters apical dicarboxylate transport and through which signal transduction pathways this occurs. Our results indicate that in the proximal tubule CaSR adjusts apical dicarboxylate transport, and does so via a CaSR → G_q → PKC signaling pathway. Thus, the CaSR may decrease the propensity for stone formation via actions in both proximal and distal nephron segments.

Rapid signaling responses in Sertoli cell membranes induced by follicle stimulating hormone and testosterone: calcium inflow and electrophysiological changes

This minireview describes the rapid signaling actions of follicle stimulating hormone (FSH) and testosterone in immature Sertoli cells mainly related to Ca(2+) inflow and the electrophysiological changes produced by hormones. The rapid membrane actions of FSH occur in a time frame of seconds to minutes, which include membrane depolarization and the stimulation of (45)Ca(2+) uptake. These effects can be prevented by pertussis toxin (PTX), suggesting that they are likely mediated by Gi-protein coupled receptor activation. Furthermore, these effects were inhibited by verapamil, a blocker of the L-type voltage-dependent Ca(2+) channel (VDCC). Finally, FSH stimulation of (45)Ca(2+) uptake was inhibited by the (phosphoinositide 3-kinase) PI3K inhibitor wortmannin. These results suggest that the rapid action of FSH on L-type Ca(2+) channel activity in Sertoli cells from pre-pubertal rats is mediated by the Gi/Gβγ/PI3Kγ pathway, independent of its effects on insulin-like growth factor type I (IGF-I). Testosterone depolarizes the membrane potential and increases the resistance and the (45)Ca(2+) uptake in Sertoli cells of the seminiferous tubules of immature rats. These actions were nullified by diazoxide (K(+)(ATP) channel opener). Testosterone actions were blocked by both PTX and the phospholipase C (PLC) inhibitor U73122, suggesting the involvement of PLC - phosphatidylinositol 4-5 bisphosphate (PIP2) hydrolysis via the Gq protein in the testosterone-mediated pathway. These results indicate that testosterone acts on the Sertoli cell membrane through the K(+)(ATP) channels and PLC-PIP2 hydrolysis, which closes the channel, depolarizes the membrane and stimulates (45)Ca(2+) uptake. These results demonstrate the existence of rapid non-classical pathways in immature Sertoli cells regulated by FSH and testosterone.

Pertussis toxin nullifies the depolarization of the membrane potential and the stimulation of the rapid phase of Ca entry through L-type calcium channels that are produced by follicle stimulating hormone in 10- to 12-day-old rat Sertoli cells

The aim of this study was to evaluate the effect of pertussis toxin (PTX) on the depolarizing component of the action of follicle stimulating hormone (FSH) on the membrane potential (MP) of Sertoli cells, which is linked to the rapid entry of Ca²⁺ into cells and to the Ca²⁺-dependent transport of neutral amino acids by the A system. This model allowed us to analyze the involvement of Gi proteins in the action of FSH in these phenomena. In parallel, using an inactive analog of insulin-like growth factor type I (IGF-1), JB1, and an anti-IGF-I antibody we investigated the possible mediating role of IGF-I on these effects of FSH because IGF-I is produced and released by testicular cells in response to stimulation by FSH and shows depolarization effects on MP similar to those from FSH. Our results have the following implications: (a) the rapid membrane actions of FSH, which occur in a time-frame of seconds to minutes and include the depolarization of the MP, and stimulation of ⁴⁵Ca²⁺ uptake and [¹⁴C]-methyl aminoisobutyric acid ([¹⁴C]-MeAIB) transport, are nullified by the action of PTX and, therefore, are probably mediated by GiPCR activation; (b) the effects of FSH were also nullified by verapamil, an L-type voltage-dependent Ca²⁺ channel blocker; (c) wortmannin, an inhibitor of phosphoinositide 3-kinase (PI3K), prevented FSH stimulation of ⁴⁵Ca²⁺ entry and [¹⁴C]-MeAIB transport; and (d) these FSH actions are independent of the IGF-I effects. In conclusion, these results strongly suggest that the rapid action of FSH on L-type Ca²⁺ channel activity in Sertoli cells from 10- to 12-day-old rats is mediated by the Gi/βγ/PI3Kγ pathway, independent of the effects of IGF-I.

A role for chemokine receptor transactivation in growth factor signaling

Complex cell responses require the integration of signals delivered through different pathways. We show that insulin-like growth factor (IGF)-I induces specific transactivation of the Gi-coupled chemokine receptor CCR5, triggering its tyrosine phosphorylation and Galpha recruitment. This transactivation occurs via a mechanism involving transcriptional upregulation and secretion of RANTES, the natural CCR5 ligand. CCR5 transactivation is an essential downstream signal in IGF-I-induced cell chemotaxis, as abrogation of CCR5 function with a transdominant-negative KDELccr5A32 mutant abolishes IGF-I-induced migration. The relevance of this transactivation pathway was shown in vivo, as KDELccr5A32 overexpression prevents invasion by highly metastatic tumor cells; conversely, RANTES overexpression confers built-in invasive capacity on a non-invasive tumor cell line. Our results suggest that this extracellular growth factor-chemokine network represents a general mechanism connecting tumorigenesis and inflammation.

Association of heterotrimeric G(i) with the insulin-like growth factor-I receptor. Release of G(betagamma) subunits upon receptor activation

The insulin-like growth factor-I receptor (IGF-IR) is a key regulator of cell proliferation and survival. Activation of the IGF-IR induces tyrosine autophosphorylation and the binding of a series of adaptor molecules, thereby leading to the activation of MAPK. It has been demonstrated that pertussis toxin, which inactivates the G(i) class of GTP-binding proteins, inhibits IGF-I-mediated activation of MAPK, and a specific role for G(betagamma) subunits in IGF-I signaling was shown. In the present study, we have investigated the role of heterotrimeric G(i) in IGF-IR signaling in neuronal cells. Pertussis toxin inhibited IGF-I-induced activation of MAPK in rat cerebellar granule neurons and NG-108 neuronal cells. G(alphai) and G(beta) subunits were associated with IGF-IR immunoprecipitates. Similarly, in IGF-IR-null mouse embryo fibroblasts transfected with the human IGF-IR, G(i) was complexed with the IGF-IR. G(alphas) was not associated with the IGF-IR in any cell type. IGF-I induced the release of the G(beta) subunits from the IGF-IR but had no effect on the association of G(alphai). These results demonstrate an association of heterotrimeric G(i) with the IGF-IR and identify a discrete pool of G(betagamma) subunits available for downstream signaling following stimulation with IGF-I.

Signal pathways that transduce growth factor-stimulated mitogenesis in bone cells

This investigation examined which signal pathways are of relevance in growth factor-stimulated bone cell mitogenesis. Platelet-derived growth factor (PDGF) and insulin-like growth factor-II (IGF-II) were potent mitogens for both the MG-63 osteoblast cell line and for primary cultures of human osteoblasts (HObs). The mitogenic action of both IGF-II and PDGF was attenuated by pertussis toxin (Ptx), by indomethacin, and by the lipoxygenase inhibitors BW755C74 and BW4AC. A combination of Ptx and indomethacin caused much greater inhibition but failed to abolish mitogenesis completely. PDGF significantly elevated inositol phosphates levels in both cell types; IGF-II had no effect on this pathway. In MG-63 cells, we demonstrated tyrosine phosphorylation of high-molecular-weight substrates elicited by both PDGF and IGF-II. Genistein inhibited the phosphorylation and mitogenic response to PDGF, but had no effect on IGF-II-induced tyrosine phosphorylation or mitogenesis. Another inhibitor of tyrosine kinases, methyl 2,5-dihydroxycinnamate, (MDHC), inhibited PDGF-stimulated mitogenesis effectively in both cell types but only blocked IGF-II-induced mitogenesis in MG-63 cells. The specificity of these inhibitors suggests that particular tyrosine kinases may regulate growth factor-induced stimulation of bone cells.

A selective inverse agonist for central cannabinoid receptor inhibits mitogen-activated protein kinase activation stimulated by insulin or insulin-like growth factor 1. Evidence for a new model of receptor/ligand interactions

In the present study, we showed that Chinese hamster ovary (CHO) cells transfected with human central cannabinoid receptor (CB1) exhibit high constitutive activity at both levels of mitogen-activated protein kinase (MAPK) and adenylyl cyclase. These activities could be blocked by the CB1-selective ligand, SR 141716A, that functions as an inverse agonist. Moreover, binding studies showed that guanine nucleotides decreased the binding of the agonist CP-55,940, an effect usually observed with agonists, whereas it enhanced the binding of SR 141716A, a property of inverse agonists. Unexpectedly, we found that CB1-mediated effects of SR 141716A included inhibition of MAPK activation by pertussis toxin-sensitive receptor-tyrosine kinase such as insulin or insulin-like growth factor 1 receptors but not by pertussis toxin-insensitive receptor-tyrosine kinase such as the fibroblast growth factor receptor. We also observed similar results when cells were stimulated with Mas-7, a mastoparan analog, that directly activates the Gi protein. Furthermore, SR 141716A inhibited guanosine 5'-0-(thiotriphosphate) uptake induced by CP-55,940 or Mas-7 in CHO-CB1 cell membranes. This indicates that, in addition to the inhibition of autoactivated CB1, SR 141716A can deliver a biological signal that blocks the Gi protein and consequently abrogates most of the Gi-mediated responses. By contrast, SR 141716A had no effect on MAPK activation by insulin or IGF1 in CHO cells lacking CB1 receptors, ruling out the possibility of a direct interaction of SR 141716A with the Gi protein. This supports the notion that the Gi protein may act as a negative intracellular signaling cross-talk molecule. From these original results, which considerably enlarge the biological properties of the inverse agonist, we propose a novel model for receptor/ligand interactions.