Connexin43 reverses the phenotype of transformed cells and alters their expression of cyclin/cyclin-dependent kinases

Communication between adjacent cells through gap junctions is believed to be involved in the regulation of cell proliferation. This stems in part from the observation that transfection and overexpression of connexin (cx) 32 or cx43 genes into neoplastic cells lead to normalization of growth and decrease their tumorigenicity. The molecular mechanism(s) responsible for this phenomenon has not been characterized. We transfected the rat cx43 gene into a phenotypically transformed dog kidney epithelial cell line, TRMP, and were successful in restoring gap junctional communication as measured by dye coupling. In addition, cx43-transfected clones reverted to a flat morphology and were sensitive to density-dependent inhibition of proliferation with their G1 and S phase duration almost doubled. These cx43-induced effects were coupled with a decreased expression of specific cell cycle regulatory genes critical to cell cycle progression in nonneoplastic cells including cyclin A, D1, D2, and the cyclin-dependent kinases (CDK) 5 and CDK6. The protein levels of cyclin E, CDK2, and CDK4 were not affected. These results suggest that overexpression of cx43 and the formation of gap junctions with the establishment of gap junctional communication can affect the phenotype of transformed cells and alter specific gene expressions involved in cell cycle regulation.

Comparison of prostate specific antigen, prostate specific membrane antigen, and LNCaP-based enzyme-linked immunosorbent assays in prostatic cancer patients and patients with benign prostatic enlargement

Serum assays for prostate specific antigen (PSA; monoclonal), for prostate specific membrane antigen (PSM; Western blot), and a LNCaP/7E11.C5-based competitive enzyme-linked immunosorbent assay (ELISA) were evaluated in a small number of prostate cancer patients with localized or disseminated disease, and judged to be in clinical progression or remission based on National Prostate Cancer Project (NPCP) criteria. PSA values recognized the presence of clinical progression in localized disease (B1-C) and to a lesser degree disseminated disease (D1-D2). In contrast, to a limited degree the ELISA test recognized clinical progression mainly in disseminated disease and chiefly in stage D2. PSM values were elevated in both D1 and D2 but not in a linear fashion as observed with PSA. The ELISA and PSM results may be assessing a different clinical response to prostatic cancer than that recognized by PSA. This could reflect a developing clone of resistant prostatic cells as previously postulated. To further pursue this possibility a secondary generation of monoclonal antibodies to PSM is being developed. The ELISA levels for benign prostatic enlargement were not elevated above normal. In contrast both with PSA and PSM the assays reflected levels significantly above the normal range in benign prostatic hypertrophy (BPH).

Western blot assay for prostate-specific membrane antigen in serum of prostate cancer patients

There is a need for the development of new diagnostic tools for the early detection of prostate cancer. A candidate molecule for a new screening test is a prostate-specific membrane antigen (PSM) recognized by the monoclonal antibody 7E11.C5. We carried out studies aimed at identifying PSM in the serum of normal and benign prostatic hyperplasia (BPH) donors and patients with adenocarcinoma of the prostate, in order to judge whether the development of a serum assay using this marker was feasible. By Western blotting, we found significant levels of PSM in serum samples from prostatic cancer patients, in the seminal fluid of pooled normal donors, in BPH patients, and in normal male sera. Similar to prostate-specific antigen (PSA), PSM was present in seminal plasma in higher concentrations than in serum, and PSM levels in prostatic cancer patients were significantly higher than in normal controls. These data suggest that the development of an assay utilizing the PSM and new monoclonal antibodies directed against the antigen, could provide a feasible test for prostatic cancers.

Dissociation of PDGF receptor tyrosine kinase activity from PDGF-mediated inhibition of gap junctional communication

Gap junction-mediated intercellular communication (GJC) may play an important role in cell proliferation and transformation since GJC is inhibited by growth factors, oncogenes, tumor promoters, and carcinogens. We have studied inhibition of GJC by platelet-derived growth factor-BB (PDGF) in the mouse fibroblast cell line C3H/10T1/2 and have sought to determine whether PDGF-induced inhibition of GJC is mediated by the PDGF receptor tyrosine kinase (RTK). PDGF-mediated inhibition of GJC was rapid and transient, with maximal inhibition occurring 40 min after PDGF addition and GJC returning to control levels after 70 min. The effect of PDGF on GJC was concentration-dependent, with maximal inhibition of 90% or greater occurring at 10 ng/ml PDGF. Stimulation of RTK activity, as determined by antiphosphotyrosine immunoblot analysis of PDGF receptor and the receptor substrates phospholipase C-gamma I (PLC-gamma I) and guanosine triphosphatase activating protein (GAP), was also concentration-dependent. Inhibition of GJC required a greater concentration of PDGF than did stimulation of RTK activity. The tyrosine kinase inhibitor genistein blocked PDGF-induced RTK activity, as measured by PDGF receptor, PLC-gamma I, and GAP tyrosine phosphorylation, in a concentration-dependent manner but did not affect PDGF-mediated inhibition of GJC. Genistein alone had no effect on GJC or PDGF receptor expression. PDGF treatment in the presence or absence of genistein resulted in phosphorylation of the connexin 43 protein on nontyrosine residues. These results suggest that inhibition of GJC by ligand-activated PDGF receptor is dissociable from the RTK activity responsible for PDGF, PLC-gamma I, and GAP phosphorylation.

Characterization of natural toxins with inhibitory activity against serine/threonine protein phosphatases

Recent studies suggest that the ability to inhibit the activity of certain serine/threonine protein phosphatases underlies the toxicity of several natural compounds including: okadaic acid, microcystin-LR, nodularin, calyculin A and tautomycin. To characterize further the actions of these toxins, this study compares the inhibitory effects of okadaic acid, chemical derivatives of okadaic acid, microcystin-LR, microcystin-LA, nodularin, calyculin A and tautomycin on the activity of serine/threonine protein phosphatases types 1 (PP1), 2A (PP2A) and a recently identified protein phosphatase purified from bovine brain (PP3). This study shows that, like PP1 and PP2A, the activity of PP3 is potently inhibited by okadaic acid, both microcystins, nodularin, calyculin A and tautomycin. Further characterization of the toxins employing the purified catalytic subunits of PP1, PP2A and PP3 under identical experimental conditions indicates that: (a) okadaic acid, microcystin-LR, and microcystin-LA inhibit PP2A and PP3 more potently than PP1 (order of potency PP2A > PP3 > PP1); (b) nodularin inhibits PP1 and PP3 at a similar concentration that is slightly higher than that which affects PP2A, and (c) both calyculin A and tautomycin show little selectivity among the phosphatases tested. This study also shows that the chemical modification of the (C1) carboxyl group of okadaic acid can have a profound influence on the inhibitory activity of this toxin. Esterification of okadaic acid, producing methyl okadaate, or reduction, producing okadaol, greatly decreases the inhibitory effects against all three enzymes tested. Further reduction, producing 1-nor-okadaone, or acetylation, producing okadaic acid tetraacetate, results in compounds with no inhibitory activity. In contrast, the substitution of alanine (-LA) for arginine (-LR) in microcystin has no apparent effect on the inhibitory activity against PP1, PP2A or PP3.

Epidermal growth factor disrupts gap-junctional communication and induces phosphorylation of connexin43 on serine

Growth factors regulate cellular proliferation and differentiation by activating plasma membrane tyrosine kinase receptors and triggering a cascade of events mediated by intracellular signaling proteins. The mechanism underlying growth factor modification of cellular functions, such as gap-junctional communication (gjc), has not been established clearly. Addition of epidermal growth factor (EGF) to T51B rat liver epithelial cells resulted in the rapid activation of EGF receptor tyrosine kinase activity followed by a transient dose-dependent disruption of gjc. This change did not result from the gross disturbance of membrane gap junction plaques as measured by immunofluorescence microscopy, but instead correlated with markedly elevated phosphorylation of the connexin43 (cx43) gap junction protein, a profound shift to predominantly phosphorylated forms of cx43, and the appearance of a novel phosphorylated cx43 protein. These changes in cx43 phosphorylation involved only serine residues. On restoration of gjc, these alterations in cx43 phosphorylation reverted to the pre-EGF treatment state. Both events were inhibited by the serine/threonine protein phosphatase inhibitor, okadaic acid. Therefore, unlike the case for pp60v-src, EGF-induced disruption of gjc is not associated with tyrosine phosphorylation of cx43, but instead may result from phosphorylation of cx43 by activated intracellular signaling serine protein kinase(s).

Inhibition of cellular transformation by triphenylmethane: a novel chemopreventive agent

Triphenylmethane (TPM) was found to inhibit 3-methyl-cholanthrene-induced neoplastic transformation of 10T1/2 cells in a dose-dependent manner (ED50 = 2.8 microM). This activity was independent of any effect on intercellular communication and did not appear to be directly related to the general antioxidant properties of TPM as measured by cellular thiobarbituric acid-reactive substances. Triphenylmethanol (TPMOL) and diphenylmethane also inhibited transformation (ED50 = 6.9 and 90 microM respectively). TPM had no effect on the proliferation of exponentially growing cells. At higher concentrations TPM and its analogues enhanced plating efficiency of cells indicating no significant toxicity for these compounds at levels up to 50 microM. The inhibitory effects of TPM on transformation were reversible when TPM was removed from the medium. While TPM had no effect on the growth of fully transformed cell lines, it was able to inhibit the growth of 1/3 neoplastic foci in the presence (but not absence) of 10T1/2 cells. TPM was found to stimulate protein kinase C (PKC) activity for both crude C3H10T1/2 cytosolic PKC and purified PKC obtained from rat brain. The ability of TPM to stimulate PKC activity appeared to be dependent on [CaCl2] and the order of reagent addition in the assay. Tamoxifen, a structurally related compound to TPM, was also found to enhance PKC activity over the same concentration range but was less potent than TPM. The biological effects of TPM and related compounds indicate that they function in a manner distinct from other highly unsaturated transformation inhibitors such as carotenoids and retinoids. The inability of triphenylene to inhibit transformation suggests that a reactive methyl carbon may be essential for activity.

Cyanobacterial nodularin is a potent inhibitor of type 1 and type 2A protein phosphatases

The present study characterizes the inhibitory effects of nodularin, a recently isolated hepatotoxic compound from the cyanobacterium Nodularia spumigena, on type 1 (PP1), type 2A, (PP2A), type 2B (PP2B), and type 2C (PP2C) protein phosphatases. Both PP2A and PP1 were potently inhibited (IC50 = 0.026 and 1.8 nM, respectively) by nodularin, whereas PP2B was inhibited to a lesser extent (IC50 = 8.7 microM). Nodularin had no apparent effect on PP2C, alkaline phosphatase, acid phosphatase, insulin receptor tyrosine kinase, protein kinase A, phosphorylase kinase, or protein kinase C. In a whole-cell extract of T51B liver cells, nodularin inhibited PP1 and PP2A activity with a potency similar to that seen with their purified catalytic subunits. Thus, due to the high specificity of nodularin for PP2A and PP1, this hepatotoxin may prove to be useful as a probe for distinguishing the activity of these protein phosphatases in cell extracts.

Identification, purification, and characterization of a novel serine/threonine protein phosphatase from bovine brain

A novel serine/threonine protein phosphatase is identified, and the catalytic subunit, obtained from a detergent extraction of the pellet generated by a 100,000 x g centrifugation of a whole bovine brain homogenate, is purified and characterized. The protein phosphatase, designated as PP3, has a Mr of 36,000, does not require divalent cations for activity, is stimulated rather than inhibited by inhibitor 2, is inhibited by both okadaic acid and microcystin-LR with an intermediate IC50 compared to type 1 and type 2A protein phosphatases, and preferentially dephosphorylates the beta subunit of phosphorylase kinase. Substrate specificity, immunoblotting with type-specific antisera, and the amino acid sequences of peptides derived from PP3 indicate that PP3 is not an isoform of any known serine/threonine protein phosphatase.

Okadaic acid inhibits PDGF-induced proliferation and decreases PDGF receptor number in C3H/10T1/2 mouse fibroblasts

Okadaic acid is both a potent inhibitor of protein serine/threonine phosphatases and a tumor promoter in the mouse skin model. We have previously shown that at non-toxic nanomolar concentrations okadaic acid reversibly inhibits induction (promotion) by PDGF of transformed cells by the 'complete' and 'two-stage' protocols in the C3H/10T1/2 mouse fibroblast transformation assay. In the present study we have demonstrated that treatment of confluent and proliferatively quiescent C3H/10T1/2 mouse fibroblasts with low doses of okadaic acid inhibits the platelet-derived growth factor (PDGF)-induced mitogenic response. This inhibition is accompanied by a loss of PDGF binding sites, a decreased PDGF-induced phosphatidylinositol turnover and a decrease in the PDGF-induced intracellular calcium signal. The decrease in the PDGF-generated intracellular signalling processes represents a mechanism by which okadaic acid inhibits PDGF-induced proliferation and the promotion of in vitro neoplastic transformation by PDGF.

Inhibition of PDGF-induced c-jun and c-fos expression by a tyrosine protein kinase inhibitor

The expression of the proto-oncogenes c-fos, c-jun, jun B and jun D was monitored in quiescent C3H10T1/2 fibroblasts after stimulation with PDGF. The mRNA level of c-fos, c-jun and jun B, but not of jun D, was stimulated by PDGF. The inductions were abolished when genistein, a specific tyrosine protein kinase inhibitor, was added concomitantly with PDGF, a condition in which DNA synthesis is known to be inhibited. As already shown previously, treatment with PDGF and genistein for 4h followed by the replacement with fresh medium induces the progression of the cells through the G1 phase of their growth-division cycle, without phospholipase C activation. The removal of PDGF and genistein was accompanied by an important increase in c-fos, c-jun and jun B mRNA expression, which correlated with the entrance of cells into G1 phase. Thus, the proto-oncogene expressions induced by PDGF are also obtained in the absence of phospholipase C activation. This result also suggests that the mRNA levels of c-jun, jun B and to a lesser degree c-fos are positively regulated by tyrosine protein kinase activity, whereas jun D is negatively regulated.

Characterization of microcystin-LR, a potent inhibitor of type 1 and type 2A protein phosphatases

The level of protein phosphorylation is dependent on the relative activities of both protein kinases and protein phosphatases. By comparison with protein kinases, however, there have been considerably fewer studies on the functions of serine/threonine protein phosphatases. This is partly due to a lack of specific protein phosphatase inhibitors that can be used as probes. In the present study we characterize the inhibitory effects of microcystin-LR, a hepatotoxic cyclic peptide associated with most strains of the blue-green algae Microcystis aeruginosa found in the Northern hemisphere, that proves to be a potent inhibitor of type 1 (IC50 = 1.7 nM) and type 2A (IC50 = 0.04 nM) protein phosphatases. Microcystin-LR inhibited the activity of both type 1 and type 2A phosphatases greater than 10-fold more potently than okadaic acid under the same conditions. Type 2A protein phosphatases in dilute mammalian cell extracts were found to be completely inhibited by 0.5 nM microcystin-LR while type 1 protein phosphatases were only slightly affected at this concentration. Thus, microcystin-LR may prove to be a useful probe for the study and identification cellular processes which are mediated by protein phosphatases.

Retinyl acetate inhibits platelet-derived growth factor-induced Ca2+ signals in C3H 10T1/2 fibroblasts

Mitogenic stimulation of density-arrested C3H 10T1/2 mouse fibroblasts by serum or purified platelet-derived growth factor (PDGF) was potently inhibited by retinyl acetate (RAc; IC50 = 0.1 microgram/ml, 0.3 x 10(-6) M) when administered during the first 2 hours of mitogen exposure. This inhibitory effect of RAc coincided with a period early in the cell growth-division cycle when density-arrested C3H 10T1/2 cells stimulated by PDGF were found to require physiological levels of extracellular Ca2+ for the transition from G0 to G1 of the cell cycle. To determine if the inhibitory effect of RAc was mediated through alterations in the Ca2+ signaling pathway induced by mitogens, we examined Fura-2-loaded fibroblasts for changes in the Ca2+ response elicited by PDGF. Addition of PDGF (5 ng/ml) induced a transient increase in the [Ca2+]i that was not significantly effected by the extracellular Ca2+ concentration. Treatment of cells with RAc caused a concentration- and time-dependent inhibition of this PDGF-stimulated Ca2+ flux (IC50 = 0.45 microgram/ml or 1.5 x 10(-6) M; t1/2 = 15 min), whereas release of intracellularly stored Ca2+ by thrombin was unaffected by RAc (1.2 micrograms/ml, 4 x 10(-6) M). Treatment with RAc did not significantly affect PDGF binding to cell surface receptors or the generation of inositol phosphates. These results suggest that the mechanism by which RAc inhibits PDGF- or serum-induced mitogenesis is through modulation of the Ca2+ signal stimulated by PDGF, and thereby depriving the cell of a rise in intracellular Ca2+ necessary for progression through the cell cycle.

Angiotensin II causes phosphatidylinositol turnover and increases 1,2-diacylglycerol mass but is not mitogenic in rat liver T51B cells

Proliferation in rat liver T51B cells has previously been shown to be initiated by the tyrosine-kinase activator epidermal growth factor. We have found that T51B cells also contain angiotensin II receptors, and, as the transforming mas oncogene has been identified as a functional angiotensin receptor [Jackson, Blair, Marshall, Goedert & Hanley (1988) Nature (London) 335, 437-440], we have investigated the possibility that angiotensin II might also regulate proliferation of T51B cells. Angiotensin II at concentrations up to 10 microM did not promote DNA synthesis, even in the presence of the co-mitogens serum (1%) or 12-O-tetradecanoylphorbol 13-acetate (TPA) (50 ng/ml). The addition of 1 microM angiotensin II to myo-[3H]inositol-radiolabelled T51B cells did however result in a rapid accumulation of multiple inositol phosphates as well as in an increase in intracellular Ca2+, demonstrating the coupling of the angiotensin receptor in these cells to a polyphosphoinositide-hydrolysing phospholipase C. The increases in both inositol phosphates and intracellular Ca2+ were lower in cells pretreated for 10 min with 50 ng of TPA/ml and potentiated by a 24 h pretreatment with TPA. In addition, angiotensin II increased 1,2-diacylglycerol levels. These results demonstrate that, although angiotensin II is capable of increasing phosphoinositide-derived second messengers in T51B cells, these responses are not sufficient to trigger DNA synthesis.

PDGF-induced activation of phospholipase C is not required for induction of DNA synthesis

Platelet-derived growth factor (PDGF) induction of DNA synthesis is believed to involve activation of phospholipase C (PLC) and subsequent accumulation of inositol 1,4,5-triphosphate [I(1,4,5)P3], increase in intracellular Ca2+, activation of protein kinase C (PKC), and receptor down regulation. Generation of these events is triggered by the tyrosine protein kinase (TPK) activity of the PDGF receptor. The TPK inhibitor genistein blocked PDGF induction of these events, including DNA synthesis, with the exception of receptor down regulation. PDGF-induced phosphotyrosine phosphorylations, including receptor autophosphorylation, were inhibited by genistein. Removal of genistein and PDGF resulted in DNA synthesis without the occurrence of PLC activation. These findings indicate that these early events, with the exception of receptor down regulation, are not necessary for PDGF-induced DNA synthesis.

Stimulation of DNA synthesis by an inositol polyphosphate-activated protein phosphatase in calcium-deprived rat liver cells

Specific stages of the prereplicative G1 phase of the cell cycle in nonneoplastic cells requires extracellular Ca2+ for successful transition. These are the G0-G1 and the G1-S transitions. A variety of agents are able to replace Ca2+ and to at least partially stimulate cells to replicate their chromosomes. One of these agonist, inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4], has been demonstrated by us to also stimulate the activity of a phosphoprotein phosphatase. The addition of a purified preparation of the protein phosphatase to Ca2(+)-deprived G1-S-blocked T51B rat liver cells stimulates a rapidly responding fraction of cells to enter their S phase, and this effect is blocked by protein phosphatase inhibitors heparin and inhibitor 2.

Inositol tetrakisphosphate-induced sequestration of Ca2+ replenishes an intracellular pool sensitive to inositol trisphosphate

In a permeable neoplastic rat liver epithelial (261B) cell system, inositol 1,3,4,5-tetrakisphosphate--Ins(1,3,4,5)P4--induces sequestration of Ca2+ released by inositol 2,4,5-trisphosphate--Ins(2,4,5)P3; a non-metabolized inositol trisphosphate (InsP3) isomer--and Ca2+ added exogenously in the form of CaCl2. Studies were performed to identify the Ca2+ pool filled after Ins(1,3,4,5)P4 treatment. Both Ins(2,4,5)P3 and inositol 1,4,5-trisphosphate--Ins(1,4,5)P3--dose-dependently release Ca2+ from permeable 261B cells--Ins(1,4,5)P3 having a threefold greater potency--but differ in that Ca2+ released by Ins(1,4,5)P3 is readily sequestered, while the Ca2+ released by Ins(2,4,5)P3 is not. Maximal release of Ca2+ by 6 microM Ins(2,4,5)P3 blocked the action of Ins(1,4,5)P3, demonstrating that these two isomers influence the same intracellular Ca2+ pool through a shared membrane receptor. Addition of 2 microM Ins(2,4,5)P3 to discharge partially the Ca2+ pool reduced the amount of Ca2+ released by a maximal dose of Ins(1,4,5)P3 (2 microM). Ins(1,3,4,5)P4 combined with Ins(2,4,5)P3 produced a Ca2+ release and sequestration response, which replenished the InsP3-sensitive pool as indicated by a recovery of full Ca2+ release by 2 microM Ins(1,4,5)P3. Induction of Ca2+ sequestration by Ins(1,3,4,5)P4 occurred dose-dependently, with a half-maximal response elicited at a dose of 0.9 microM. Further studies of the effect of Ins(1,3,4,5)P4 apart from the influence of Ins(2,4,5)P3 using a model in which the Ca2+ levels are raised by an exogenous addition of CaCl2 showed that Ins(1,4,5)P3 released twice the amount of Ca2+ from the storage pool following Ins(1,3,4,5)P4-induced Ca2+ sequestration. These results demonstrate that the Ca2+ uptake induced by Ins(1,3,4,5)P4 preferentially replenishes the intracellular Ca2+ storage sites regulated by Ins(1,4,5)P3 and Ins(2,4,5)P3.

Okadaic acid: a reversible inhibitor of neoplastic transformation of mouse fibroblasts

Okadaic acid (OA) is a potent inhibitor of serine/threonine-specific protein phosphatases types 1 and 2A at nanomolar concentrations in cell-free assays and has tumor promoting activity in vivo. We have found that at non-toxic, nanomolar concentrations, OA concentration dependently inhibits the induction of focus-forming transformed cells by the "complete" and "two-stage" protocols in the C3H/10T1/2 mouse fibroblast transformation assay. This inhibitory effect was fully reversible upon removal of OA from the culture medium of carcinogen-treated cells, indicating that OA was not selectively toxic to initiated or transformed cells. Additional treatment with the phorbol ester tumor promoter, TPA, was required to promote the induction of transformed cells after the removal of OA in the two-stage transformation assay. At concentrations that inhibited neoplastic transformation, OA inhibited a type 2A-like phosphohistone protein phosphatase in homogenates of C3H/10T1/2 cells. It is postulated that OA inhibited an early protein phosphatase-sensitive event in the process of in vitro neoplastic transformation by C3H/10T1/2 fibroblasts and had the effect of maintaining carcinogen-treated cells in an initiated state.

Epidermal growth factor induces the accumulation of calpactin II on the cell surface during membrane ruffling

Confluent and proliferatively quiescent T51B rat liver epithelial cells provide a cellular model for the study of epidermal growth factor (EGF) effects in non-neoplastic cells. Immunoreactive calpactin II, a well-known substrate for EGF-receptor kinase, was found predominantly in the cytosol, although a second immunoreactive pool was found in a Triton X-100-extractable membrane fraction. Stimulation with EGF resulted in a rapid and transient (2-5 min) formation of ruffles at the cell surface and at the cell-cell contacts. Both calpactin II and filamentous actin were found co-localized at the membrane ruffles. Immunoprecipitations of membrane-bound calpactin II from 32P-labeled cells indicate a transient EGF-dependent phosphorylation of calpactin II correlating with membrane ruffling. These results suggest a temporal (2-5 min) function for calpactin II at the plasma membrane during the EGF-induced mitogenesis of T51B cells.

Effects of the tyrosine-kinase inhibitor genistein on DNA synthesis and phospholipid-derived second messenger generation in mouse 10T1/2 fibroblasts and rat liver T51B cells

We have examined the effects of the tyrosine kinase inhibitor genistein on hormone dependent cell proliferation and intracellular signalling in mouse 10T1/2 fibroblasts and rat liver T51B epithelial cells. Genistein inhibits both PDGF and EGF induced mitogenesis with an IC50 of 40 uM and 10 uM respectively. Genistein also inhibits inositol phosphate generation and calcium signalling in response to PDGF, and 1,2-diacylglycerol generation and calpactin II translocation in response to EGF. By contrast genistein does not inhibit inositol phosphate production, Ca2+ signalling or 1,2-diacylglycerol generation in response to ATP or angiotensin II. These data demonstrate that genistein selectively inhibits tyrosine kinase dependent processes without effecting similar responses obtained to hormones which are not dependent upon tyrosine kinase activation.

Tumor promoter-dependent phosphorylation of a Triton X-100 extractable form of lipocortin I in T51B rat liver cells

The phosphorylation of lipocortin (a substrate of EGF-receptor kinase, and a putative phospholipase A2 inhibitor) was examined in T51B cells. By using Western blot procedures and antisera specific to lipocortin I, we found that most immunoreactive lipocortin I was located in the cytosol (lipocortin(cvt] of cells extracted in Ca2+-free buffers These cells however had another pool of immunoreactive lipocortin I located in the particulate fraction that was Triton X-100 extractable (lipocortin(mem]. Increasing Ca2+ concentrations in the extraction buffer resulted in more lipocortin(mem) recovered. In vitro phosphorylation of endogenous proteins demonstrated that lipocortin I became phosphorylated in a Ca2+ and phosphatidylserine-dependent manner, suggesting an involvement of protein kinase C. Treatment of cells with 100 ng/ml 12-0-tetradecanoylphorbol-13-acetate (TPA) but not with 4 alpha-phorbol 12,13-didecanoate (4 alpha-PDD) resulted in the in vitro phosphorylation of lipocortin(mem) by protein kinase C. TPA also increased the phosphorylation of lipocortin(mem) in [32P]phosphate-labeled cells.

Inositol 1,3,4,5-tetrakisphosphate stimulates the initiation of DNA synthesis in Ca2+-deprived rat liver cells

The G1-S boundary of non-neoplastic cells requires extracellular Ca2+ for successful transition. Inositol 1,3,4,5-tetrakisphosphate but not inositol 1,4,5-trisphosphate can partially replace Ca2+ and stimulate the initiation of DNA synthesis of Ca2+-deprived T51B rat liver cells but only if sufficient extracellular Ca2+ (i.e., 0.075 mM) is present. The potent tumor promoter and protein kinase C activator 12-O-tetradecanoylphorbol acetate is also capable of replacing extracellular Ca2+ and partially stimulating the initiation of DNA synthesis. In addition, both inositol-1,3,4,5-tetrakisphosphate and 12-O-tetradecanoylphorbol acetate added together elicit a full DNA synthetic response.

Extracellular ATP mobilizes intracellular Ca2+ in T51B rat liver epithelial cells: a study involving single cell measurements

T51B rat liver epithelial cells were stimulated with extracellular ATP. Changes in cytoplasmic free Ca2+ concentration [( Ca2+]i) were measured by fura-2 both in a large population of cells on coverslips in a cuvette and in single cells in a microscopic system. Extracellular ATP evoked a prompt increase in [Ca2+]i in both the presence and absence of extracellular Ca2+, although the effect was less pronounced in the latter case. These findings indicate that at least part of the [Ca2+]i increase is due to mobilization of intracellularly bound calcium. Stimulation with ATP did not mobilize the total pool of intracellular releasable Ca2+, as evidenced from experiments where subsequent addition of ionomycin evoked a pronounced increase in [Ca2+]i in the absence of extracellular Ca2+. The effect of ATP was maintained at room temperature but was markedly impaired in the absence of continuous stirring of the buffer solution. In the absence of stirring, ATP had to be increased to the millimolar range in order to evoke a pronounced effect. Single cell measurements revealed a heterogenous Ca2+ response to ATP, with some cells failing to respond with a detectable increase in [Ca2+]i. The actual increase in [Ca2+]i was not uniform throughout the cytoplasm, but seemed to start in one part of the cell. Even if part of the [Ca2+]i increase might be accounted for by ATP promoting the hydrolysis of phosphatidylinositol 4,5-bisphosphate and thereby a generation of InsP3 and diacylglycerol, there was no initiation of DNA synthesis under the present conditions. Hence, extracellular growth factors exert either a quantitative difference in second messenger production or additional stimulatory effects by activating intracellular signal pathways beyond these represented by [Ca2+]i and protein kinase C.