Neurotensin Induces Protein Kinase C-dependent Protein Kinase D Activation and DNA Synthesis in Human Pancreatic Carcinoma Cell Line PANC-1

Cell Culture.

PANC-1, obtained from American Type Culture Collection, is a less well-differentiated line established from human ductal pancreatic adenocarcinoma. PANC-1 cells were grown in DMEM (Life Technologies, Inc.) with 4 mm of glutamine, 1 mm of Na-pyruvate, and 10% FBS at 37°C with a humidified atmosphere containing 10% CO₂.

Immunoprecipitation.

PANC-1 cells cultured for 4–6 days were washed twice in ice-cold PBS (Sigma Chemical Co.) and lysed in 50 mm of Tris-HCl (pH 7.6), 2 mm of EGTA, 2 mm of EDTA, 1 mm of DTT, 10 μg/ml aprotinin, 10 μg/ml leupeptin, 1 mm of 4-(2-aminoethyl)-benzenesulfonyl fluoride hydrochloride, and 1% Triton X-100 (lysis buffer). The immunoprecipitation was carried out with the PA-1 antipeptide antiserum (1:100) in the absence or the presence of the immunizing peptide (10 μg/ml) as described before (26 , 31) .

Kinase Assay of PKD.

PKD autophosphorylation was determined in an IVK assay by mixing 20 μl of PKD immunocomplexes with 10 μl of a phosphorylation mixture containing (final concentration) 100 μm [γ-³²P]ATP (specific activity 400–600 cpm/pmol), 30 mm Tris-HCl (pH 7.4), 10 mm MgCl₂, and 1 mm DTT, and followed by SDS-PAGE analysis (26 , 31) . The gels were dried, and the 110-kDa radioactive band corresponding to autophosphorylated PKD was visualized by autoradiography. Autoradiographs were scanned in a GS-710 Calibrated Imaging Densitometer (Bio-Rad), and the labeled band was quantified with the Quantity One software program.

Western Blot Analysis.

Serum-starved cultures of PANC-1 cells grown on 100-mm dishes were washed twice with PBS then replaced with DMEM and treated as described in individual experiments. To detect different PKC isoenzymes and PKD, the cells were directly lysed in 2 × SDS-PAGE sample buffer, followed by SDS-PAGE on 8% gels and transfer to Immobilon-P membranes (Millipore). For Western blot analysis of PKD IP, the samples were washed twice in lysis buffer followed by twice in kinase buffer. Western blots were then performed on membranes incubated overnight with antibodies that recognize the different PKC isoforms and PKD (C-20) at a dilution of 1 μg/ml, or an antiserum that specifically recognizes the dual-phosphorylated state of S744/S748 in PKD (pS744/S748) at a concentration of 1 μg/ml, or for 3 h with an antiserum that specifically recognizes the phosphorylated state of serine 916 position in PKD (pS916) at a dilution of 1:500 in PBS containing 5% nonfat dried milk and 0.1% Tween 20. The immunoreactive bands were detected with enhanced chemiluminescence reagents (Amersham).

Immunocytochemistry.

PANC-1 cells plated in Lab-Tek (Nalge Nunc International, Naperville, IL) double-chambers per each slide were fixed for 15 min at 25°C in 10% buffered formalin phosphate and permeabilized with 0.2% Triton X-100 in PBS for 5 min at 25°C. The fixed cells were incubated for 18 h at 25°C in BB (PBS-1% gelatin-0.05% Tween 20) and then stained at 37°C for 60 min with rabbit polyclonal PKD antibody (C-20) diluted in BB. Subsequently, the cells were washed with PBS containing 0.05% Tween 20 at 25°C and stained at 37°C for 60 min with fluorescein-conjugated goat-anti rabbit antibody diluted in BB. After washing with PBS containing 0.05% Tween 20 and PBS, the samples were mounted with a gelvatol-glycerol solution containing 2.5% 1,4-diazobicyclo-[2.2.2]octane.

Confocal Imaging.

The samples were examined with a Leica TCS-SP upright laser-scanning confocal microscope (Leica, Heidelberg, Germany) using a 63 × 1.2 w HCX PL APO oil immersion objective. Images were collected using an argon gas laser with excitation filter set at 488 nm and emission filter set between 500 and 550 nm. The images were captured as uncompressed 8-bit TIFF files using Leica TCS-NT software (version 1.6.587). One hundred cells were analyzed per experiment, and each experiment was performed at least in duplicate.

[³H]Thymidine Incorporation.

PANC-1 cells (5 × 10⁴) were plated and grown in 3.5-cm tissue culture plates for 5 days in DMEM (Life Technologies, Inc.) with 4 mm of glutamine, 1 mm of Na-pyruvate, and 10% FBS. The cells were washed twice and incubated in serum-free medium for 6 h. To start the experiment, fresh medium containing the specified concentration of agonist/antagonist or solvent was added after washing twice with PBS (four cultures for each condition), and the cells were incubated for 16 h then pulsed for 6 h with [³H]thymidine (0.25 μCi/ml). The cells were fixed with 5% trichloroacetic acid and washed twice with ethanol. Finally, cell residues were dissolved in 0.1 n NaOH with 1% SDS and counted in a liquid scintillation counter.

Materials.

[γ-³²P]ATP (370 MBq/ml) was obtained from Amersham International. Bisindolylmaleimide I (GF-1), Bisindolylmaleimide V (GF-V), Ro 31–8220, wortmannin, PD 98059, and genistein were purchased from Calbiochem. NT, PDB, and rapamycin were obtained from Sigma Chemical Co. Protein-A agarose was from Boehringer Mannheim-Roche. PA-1 antiserum was raised against the synthetic peptide EEREMKALSERVSIL that corresponds to the COOH-terminal region of the predicted amino acid sequence of PKD, as described previously (26 , 31) . Antibodies (PKD C-20, PKCζ C-20, PKCε C-15, PKCηC-15, PKCδ C-15, and PKCα C-15) used in Western blot analysis were obtained from Santa Cruz Biotechnology (Palo Alto, CA). The phospho-S744/S748 PKD/PKCμ antibody was obtained from Cell Signaling Technology (Beverly, MA). An antiserum that specifically recognizes pS916 was generously provided by Dr. Doreen Cantrell (Imperial Cancer Research Institute, London, United Kingdom). Fluorescence-conjugated goat-anti rabbit antibody was obtained from Sigma Chemical Co. Other items were from standard suppliers or as indicated in text.

Expression of Multiple PKC Isoforms, PKD, and PDB-mediated Activation of PKD in PANC-1 Cells.

PANC-1 cells have been used extensively as a model system to study the effects of growth factors on the biological behavior of human pancreatic cancer cells (7, 8, 9, 10, 11, 12, 13) . To identify PKC isoforms expressed in PANC-1 cells, lysates from these cells were subjected to Western blot analysis using PKC isoform-specific antibodies. As shown in Fig. 1A ⇓ , a major immunoreactive band corresponding to the predicted molecular masses of PKCα, PKCβ₁, PKCε, PKCη, and PKCζ were detected in PANC-1 cells. Isoform-specific matched controls were used to detect the presence of a band corresponding to different PKCs (results not shown). PKCδ, which inhibits growth and induces apoptosis in certain cancer cells (46 , 47) , is notably not detected by Western blot analysis in these cells as compared with a control lysate from IEC-6 cells, known to express PKCδ (48) . A similar pattern of expression of PKC isoforms was noted in other human pancreatic ductal adenocarcinoma cell lines, HPAF-II and MIAPaCa-2 (results not shown). Our results differ from an earlier report that indicated expression of only PKCα and PKCζ in human pancreatic cancer cell lines, including PANC-1 (22 , 49) . We have demonstrated here that the expression of PKCs in human pancreatic cancer cell line PANC-1 is not restrictive, similar to that observed in HPAC cells (20) .

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Fig. 1.

Detection of PKC isoforms, PKD, and PDB-mediated activation of PKD in PANC-1 cells. A, Western blot analysis of the cell lysates. Confluent cultures of PANC-1 cells were washed twice with Dulbecco’s PBS. The cells were then solubilized in 2 × SDS-PAGE sample buffer (as indicated in “Materials and Methods”). The lysates were analyzed by SDS-PAGE and transferred to Immobilon membranes. Western blots were performed with the use of isoform-specific polyclonal antisera against the different PKCs, as described in “Materials and Methods.” Isoform-specific matched controls were used to detect the presence or absence of a band corresponding to different PKCs (data not shown). Equivalent amount of protein (measured by Bradford assay) was loaded in each lane except for PKC α, which was one-third of the rest. B, Western blot analysis of the cell lysates, PKD IP and PKD activation. Left panel, confluent cultures of PANC-1 cells were washed twice with Dulbecco’s PBS. The cells were then solubilized in 2 × SDS-PAGE sample buffer (as indicated in “Materials and Methods”). The lysates were analyzed by SDS-PAGE and transferred to Immobilon membranes. Western blot was performed using the polyclonal antibody against the COOH-terminal region of mouse PKD (C-20). Equivalent amount of protein (measured by Bradford assay) was loaded. Middle panel, confluent cultures PANC-1 cells were lysed with lysis buffer (as indicated in “Materials and Methods”). The lysates were immunoprecipitated with the PA-1 antiserum in the absence (−) or presence (+) of the immunizing peptide (10 μg/ml) that corresponds to the COOH-terminal region of the predicted amino acid sequence of PKD. The IPs were analyzed by Western blotting using the polyclonal PKD antiserum (C-20) as described in “Materials and Methods.” Right panel, PKD activation in PANC-1 cells. Confluent cultures of PANC-1 cells, washed with PBS and serum-free medium (DMEM), were incubated in the absence (−) or in the presence (+) of 200 nm PDB for 10 min at 37°C. The cells were lysed with lysis buffer and immunoprecipitated with PA-1 antiserum in the absence (−) or presence (+) of the immunizing peptide (10 μg/ml). The PKD activity in the immunocomplexes was determined by IVK assay as described in “Materials and Methods,” followed by SDS-PAGE and autoradiography.

To demonstrate that PKD is expressed in PANC-1 cells, lysates from these cells were analyzed by Western blotting using a specific antibody that recognizes the COOH-terminal region of PKD (C-20). As shown in Fig. 1B ⇓ (left panel), a pair of immunoreactive band of ∼110 kDa, which corresponds to the molecular mass of PKD (23) , was detected by SDS-PAGE in PANC-1 cells.

A pair of prominent band migrating with an identical molecular mass (∼110 kDa) was also obtained when lysates from PANC-1 cells were immunoprecipitated with the PA-1 antiserum (see “Materials and Methods”), and the IPs were analyzed by Western blot using the polyclonal PKD antiserum, C-20. Detection of the 110 kDa doublet was completely blocked by the inclusion of the synthetic peptide EEREMKALSERVSIL (immunizing peptide) that corresponds to the COOH-terminal region of the predicted amino acid sequence of PKD during the immunoprecipitation (Fig. 1B ⇓ , middle panel). The band migrating with a lower molecular mass, the detection of which was also completely extinguished by the immunizing peptide, could represent either a proteolytic fragment of PKD or PKD2, a novel homologue with predicted molecular mass of 105 kDa (34) . These results clearly demonstrate the expression of PKD in PANC-1 cells.

Next, we examined whether phorbol esters can induce PKD activation in intact PANC-1 cells. We treated serum-starved cultures of cells with PDB for 10 min, lysed the cells, and immunoprecipitated PKD with the PA-1 antiserum. The resulting immunocomplexes were incubated with [γ-³²P]ATP, and the incorporation of ³²P into PKD was analyzed by SDS-PAGE and autoradiography. As shown in Fig. 1B ⇓ , right panel, stimulation of intact cells with 200 nm of PDB induced a striking increase in PKD activity (measured by PKD autophosphorylation), which was maintained during cell lysis and immunoprecipitation. We verified that the immunoprecipitation of the kinase activity in PKD immunocomplexes was also abolished by the inclusion of the immunizing peptide. These results demonstrate that PDB stimulation of intact human pancreatic cancer cells induces the conversion of PKD from a low basal catalytic activity to a state of high activity that persists during cell disruption and protein isolation by the immunoprecipitation procedure.

NT Induces PKD Activation in PANC-1 Cells.

We reported recently that NT stimulated rapid Ca²⁺ fluxes, ERK activation, and DNA synthesis in PANC-1 cells through NTR-1 (7) . To additionally elucidate the signaling events downstream of NTR-1 in PANC-1 cells, we examined whether NT can stimulate PKD activation in intact PANC-1 cells. We treated serum-starved cultures of PANC-1 cells with NT for increasing times, lysed the cells, immunoprecipitated PKD with the PA-1 antiserum, and performed IVK assay. As shown in Fig. 2A ⇓ , top panel, PKD isolated from unstimulated PANC-1 cells had low basal catalytic activity. Treatment of PANC-1 cells with NT induced a rapid and striking increase in PKD kinase activity that was maintained during cell lysis and immunoprecipitation. PKD activation was detectable within 1 min and reached a maximum (∼9-fold) after 15 min of NT stimulation. These results demonstrate that PKD activation is one of the early events induced by NT in pancreatic cancer cells.

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Fig. 2.

NT activates PKD in PANC-1 cells in a time- and dose-dependent manner. Confluent PANC-1 cells were washed with PBS and incubated in serum-free medium (DMEM) for 6 h. A, time course. Serum-starved cells were stimulated with 50 nm NT for the indicated time periods at 37°C. Top, cells were lysed with lysis buffer and immunoprecipitated with PA-1 antiserum. The PKD activity in the immunocomplexes was determined by IVK assay as described in “Materials and Methods,” followed by SDS-PAGE and autoradiography. The autoradiogram shown is representative of three independent experiments. Middle, an identical experiment was carried out in parallel where Western blot analysis using pS916 antiserum was performed after lysis of the treated cells with 2 × SDS-PAGE sample buffer as described in “Materials and Methods.” The same membrane was stripped and probed with polyclonal PKD antibody (C-20) in a similar fashion. The Western blot shown is representative of at least three independent experiments. Bottom, scanning densitometry. The results shown are means (n = 3) of the level of PKD activation by IVK obtained from scanning densitometry and are expressed as percentages of the maximum increase in phosphorylation obtained from cells incubated with 50 nm NT for 15 min at 37°C; bars, ±SE. B, dose response. Serum-starved cells were stimulated with various concentrations of NT for 10 min at 37°C. Top, cells were lysed with lysis buffer and immunoprecipitated with PA-1 antiserum. The PKD activity in the immunocomplexes was determined by IVK assay as described in “Materials and Methods,” followed by SDS-PAGE and autoradiography. The autoradiogram shown is representative of three independent experiments. Middle, an identical experiment was carried out in parallel where Western blot analysis using pS916 antiserum was performed after lysis of the treated cells with 2 × SDS-PAGE sample buffer as described in “Materials and Methods.” The same membrane was stripped and probed with polyclonal PKD antibody (C-20) in a similar fashion. The Western blot shown is representative of at least three independent experiments. Bottom, scanning densitometry. The results shown are means (n = 3) of the level of PKD activation by IVK obtained from scanning densitometry and are expressed as percentages of the maximum increase in phosphorylation obtained with 100 nm NT at 37°C; bars, ±SE.

Recently, an antiserum specifically recognizing the phosphorylated form of a PKD COOH-terminal residue, serine 916, was developed and used to detect in vivo autophosphorylation at this site by active PKD (36) . Thus, pS916 antiserum provides a novel approach for detecting conversion of PKD to an active form within cells. Here, lysates from PANC-1 cells stimulated with NT for various times were analyzed by SDS-PAGE followed by Western blot analysis with the pS916 antiserum. NT stimulation induced a dramatic increase in the immunoreactivity of the PKD band indicative of phosphorylation at serine 916 (Fig. 2A ⇓ , middle panel). An increase in PKD autophosphorylation was detectable within 2.5 min and reached a maximum (∼10-fold) after 10–15 min of NT stimulation. NT-induced PKD activation within cells declined gradually toward baseline levels. In contrast, PKD activation induced by PDB persisted for at least 180 min of treatment (results not shown). Thus, NT-induced PKD activation is transient when compared with that induced by PDB. Western blotting with PKD specific antiserum (C-20) confirmed that similar amounts of PKD protein were loaded into the gel after treatment with NT for different times (Fig. 2A ⇓ , bottom panel).

Stimulation of intact PANC-1 cells with increasing concentrations of NT for 10 min induced a striking dose-dependent increase in PKD activation, as judged by assays of IVK activity after immunoprecipitation or by Western blotting with pS916 antibody to detect autophosphorylation in intact cells (Fig. 2B) ⇓ . Again, Western blot analysis with a PKD-specific antibody (C-20) confirmed that equal amounts of PKD protein were loaded into the gel after treatment with increasing concentrations of NT for 10 min. Half-maximal PKD activation by NT was achieved at 10 nm (Fig. 2B) ⇓ .

NT Increases Transient and Rapid Translocation of PKD from the Cytosol to the Plasma Membrane in PANC-1 Cells.

In unstimulated cells, PKD has been localized to the cytosol and to several intracellular compartments including Golgi and mitochondria (29 , 50, 51, 52, 53, 54, 55) . It has been demonstrated recently that treatment of Swiss 3T3 fibroblasts with the GPCR agonist bombesin induces a striking and transient translocation of PKD from the cytosol to the plasma membrane (29 , 44 , 55) . Here, we analyzed the effect of NT stimulation on the intracellular distribution of endogenous PKD in PANC-1 cells. As illustrated by the confocal image presented in Fig. 3A ⇓ , endogenous PKD expressed in unstimulated PANC-1 cells was distributed throughout the cytosol and mostly excluded from the nucleus with basal fluorescent signal localized to the plasma membrane. The basal level of fluorescence seen in unstimulated cells (Fig. 3A) ⇓ represents plasma membrane localized serum-activated PKD. Stimulation with NT for 5 min induced rapid translocation of PKD to the plasma membrane, causing a striking increase in localized fluorescence in the plasma membrane at the cell periphery (Fig. 3B) ⇓ . The association of endogenous PKD with the plasma membrane was transient. The reverse translocation from the plasma membrane to the cytosol was nearly complete within 15 min of NT stimulation (Fig. 3C) ⇓ . The results presented in Fig. 3 ⇓ demonstrate that NT induces a prominent and transient translocation of endogenous PKD to the plasma membrane in PANC-1 cells.

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Fig. 3.

NT increases rapid PKD translocation to the membrane in PANC-1 cells. Confluent cultures of PANC-1 cells were washed with PBS and were trypsinized to determine the count in the Coulter Counting chamber (Z-1 model; Coulter Corporation). Approximately 80,000 cells/chamber were plated in the Lab-Tek double-chamber slides. NT-stimulated and unstimulated cells were subsequently fixed for 15 min at 25°C in 10% buffered formalin phosphate and permeabilized with 0.2% Triton X-100 in PBS for 5 min at 25°C. The fixed cells were stained with rabbit polyclonal PKD antibody (C-20) and subsequently with fluorescein-conjugated goat-antirabbit antibody as described in “Materials and Methods.” The samples were examined with a Leica TCS-SP upright laser-scanning confocal microscope (Leica), and images were captured as described in “Materials and Methods.” Bar, 10 μm. A, unstimulated cells; B, cells stimulated with NT for 5 min; and C, cells stimulated with NT for 15 min. The images shown are representative of three independent experiments.

NT Induces PKD Activation in PANC-1 Cells through a PKC-dependent Pathway.

Having established that NT induces striking PKD activation in PANC-1 cells, our next step was to determine the role of PKCs in mediating PKD activation in this system. Cultures of PANC-1 cells were treated with various concentrations of either GF-1 or Ro 31–8220, selective inhibitors of phorbol ester-sensitive isoforms of PKC (56, 57, 58) but not PKD (31 , 38 , 41) . As shown in Fig. 4A ⇓ , treatment with GF-1 potently blocked PKD activation induced by subsequent addition of NT in a concentration-dependent manner. In contrast, GF-1 added directly to the IVK assay, even at concentrations (0.1–1 μm) that abrogated NT-induced PKD activation in intact PANC-1 cells did not inhibit PKD activity. Fig. 4B ⇓ depicts similar results obtained with a structurally distinct PKC inhibitor Ro 31–8220 instead of GF-1. Ro 31–8220 blocked NT-induced PKD activation at a lower concentration than GF-1 (half-maximal inhibition obtained with Ro was at 0.25 μm as compared with 0.5 μm of GF-1). These results imply that GF-1 and Ro 31–8220 do not inhibit PKD activity directly but interfere with NT-mediated PKD activation in intact PANC-1 cells by blocking PKC.

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Fig. 4.

The PKC inhibitors GF-1 and Ro 31–8220 prevent PKD activation by NT in PANC-1 cells. Confluent PANC-1 cells were washed with PBS and incubated in serum-free medium (DMEM) for 6 h. A, GF-1 inhibits PKD activity. Serum-starved PANC-1 cells were incubated for 1 h with different concentrations of the PKC inhibitor GF-1 (top blot and •). Control cells (−) received equivalent amounts of solvent. The cultures were subsequently stimulated with 50 nm NT for 10 min at 37°C. Cells were lysed with lysis buffer and immunoprecipitated with PA-1 antiserum. The PKD activity in the immunocomplexes was determined by IVK assay as described in “Materials and Methods,” followed by SDS-PAGE and autoradiography (in vivo). Parallel cultures were treated with 50 nm NT for 10 min at 37°C (bottom blot and ○), lysed with lysis buffer and immunoprecipitated with PA-1 antiserum. The PKD activity was then determined by IVK assay in the absence or presence of indicated concentrations of GF-1 (in vitro). Graphed data represent average values (n = 3) of the level of PKD activation obtained from scanning densitometry and are expressed as percentages of the maximum increase in phosphorylation obtained with 50 nm NT for 10 min at 37°C; bars, ±SE. B, Ro 31–8220 inhibits PKD activity. Serum-starved PANC-1 cells were incubated for 1 h with different concentrations of a structurally unrelated PKC inhibitor Ro 31–8220 (top blot and •). Control cells (−) received equivalent amounts of solvent. The cultures were subsequently stimulated with 50 nm NT for 10 min at 37°C. Cells were lysed with lysis buffer and immunoprecipitated with PA-1 antiserum. The PKD activity in the immunocomplexes was determined by IVK assay as described in “Materials and Methods,” followed by SDS-PAGE and autoradiography (in vivo). Parallel cultures were treated with 50 nm NT for 10 min at 37°C (bottom blot and ○), lysed with lysis buffer, and immunoprecipitated with PA-1 antiserum. The PKD activity was then determined by IVK assay in the absence or presence of indicated concentrations of Ro 31–8220 (in vitro). Graphed data represent average values (n = 3) of the level of PKD activation obtained from scanning densitometry and are expressed as percentages of the maximum increase in phosphorylation obtained with 50 nm NT for 10 min at 37°C; bars, ±SE.

To confirm the requirement of PKC in mediating NT-induced PKD activation, we treated PANC-1 cells with PDB for 36 h to produce extensive depletion of classic and novel PKCs but not of PKD (48) and then challenged with NT for 10 min. In a parallel set of experiments, cells were stimulated with either NT or PDB for 10 min. As shown in Fig. 5A ⇓ , the increase in PKD activity induced by acute exposure to PDB or NT was completely abrogated by previous chronic treatment with PDB for 36 h. This result provides additional evidence indicating the requirement of PKC(s) for PKD activation.

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Fig. 5.

NT induces PKD activation in PANC-1 cells through a PKC-dependent pathway. A, NT induced PKD activity after prolonged PDB treatment. Confluent cultures of PANC-1 cells were incubated for 36 h either without (−) or with 200 nm PDB. The cultures were washed with DMEM and incubated for 2 h at 37°C in DMEM. The cells were then challenged for an additional 10 min with 50 nm NT at 37°C. Parallel set of cultures were incubated without (−) or with 200 nm PDB for 10 min at 37°C. Cells were lysed with lysis buffer and immunoprecipitated with PA-1 antiserum. The PKD activity in the immunocomplexes was determined by IVK assay as described in “Materials and Methods,” followed by SDS-PAGE and autoradiography. The autoradiogram shown is representative of two independent experiments. B, Wortmannin (Wort), rapamycin (Rap), genistein (Gen), and PD 98059 (PD) do not inhibit PKD activity. Confluent cultures of PANC-1 cells were washed with PBS and incubated in serum-free DMEM for 6 h. Serum-starved cells were incubated for 1 h with either 3.5 μm GF-1, 100 nm Wort, 20 nm Rap, 50 μm Gen, 10 μm PD, or an equivalent amount of solvent (−). Cells were subsequently challenged for 10 min with 50 nm NT at 37°C and lysed with lysis buffer followed by immunoprecipitation with PA-1 antiserum. The PKD activity in the immunocomplexes was determined by IVK assay as described in “Materials and Methods,” followed by SDS-PAGE and autoradiography. The autoradiogram shown is representative of three independent experiments. C, NT induces rapid and transient PKC-dependent phosphorylation of Ser744/Ser748 in PKD activation loop. Serum-starved PANC-1 cells were incubated with 50 nm NT for the indicated time periods at 37°C (top blot). The cells were lysed in 2 × SDS-PAGE sample buffer, and Western blot analysis was performed using pS744/S748 dual-specific antiserum as described in “Materials and Methods.” Parallel cultures were incubated for 1 h with indicated concentrations of either GF-1 or Ro 31–8220 and an equivalent amount of solvent (−). The cells were then stimulated without (−) and either with 50 nm NT (+) or 200 nm PDB (+) for 10 min at 37°C (bottom blot). Western blot analysis using pS744/S748 dual-specific antiserum was performed after lysis of the treated cells with 2 × SDS-PAGE sample buffer as described in “Materials and Methods.” Shown here is a representative autoluminogram from one experiment. Similar results were obtained in three independent experiments.

To additionally substantiate the specificity of the results obtained with PKC inhibitors GF-1 and Ro 31–8220, we treated PANC-1 cells with inhibitors of multiple pathways and examined PKD activation after the subsequent challenge with NT. Treatment with the phosphatidyl 3′-kinase inhibitor wortmannin (59 , 60) , the p70 ribosomal S6 kinase inhibitor rapamycin (61, 62, 63) , the broad spectrum protein tyrosine kinase inhibitor genistein (64) , and the mitogen-activated protein/ERK kinase inhibitor PD 98059 (65) , which prevents activation of ERKs, did not affect PKD activation in response to NT in PANC-1 cells (Fig. 5B) ⇓ . In contrast, GF-1, a selective PKC inhibitor, abrogated NT-induced PKD activation as measured both by the IVK assay and pS916 immunoreactivity in Western blot (Fig. 5B) ⇓ . These results demonstrate the specificity of the PKC inhibitors and indicate that these kinases are not upstream regulators of PKD. So, activation of PKD by NT in PANC-1 cells is dependent on phorbol ester-regulated PKC(s).

It has been reported recently that Ser744 and Ser748 located in the activation loop of PKD are transphosphorylated in vivo resulting in PKD activation (43) . To determine whether a similar mechanism operates in human pancreatic cancer cells, we examined whether NT stimulated Ser744/Ser748 phosphorylation in PANC-1 cells. As shown in Fig. 5C ⇓ (upper panel), NT induced a rapid and transient increase in phospho-Ser744/Ser748 immunoreactivity (indicative of Ser744/Ser748 phosphorylation). Western blot also demonstrated that treatment with this agonist induced an electrophoretic mobility shift of PKD Ser744/Ser748, which is indicative of their phosphorylation and subsequent activation. Next, we assessed the role of PKC in the phosphorylation of Ser744/Ser748 of PKD after stimulation with NT and PDB in PANC-1 cells. NT-induced phosphorylation of Ser744/Ser748 was potently blocked by the PKC inhibitors GF-1 and Ro 31–8220 in PANC-1 cells (Fig. 5C ⇓ , bottom panel). A similar result was obtained when the cells were stimulated with PDB instead of NT (Fig. 5C ⇓ , bottom panel). These results demonstrate that activation of PKD by NT is a rapid and transient event, and support a model of PKC-PKD phosphorylation cascade in PANC-1 cells.

PKD Reverse Translocation from the Plasma Membrane Is PKC-dependent in PANC-1 Cells.

We have shown in Fig. 3 ⇓ that NT induced a rapid translocation of PKD from the cytosol to the plasma membrane. As shown in Fig. 3C ⇓ , the rapid translocation of PKD to the plasma membrane is transient. PKD returns to the cytosol after 15 min of exposure to NT. Next, we examined whether PKCs are required for these processes. PANC-1 cells grown in serum were treated with PKC inhibitors GF-1 (Fig. 6, A–C) ⇓ and Ro 31–8220 (Fig. 6, D–F) ⇓ and subsequently challenged with NT for 5 min (Fig. 6, B and E) ⇓ and 15 min (Fig. 6, C and F) ⇓ . Cells treated with GF-1 (Fig. 6A) ⇓ or Ro 31–8220 (Fig. 6D) ⇓ alone demonstrated basal level of immunofluorescence indicative of serum-stimulated PKD localized to the plasma membrane. On stimulation with NT for 5 min, there was a significant increase in the level of localized fluorescence in the plasma membrane at the cell periphery (Fig. 6, B and E) ⇓ . These results indicate that PKC is not required for NT-induced translocation of PKD to the plasma membrane.

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Fig. 6.

NT-induced PKD reverse translocation from the membrane is PKC-dependent in PANC-1 cells. Confluent cultures of PANC-1 cells were washed with PBS and were trypsinized to determine the count in the Coulter Counting chamber (Z-1 model; Coulter Corporation). Approximately, 80,000 cells/chamber were plated in the Lab-Tek double-chamber slides. The cells were initially incubated with GF-1 or Ro 31–8220 and were subsequently challenged with NT for the indicated time periods as mentioned below. NT-stimulated and unstimulated cells were subsequently fixed for 15 min at 25°C in 10% buffered formalin phosphate and permeabilized with 0.2% Triton X-100 in PBS for 5 min at 25°C. The fixed cells were stained with rabbit polyclonal PKD antibody (C-20) and subsequently with fluorescein-conjugated goat-antirabbit antibody as described in “Materials and Methods.” The samples were examined with a Leica TCS-SP upright laser-scanning confocal microscope (Leica) and images were captured as described in “Materials and Methods.” Bar, 10 μm. A, cells were incubated with 1 μm GF-1 alone for 1 h at 37°C; B, cells were incubated with 1 μm GF-1 for 1 h and then challenged with 50 nm NT for 5 min at 37°C; C, cells were incubated with 1 μm GF-1 for 1 h and then challenged with 50 nm NT for 15 min at 37°C; D, cells were incubated with 1 μm Ro 31–8220 alone for 1 h at 37°C; E, cells were incubated with 1 μm Ro 31–8220 for 1 h and then challenged with 50 nm NT for 5 min at 37°C; and F, cells were incubated with 1 μm Ro 31–8220 for 1 h and then challenged with 50 nm NT for 15 min at 37°C. The images shown are representative of three independent experiments.

Interestingly, the level of localized immunofluorescence noted on the plasma membrane in cells treated with PKC inhibitors did not appreciably decrease after stimulation with NT for 15 min (Fig. 6, C and F) ⇓ in contrast to Fig. 3C ⇓ . This demonstrates that PKC activity is required for the rapid reverse translocation of PKD from the plasma membrane to the cytosol, which is consistent with recent data shown in other nontransformed cells (44) .

Effect of the PKC Inhibitor GF-1 on NT-induced DNA Synthesis in PANC-1 Cells.

The effect of receptor-mediated PKC activation on the regulation of DNA synthesis has not been examined previously in human pancreatic cancer cells. We have reported earlier that NT promotes DNA synthesis in PANC-1 cells in a dose-dependent manner signaling via NTR-1 receptor subtype (7) . Now, we examined whether NT receptor-mediated DNA synthesis is dependent on PKC activity in these cells.

Cultures of PANC-1 cells grown in medium containing 10% FBS were washed and transferred to serum-free medium for 6 h. To start the experiment, fresh serum-free medium containing NT, 10% FBS, or solvent were added to parallel cultures (n = 4 for each condition). After 16 h of incubation, the cultures were pulse labeled for 6 h with [³H]thymidine. As shown in Fig. 7A ⇓ , NT induced a significant increase in the incorporation of [³H]thymidine into acid-insoluble pools. These results demonstrate that NT, at concentrations that induced PKC/PKD activation, also induced DNA synthesis in PANC-1 cells.

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Fig. 7.

NT-induced DNA synthesis in PANC-1 cells is PKC-dependent. Cells incubated in serum-free medium were pulse-labeled with [³H]thymidine (0.25 μCi/ml), and the radioactivity incorporated into acid-insoluble pools was counted in a scintillation counter as described in “Materials and Methods.” A, DNA synthesis in cells treated with 50 nm NT, 10% FBS, and an equivalent amount of solvent (−). B, effect of PKC inhibitor, GF-1, on DNA synthesis in cells stimulated with NT. Cells were incubated with 1 μm GF-V (+) or 1 μm GF-1 (+) and an equivalent amount of solvent (−) for 1 h at 37°C. DNA synthesis was then measured in cells additionally stimulated without (−) and with (+) 50 nm NT at 37°C. C, dose-response for increasing concentration of GF-1 on NT-stimulated DNA synthesis. Cells were incubated with 1 μm GF-V (+) or indicated concentrations of GF-1 for 1 h at 37°C. DNA synthesis was measured in cells additionally stimulated with 50 nm NT at 37°C. The plot represents percentage maximal inhibition of DNA synthesis with GF-1 as compared with GF-V in NT-stimulated cells. Shown here is a representative plot from one experiment for each panel. Similar results were obtained in three independent experiments; bars, ±SE.

To determine the role of PKCs in NT-induced DNA synthesis, PANC-1 cells were treated with the selective PKC inhibitor, GF-1, and GF-V, an inactive analogue of GF-1, before addition of NT. DNA synthesis, as measured by [³H]thymidine incorporation, was again induced by NT and was not affected by addition of GF-V (Fig. 7B) ⇓ . NT alone stimulated DNA synthesis to the same level as depicted in Fig. 8A (result not shown). In striking contrast, GF-1 blocked NT-induced DNA synthesis in PANC-1 cells (Fig. 7B) ⇓ . The addition of GF-1 prevented the NT-induced increase in the incorporation of [³H]thymidine in a dose-dependent fashion (Fig. 7C) ⇓ . Increasing concentrations of GF-1 attenuated NT-induced DNA synthesis as compared with cells treated with highest concentration of GF-V (Fig. 7C) ⇓ . The results demonstrate that NT receptor-mediated activation of DNA synthesis is PKC-dependent in PANC-1 cells.