INTRODUCTION

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The nuclear lamins are karyophilic proteins located at the nucleoplasmic surface of the inner nuclear membrane where they assemble in a polymeric structure referred to as the nuclear lamina (for review, see Refs. 1-3). Lamins belong to the family of intermediate filaments, which share a tripartite organization consisting of a central -helical rod domain of conserved size, flanked by N- and C-terminal non--helical end domains of variable size and sequence (see Fig. 9). The lamina has been suggested to serve as a major chromatin anchoring site of nuclear scaffold-associated regions during interphase and possibly to be involved in organizing higher order chromatin domains. The lamina is a dynamic structure regulated by phosphorylation. Phosphorylation by p34^cdc2 kinase is key to the dissolution of the nuclear lamina during mitosis. Other lamin kinases include mitogen-associated protein kinases, c-AMP-dependent protein kinase (PKA)¹ and protein kinase C (PKC) (1-3). Major PKC phosphorylation sites have been mapped to serine residues located in close proximity to the nuclear localization signal in the C-terminal non--helical region, and phosphorylation of these residues interferes with the nuclear transport of lamin B (2). The p34^cdc2 phosphorylation sites are on both sides of the central -helical rod domain. While many mammalian cells contains three distinct lamins (lamins A, B, and C), human leukemia HL60 cells express primarily lamin B (4).

Lamin proteolysis during apoptosis has been reported in various cell lines treated with different stimuli. In human leukemia HL60 cells treated with etoposide (VP-16) (5) or camptothecin (CPT) (6), apoptosis is accompanied by diminished levels of lamin B. Etoposide is a topoisomerase II inhibitor (7) and CPT a topoisomerase I inhibitor (8). Both drugs are effective anti-cancer agents. Lamin B1 degradation was also reported to precede DNA fragmentation in apoptotic thymocytes (9) and in HeLa cells treated with anti-CD95 antibody (10). Lamin A and B proteolysis into 45-kDa fragments is also observed in apoptosis induced by serum starvation of ras transformed primary rat embryo cells (11) and in reconstituted cell-free systems (6, 12). The site of lamin A and B cleavage yielding the 45-kDa fragment has recently been mapped to a conserved aspartate residue at position 230 (13, 14) corresponding to a consensus sequence for caspases. Furthermore, lamin A has been shown to be cleaved by caspase 6 (Mch-2) but not caspase 3 (CPP32/YAMA) (13, 15).

The death-related cysteine proteases of the caspase family play a central role in the execution phase of apoptosis (16-19). Each caspase cleaves selectively a subset of cellular proteins. For instance, poly(ADP-ribose)polymerase is preferentially cleaved by caspase 3 (CPP32/Yama) (20, 21), and lamin A can be cleaved by caspase 6 (Mch-2) (13, 15). Interestingly, recent observations demonstrated that overexpression of mutant lamins A or B resistant to caspase cleavage delayed DNA fragmentation, suggesting that lamin cleavage participates in the activation of DNA fragmentation and nuclear apoptosis (14). Thus, lamins are presently the only known caspase substrates known to be directly involved in the execution phase of apoptosis.

Protein phosphorylation is probably important to regulate apoptosis (22). For instance, unscheduled activation of p34^cdc2 kinase, one of the lamin kinases (2), is associated with cytotoxic T lymphocyte-mediated apoptosis (23) and precedes CPT- and DNA damage-induced apoptosis in HL60 cells (24). In the present study, we investigated lamin B phosphorylation and degradation during apoptosis in response to camptothecin in HL60 cells and in a previously described cell-free system (6, 25-27). The identity of the lamin B protease is not known. Both caspases and the nuclear scaffold-associated serine protease have been suggested as candidate proteases (28, 29).

	MATERIALS AND METHODS

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Chemicals, Drugs, and Antibodies-- CPT, 7-hydroxystaurosporine (UCN-01), and flavopiridol were obtained from the NCI Drug Chemistry and Synthesis Branch. Drugs were freshly dissolved in dimethyl sulfoxide at 10 mM and further diluted in water prior to each experiment. Radiolabeled precursor [¹⁴C]thymidine (53.6 mCi/mmol) was purchased from NEN Life Science Products and [³²P]orthophosphate was obtained from Amersham Pharmacia Biotech. 3,4-Dichloroisocoumarin (DCI) was purchased from Boehringer Mannheim. All other chemicals were of reagent grade and purchased from Sigma or other local sources.

Cell Culture, Drug Treatment, DNA, and Protein Labeling-- Human promyelocytic leukemia HL60 cells were grown in suspension culture in RPMI 1640 medium supplemented with 10% fetal calf serum (Life Technologies, Inc.), 2 mM glutamine, 100 units/ml penicillin, and 100 µg/ml streptomycin at 37 °C in an atmosphere of 95% air and 5% CO₂. For filter elution assays, HL60 cells were incubated with [¹⁴C]thymidine for 1-doubling time (about 24 h). Cell cultures were then washed with fresh medium twice and chased in isotope-free medium overnight before drug treatment. Unless otherwise indicated, camptothecin treatments were with 5 µM. For in vivo phosphorylation, HL60 cells were washed twice in phosphate-free RPMI 1640 medium containing 10% dialyzed fetal calf serum, resuspended in the same medium and incubated with 250 µCi of [³²P]orthophosphate/10⁷ cells. Following 1-h incubation, the ³²P-labeled cells were washed twice and resuspended in phosphate-free RPMI 1640 with dialyzed serum for 30 min prior to drug treatment.

Isolation of Nuclei and Cytosol for Reconstituted Cell-free System Studies-- We followed our previously published procedure (25, 26). Briefly, untreated and treated cells (5 µM camptothecin for 3 h) were spun down, rinsed three times in cold PBS (phosphate-buffered saline), and resuspended at a density of approximately 10⁷ cells/ml in nucleus buffer (1 mM KH₂PO₄, 150 mM NaCl, 5 mM MgCl_2, 1 mM EGTA, 0.1 mM AEBSF, 0.15 unit/ml aprotinin, 1.0 mM Na₃VO₄, 5 mM HEPES, pH 7.4, 10% glycerol), including 0.3% Triton X-100. After incubation at 4 °C for 10 min and gentle agitation, cellular mixes were centrifuged at 2,000 × g for 10 min, rinsed once by centrifugation/resuspension in nucleus buffer without Triton X-100, and used as nuclei suspensions at a density of 1-2 × 10⁷ nuclei/ml. Supernatants were centrifuged at 10,000 × g for 10 min and used as cytosol. [¹⁴C]Thymidine-labeled cells were used to prepare nuclei for filter elution assay.

Filter Elution Assays for Measurement of DNA Fragmentation-- DNA fragmentation related to apoptosis was measured by filter elution as described previously (25, 30). Briefly, reaction mixtures were deposited onto protein-adsorbing filters (Metricel, Gelman Science, Ann Harbor, MI) and washed with 3 ml of nucleus buffer. This fraction (W) was collected. Lysis was performed with 5 ml of LS10 (2 M NaCl, 0.04 M Na₂EDTA, 0.2% Sarkosyl, pH 10) followed by washing with 5 ml of 0.02 mM Na₂EDTA, pH 10. The lysis (L) and EDTA (E) fractions were collected. All fractions (W, L, and E) and filters (F) were counted by liquid scintillation. DNA fragmentation was calculated as the percent of DNA eluting from the filter as: percent DNA fragmentation = 100 × (W + L + E)/(W + L + E + F). All experiments were repeated at least two or three times.

Immunoblotting for Lamin B-- After drug treatment, cells were washed in PBS and resuspended in reducing loading buffer (62.5 mM Tris-HCl, pH 6.8, 6 M urea, 10% glycerol, 2% SDS, 0.003% bromphenol blue, 5% 2-mercaptoethanol) and sonicated for 20 s. The lysates containing 2.5 × 10⁵ cells were heated at 65 °C for 15 min and then loaded in 12% SDS-polyacrylamide gels (precast gel from NOVEX, San Diego, CA). After electrophoresis, proteins were electrophoretically transferred from the gels to polyvinylidene difluoride membranes (Immobilon-P from Millipore Co., Bedford, MA) according to the manufacturer's protocol. Membranes were incubated at room temperature for 1 h in the primary antibody solutions after blocking in 5% non-fat dry milk solution for 1 h, followed by 1-h incubation with secondary antibody. Bands were visualized by enhanced chemiluminescence (SuperSignal, Pierce).

In Vivo Lamin B Phosphorylation-- After drug treatment, ³²P-labeled HL60 cells (10⁷ cells/sample) were washed in phosphate-free RPMI 1640 medium without serum once, and the cell pellets were lysed in buffer A (PBS containing 1% Nonidet P-40, 1 µg/ml leupeptin, 5 mM NaF, 1 mM Na₃VO₄, 2 mM AEBSF, 4 units/ml aprotinin, and 1% bovine serum albumin) with 0.4% SDS before sonication. The cell lysates were centrifuged at 14,500 rpm for 15 min, and the supernatants were mixed with 1.5 µg of anti-human lamin B antibody and 20% protein G-Sepharose suspensions in lysis buffer followed by overnight mixing at 4 °C. At the end of incubation, the immune complex was washed in buffer A and buffer A without bovine serum albumin. The immune complex was boiled for 10 min after adding 3 × SDS loading buffer. Samples were analyzed in 12% SDS-PAGE. Protein gels were dried up and subjected to autoradiography after being dried up.

In Vitro Lamin B Phosphorylation in Cell-free System-- The nuclei from untreated HL60 cells (1.5 × 10⁶ nuclei) were incubated with cytosol in the presence of [-³²P]ATP. After incubation, buffer A supplemented with 0.4% SDS was added to the samples before brief sonication. Afterward the procedures were the same as for in vivo lamin B phosphorylation.

In Vitro Histone H1 Kinase Assay for Protein Kinase C Activity-- After drug treatment, cells (10⁷ cells/sample) were harvested, washed in cold PBS once, and resuspended in 340 µl of lysis buffer (150 mM NaCl, 50 mM Tris-HCl, pH 8.0, 1% Nonidet P-40, 0.5% sodium deoxycholate, 0.1% SDS, 5 mM EGTA, 10 mM NaF, 5 µg/ml leupeptin, 1 mM Na₃VO₄, 1 mM AEBSF) followed by 30-min incubation on ice. The supernatant was collected after centrifugation at 14,500 rpm for 30 min and incubated with protein G-Sepharose and 1 µg of anti-PKC at 4 °C for 2 h to make PKC immunoprecipitates. The Sepharose beads associated with immunoprecipitates were washed in lysis buffer three times and twice in kinase buffer (50 mM Tris-HCl, pH 7.4, 10 mM NaF, 5 µg/ml leupeptin, 1 mM Na₃VO₄, 1 mM AEBSF, 2 mM MgCl₂, 0.5 mM EDTA, 0.5 mM EGTA). The washed beads were incubated with reaction buffer (20 mM Tris-HCl, pH 7.4, 10 mM MgCl₂, 10 µM ATP, 0.4 µg/ml histone H1, 5 µCi of [-³²P]ATP) in the presence of 1.2 mM CaCl₂, 40 µg/ml phosphatidylserine, and 3.3 µM dioleylglycerol at 30 °C for 10 min. After the incubation, 3 × SDS loading buffer was added to stop the kinase reaction and samples were boiled for 10 min. Samples were run on 12% SDS-PAGE, and the phosphorylated histone H1 bands were analyzed by PhosphorImager (Molecular Dynamics, Sunnyvale, CA).

Immunodepletion of PKC-- Anti-PKC antibody was incubated with protein G-Sepharose beads at 4 °C for 3 h. The beads were collected by centrifugation. After removal of the supernatant, the beads were washed once with nucleus buffer and incubated with cytosol from CPT-treated cells (CPT-cytosol:antibody = 5:1 (v/v)) overnight in a rotator at 4 °C. The beads were subsequently pelleted by centrifugation at 10,000 × g. The supernatant was subjected to immunoblotting for PKC and was used as CPT-cytosol immunodepleted of PKC. Mock-depleted CPT-cytosol was made just using nuclei buffer to replace antibody.

	RESULTS

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Lamin B Degradation and Phosphorylation in Apoptotic HL60 Cells-- HL60 cells are remarkably sensitive to various apoptotic stimuli, including chemotherapeutic DNA-damaging agents (5, 31) such as the topoisomerase I inhibitor camptothecin, protein kinase inhibitors (25, 32), and the Golgi poison, brefeldin A (33). Consistent with previous studies (31), Fig. 1A shows that camptothecin induces apoptotic DNA fragmentation in HL60 cells with rapid kinetics. Lamin B protein was cleaved with similar kinetics as the DNA fragmentation, yielding two cleavage bands (Fig. 1B) corresponding to 45- and 32-kDa polypeptides that were detected 3 h after the beginning of drug treatment.

View larger version (27K): [in this window] [in a new window]   Fig. 1.   DNA fragmentation and lamin B cleavage in apoptotic HL60 cells after treatment with camptothecin. A, [14C]thymidine-labeled cells were treated without (Control) or with camptothecin (CPT) for the indicated times. DNA fragmentation was assayed by filter elution. B, at the indicated times, cell lysates from camptothecin-treated cells were separated by SDS-PAGE. Lamin B was detected by Immunoblotting. Numbers to the left correspond to migration position of protein markers (in kilodaltons). Lamin B-specific bands are indicated to the right.

View larger version (20K): [in this window] [in a new window]   Fig. 2.   Lamin B phosphorylation in apoptotic HL60 cells. [32P]Orthophosphate-labeled HL60 cells were treated with camptothecin. Lamin B protein from cell extracts was immunoprecipitated with anti-lamin B antibody. A, samples were separated by 12% SDS-PAGE and processed by PhosphorImager. Lanes 1-4 are samples taken 0.5, 1, 2, and 3 h after addition of camptothecin. B, quantification of lamin B phosphorylation in camptothecin-treated (CPT) and control HL60 cells.

Cytosol from Apoptotic HL60 Cells Also Induced Lamin B Cleavage and Phosphorylation in Vitro-- We next used a cell-free system that we previously established to demonstrate the role of serine proteases in triggering apoptotic DNA fragmentation (25, 26, 34). Consistent with our previous results, the cytosol from apoptotic HL60 cells induced DNA fragmentation in nuclei from untreated HL60 cells (Fig. 3A). Cytosol from apoptotic cells also cleaved lamin B from naive nuclei to a 45-kDa product (Fig. 3B).

View larger version (27K): [in this window] [in a new window]   Fig. 3.   DNA fragmentation and lamin B degradation induced in untreated isolated nuclei by cytosol from apoptotic HL60 cells. A, nuclei from untreated [14C]thymidine-labeled HL60 cells were incubated at 37 °C for the indicated times with cytosol from untreated HL60 cells (Control Cytosol) or cells treated with camptothecin (CPT Cytosol). DNA fragmentation was measured by filter elution. B, naive nuclei were incubated at 37 °C with CPT-cytosol or control-cytosol for the indicated times. At the indicated times samples were analyzed by SDS-PAGE and lamin B immunoblotting.

View larger version (28K): [in this window] [in a new window]   Fig. 4.   Apoptosis-associated lamin B phosphorylation in a cell-free system. A, nuclei from untreated HL60 cells were incubated at 37 °C for the indicated times with cytosol from HL60 cells treated with camptothecin for 3 h (CPT Cytosol) or from untreated HL60 cells (Control Cytosol) in the presence of [-32P]ATP. After incubation, lamin B was immunoprecipitated from the reaction mixtures with anti-lamin B monoclonal antibody. Phosphorylated lamin B (32P Lamin B) was analyzed by a PhosphorImager after SDS-PAGE. A representative experiment is shown. B, lamin B phosphorylation induced by CPT-apoptotic cytosol was quantitated relative to untreated (control) cytosol.

Lamin B Cleavage, but Not Phosphorylation, in the Cell-free System Can Be Inhibited by the Serine Protease Inhibitor DCI-- We observed previously that DNA fragmentation induced by apoptotic cytosol could be inhibited by the serine protease inhibitor DCI (6). Fig. 5 shows while DCI blocked DNA fragmentation induced by CPT, lamin B cleavage also was abolished. The result suggested that serine protease activation was required for both DNA fragmentation and lamin B degradation. We next asked whether lamin B phosphorylation could be inhibited by DCI. Fig. 5C shows that lamin B phosphorylation was not affected by DCI. This finding suggests that protease activation does not affect lamin B phosphorylation.

View larger version (16K): [in this window] [in a new window]   Fig. 5.   Effects of the serine protease inhibitor DCI on DNA fragmentation, lamin B degradation, and phosphorylation in a cell-free system. Naive nuclei were incubated at 37 °C for 30 min with cytosol from apoptotic HL60 cells (CPT cytosol) or from untreated HL60 cells (Control cytosol) in the absence or presence of DCI. A, DNA fragmentation was measured by filter elution. B, Western blotting for lamin B protein. C, lamin B phosphorylation measured after immunoprecipitation with anti-lamin B antibody.

Investigation of the Lamin B Kinase during Apoptosis in HL60 Cells-- Cyclin B/Cdc2 (p34^cdc2) kinase is critical for lamin depolymerization during mitosis (2). We found that flavopiridol, a potent Cdk inhibitor (35, 36) could not inhibit lamin B phosphorylation induced by cytosol from apoptotic cells even at high concentrations (100 µM) (Fig. 6A). We also found that flavopiridol had not effect on either DNA fragmentation or lamin B cleavage in the cell-free system (Fig. 6, B and C). These observations suggested that p34^cdc2 kinase was not responsible for phosphorylation of lamin B by the apoptotic cytosol.

View larger version (14K): [in this window] [in a new window]   Fig. 6.   Effects of the cyclin-dependent protein kinase inhibitor flavopiridol on DNA fragmentation, lamin B degradation, and phosphorylation in vitro. Nuclei from untreated HL60 cells were incubated at 37 °C for 30 min with cytosol from apoptotic HL60 cells (CPT cytosol) or from untreated HL60 cells (Control cytosol) in the absence or presence of flavopiridol. A, lamin B phosphorylation. B, DNA fragmentation. C, Western blotting for lamin B protein.

View larger version (26K): [in this window] [in a new window]   Fig. 7.   Lamin B phosphorylation by apoptotic cytosol is prevented by the PKC inhibitor UCN-01, by anti-PKC antibody, and by immunodepletion of cytosolic PKC and is restored by purified PKC. Nuclei from untreated HL60 cells were incubated with cytosol from apoptotic cells (CPT cytosol) and [-32P]ATP in the absence or presence of various concentration of UCN-01 (A) or anti-PKC antibody 1.9 and/or PKC (B) for 5 min at 37 °C. C, inhibition of lamin B phosphorylation by immunodepletion of PKC. Cytosol from apoptotic HL60 cells (CPT cytosol) was incubated with or without anti-PKC antibody (2 µg/100 µl), including protein G-Sepharose at 4 °C overnight. Upper panel, lamin B phosphorylation in cell-free system was measured using supernatants. Lower panel, supernatants were subjected to Immunoblotting for PKC.

Protein Kinase C Is Activated with a Similar Kinetics as Lamin B Phosphorylation during Camptothecin-induced Apoptosis in HL60 Cells-- A recent study showed that PKC is activated in cytosol from apoptotic HL60 cells (40). Now we tested whether camptothecin also induces PKC activation in whole HL60 cells. As shown in Fig. 8, PKC activity increased rapidly during the first hour after the beginning of camptothecin treatment.

View larger version (24K): [in this window] [in a new window]   Fig. 8.   Induction of PKC activity in apoptotic HL60 cells after treatment with camptothecin. PKC immunoprecipitates were prepared from camptothecin (CPT)-treated and untreated (Control) HL60 cells and tested for PKC activity using histone H1 as substrate. A, representative PhosphorImager picture. 32P-H1, radiolabeled histone H1. B, quantification of PKC activity as percent of untreated control.

	DISCUSSION

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The present study is the first report of lamin B phosphorylation during apoptosis. We found that lamin B is phosphorylated within 1 h after the addition of the apoptotic inducer (camptothecin) and that lamin B phosphorylation persists for several hours as lamin B is being cleaved, and DNA fragmentation and complete apoptosis take place (31). Various kinases are involved in lamin phosphorylation, including p34^cdc2 kinase, mitogen-associated kinase, PKC, PKA (2). In particular, in mitosis, Cdc2 kinase is believed to play a critical role in lamin phosphorylation and thus disassembly of lamin polymers (2, 41). The present data suggest that PKC is critical for lamin B phosphorylation in HL60 cell during apoptosis for the following reasons. First, the PKC inhibitor, UCN-01 (38, 42) effectively blocked the lamin kinase in vitro, while the cyclin kinase inhibitor flavopiridol (35) was inactive. Second, lamin B phosphorylation was inhibited by a monoclonal antibody directed against the active site of PKC. Third, immunodepletion of apoptotic cell extracts with anti-PKC antibody inhibited lamin B phosphorylation, while addition of excess PKC restored lamin B phosphorylation. Fourth and finally, total PKC activity increased at the time of lamin B phosphorylation (40).

The lamin B protein kinase C phosphorylation sites have been mapped to serines 395 and 405 in HL60 cells following PKC activation by bryostatin (43) (Fig. 9). These sites are adjacent to the highly conserved central -helical rod domain, which is thought to be responsible for the formation of a highly stable coiled-coil dimer between two lamin molecules. It is also next to the nuclear translocation signal sequence (NLS) (Fig. 9). In the case of chicken lamin B2, phosphorylation by PKC in this region has been shown to alter recognition of this sequence and block nuclear import of newly synthesized lamin polypeptides (2). Recently, PKC-mediated lamin B phosphorylation during interphase has been shown to promote lamin B solubilization and nuclear lamina disassembly (37). Thus, PKC-mediated lamin B phosphorylation during apoptosis is likely to affect nuclear and chromatin structure.

View larger version (11K): [in this window] [in a new window]   Fig. 9.   Schematic diagram of cleavage and PKC phosphorylation sites of human lamin B. Hypothetical cleavage sites of caspases at aspartate 231 (14) and putative nuclear scaffold protease at tyrosine 377 are indicated above the domain organization of lamin B. PKC phosphorylation sites (43) (solid arrows) and a p34cdc2 kinase phosphorylation site (open arrowhead) (2) are shown at the bottom. The nuclear localization signal (NLS) is represented as an open box and the C-terminal CaaX motif as the black box. The C-terminal tetrapeptide referred to as the CaaX box (C = Cys; a = aliphatic amino acid; X = any amino acid) is subject to three successive posttranslational modifications (farnesylation, proteolytic trimming, and carboxymethylation), which are required for association of nuclear lamins with the nuclear membrane (2).

Proteolytic lamin degradation is a common and probably functionally important biochemical feature of apoptosis. It has been observed in all the apoptotic cell systems described to date, including HL60 cells treated with chemotherapeutic agents (5, 6, 34), activation-driven cell death of T cells (44), thymocyte apoptosis (9), and serum starvation in ras transformed embryo cells (11). Lamin degradation has also been reported in apoptosis induced by drICE in insect cells (45). Interestingly, a recent study of Rao et al. (14) demonstrated that overexpression of lamin A or B delayed nuclear apoptosis and DNA fragmentation in the case of p53-dependent apoptosis in rodent cells. These observations suggest that lamin cleavage plays an active role in the execution phase of apoptosis. Caspase 6 (Mch-2) has been shown to cleave lamin A at the conserved aspartic residue at position 230 (13, 14), and the corresponding lamin B cleavage site has been mapped to Asp²³¹ (14). This site is located in the conserved -helical rod domain (Fig. 9), and its cleavage would produce two polypeptides of 40.3 and 25.9 kDa, respectively. The observed 45-kDa fragment has been shown to correspond to the predicted 40.3 fragment (13, 14). Another candidate lamin protease is the nuclear scaffold protease (46), which would be expected to cleave lamin B at tyrosine 377 and to yield two polypeptides of 43.2 and 23 kDa, respectively (Fig. 9). Therefore, it is possible that serine proteases (6) might also be involved in lamin B cleavage during apoptosis to produce the 32-kDa lamin B fragment that we observed in addition to the 45-kDa polypeptide in apoptotic HL60 cells. Since the observed sizes indicate that cleavage of lamin B during apoptosis occurs in the conserved -helical rod domain, which is essential for lamin dimerization (Fig. 9), it is likely that lamin B cleavage should promote the dissolution of the nuclear lamina and affect nuclear condensation. This recent conclusion is consistent with the works of Rao et al. (14) and Lazebnik et al. (12), demonstrating an impairment of apoptotic chromatin condensation upon inhibition of lamin proteolysis. Chromatin condensation and DNA fragmentation might be related to the important function of the nuclear lamina as an anchorage structure for the chromatin scaffold-associated regions, which would organize the chromatin loop structures. Thus, it is possible that chromatin release from the nuclear lamina might facilitate the activity of nucleases and the cleavage and release of chromatin loops during apoptosis.