Modulation of vimentin containing intermediate filament distribution and phosphorylation in living fibroblasts by the cAMP-dependent protein kinase

The function of intermediate filaments in cell shape and cytoskeletal integrity

This study describes the development and use of a specific method for disassembling intermediate filament (IF) networks in living cells. It takes advantage of the disruptive effects of mimetic peptides derived from the amino acid sequence of the helix initiation 1A domain of IF protein chains. The results demonstrate that at 1:1 molar ratios, these peptides disassemble vimentin IF into small oligomeric complexes and monomers within 30 min at room temperature in vitro. Upon microinjection into cultured fibroblasts, these same peptides induce the rapid disassembly of IF networks. The disassembly process is accompanied by a dramatic alteration in cell shape and the destabilization of microtubule and actin-stress fiber networks. These changes in cell shape and IF assembly states are reversible. The results are discussed with respect to the roles of IF in cell shape and the maintenance of the integrity and mechanical properties of the cytoplasm, as well as the stability of the other major cytoskeletal systems.

The relative roles of specific N- and C-terminal phosphorylation sites in the disassembly of intermediate filament in mitotic BHK-21 cells

Previously we identified p34cdc2 as one of two protein kinases mediating the hyperphosphorylation and disassembly of vimentin in mitotic BHK-21 cells. In this paper, we identify the second kinase as a 37 kDa protein. This p37 protein kinase phosphorylates vimentin on two adjacent residues (thr-457 and ser-458) which are located in the C-terminal non-alpha-helical domain. Contrary to the p34cdc2 mediated N-terminal phosphorylation (at ser-55) which can disassemble vimentin intermediate filaments (IF) in vitro, p37 protein kinase phosphorylates vimentin-IF without obviously affecting its structure in vitro. We have further examined the in vivo role(s) of vimentin phosphorylation in the disassembly of the IF network in mitotic BHK cells by transient transfection assays. In untransfected BHK cells, the interphase vimentin IF networks are disassembled into non-filamentous aggregates when cells enter mitosis. Transfection of cells with vimentin cDNA lacking the p34cdc2 phosphorylation site (ser55:ala) effectively prevents mitotic cells from disassembling their IF. In contrast, apparently normal disassembly takes place in cells transfected with cDNA containing mutated p37 kinase phosphorylation sites (thr457:ala/ser458:ala). Transfection of cells with vimentin cDNAs lacking both the N- and C-terminal phosphorylation sites yields a phenotype indistinguishable from that obtained with the single N-terminal mutant. Taken together, our results demonstrate that the site-specific phosphorylation of the N-terminal domain, but not the C-terminal domain of vimentin plays an important role in determining the state of IF polymerization and supramolecular organization in mitotic cells.

Dynamics of bovine glial fibrillary acidic protein phosphorylation

Recently, the dynamic features of the intermediate filaments (IF) have been revealed. The effect of phosphorylation on the dynamics of bovine glial fibrillary acidic protein (GFAP), the astroglial IF, was studied in vitro with fluorescently labeled GFAP. Soluble GFAP in low ionic strength buffer was rapidly and fully phosphorylated to be used as phosphorylated GFAP. Assembly of GFAP was observed to be inhibited in proportion to the extent of phosphorylation by mixing phosphorylated and non-phosphorylated GFAP at various ratios, and phosphorylated GFAP could not be assembled with non-phosphorylated GFAP into filaments at all. Furthermore, the subunit exchange was suppressed in proportion to the extent of phosphorylation. Phosphorylation affects the dynamic equilibrium of GFAP, and contributes to breaking down GFAP frameworks in mitotic glial cells.

Detection of protein kinase activity specifically activated at metaphase-anaphase transition

We have previously reported that Ser13 and Ser34 on glial fibrillary acidic protein (GFAP) in the cleavage furrow of glioma cells are phosphorylated during late mitotic phase (Matsuoka, Y., K. Nishizawa, T. Yano, M. Shibata, S. Ando, T. Takahashi, and M. Inagaki. 1992, EMBO (Eur. Mol. Biol. Organ.) J. 11:2895-2902). This observation implies a possibility that there is a protein kinase specifically activated at metaphase-anaphase transition. To further analyze the cell cycle-dependent GFAP phosphorylation, we prepared monoclonal antibodies KT13 and KT34 which recognize the phosphorylation of GFAP at Ser13 and Ser34, respectively. Immunocytochemical studies with KT13 and KT34 revealed that the GFAP phosphorylation in the cleavage furrow during late mitotic phase occurred not only in glioma cells but also in human SW-13 and mouse Ltk- cells in which GFAP was ectopically expressed, thus the phosphorylation can be monitored in a wide range of cell types. Furthermore, we detected kinase activity which phosphorylates GFAP at Ser13 and Ser34 in the lysates of late mitotic cells but not in those of interphase cells or early mitotic cells. These results suggest that there exists a protein kinase which is specifically activated at the transition of metaphase to anaphase not only in GFAP-expressing cells but also in cells without GFAP.

Differential targeting of protein kinase C and CaM kinase II signalings to vimentin

Hydrolysis of inositol phospholipids by receptor stimulation activates two separate signaling pathways, one leading to the activation of protein kinase C (C kinase) via formation of diacylglycerol. The other is the inositol trisphosphate (IP3)/Ca2+ pathway and a major downstream kinase which is activated is Ca2+/calmodulin-dependent protein kinase II (CaM kinase II). To examine signaling pathways of C kinase and CaM kinase II to the cytoskeletal protein vimentin, we prepared monoclonal antibodies YT33 and MO82 which recognize the phosphorylation state of vimentin by C kinase and by CaM kinase II, respectively. Ectopic expression of constitutively active C kinase or CaM kinase II in primary cultured astrocytes by microinjection of the corresponding expression vectors induced phosphorylation of vimentin at each specific phosphorylation site, followed by reorganization of vimentin filament networks. In contrast, simultaneous activation of C kinase and CaM kinase II by inositol phospholipid hydrolysis with receptor stimulation led to an exclusive phosphorylation of vimentin at the CaM kinase II site, not at the site of C kinase. These results indicate that the intracellular targeting of C kinase and CaM kinase II signalings to vimentin is regulated separately, under physiological conditions.

Distribution of the intermediate filament proteins vimentin, keratin, and desmin in the bovine ovary

The distribution of the intermediate filament (IF) proteins desmin, keratin, and vimentin was studied immunohistochemically in bovine ovaries. Special attention was paid to granulosa cells to examine possible marked changes of IF distribution in relation to folliculogenesis during ovarian development. Therefore, ovaries were used from fetuses from 3 months of gestation onward, calves, heifers, and cows. In all ovaries, desmin immunoreactivity was restricted to smooth muscle cells in blood vessel walls. Keratin appeared a characteristic of the ovarian surface epithelium. Co-localization of keratin and vimentin was observed in the epithelium of rete ovarii tubules in fetuses and calves, and in cortical cord epithelium and pregranulosa cells of primordial follicles in fetuses at 3-7 months of gestation. Vimentin was demonstrated in endothelium and in fibroblasts. In addition, vimentin immunoreactivity was present in granulosa cells of primary, secondary, and antral follicles. In antral follicles, these granulosa cells mainly had an elongated appearance and either contained an oblong or a round nucleus. Those with an oblong nucleus were characteristic for atretic antral follicles. In nonatretic follicles, numerous vimentin immunoreactive, elongated granulosa cells with a round nucleus were observed, especially in the peripheral granulosa layer and in small ( < 3 mm in diameter) antral follicles. Additionally, in antral follicles, protrusions of vimentin-positive corona radiata cells were observed, that penetrated the zona pellucida to contact the oocyte. The data show that the distribution of vimentin containing IFs is associated with various aspects of granulosa cell activity, as mitosis, atresia, and intercellular transport.

Regulation of cytoskeletal organization in tumor cells by protein phosphatases-1 and -2A

Non-metastatic Lewis lung carcinoma cells (LLC-C8) become more motile when protein phosphatases (PP-1 and -2A) are inhibited by okadaic acid, attaining the same level of motility as metastatic LLC (LLC-LN7) variants. This stimulation of LLC-C8 motility was tempered when protein kinase A activity was inhibited. We examined whether the okadaic acid-stimulated LLC-C8 motility was associated with alterations in the cytoskeletal organization so that these non-metastatic cells acquire the rounded morphology and diffuse cytoskeletal organization previously described for metastatic LLC-LN7 cells. Non-metastatic LLC-C8 are typically adherent during culture, achieving a spread morphology. Treatment of non-metastatic LLC-C8 cells with okadaic acid resulted in a contraction of most of their extended processes, formation of spikes and membrane blebs within 10 min, and complete cell rounding within 20 min for most of the cells. While the overall level of F-actin was minimally affected by the okadaic acid, its uniform distribution shifted to localization toward the periphery of the rounded cells, often concentrating at a single focus. Immunofluorescent staining for vimentin showed a similar shift to the cell periphery and similar capping. After okadaic acid treatment, the filamentous network of microtubules in non-metastatic LLC-C8 cells disappeared and was replaced with a diffusely staining distribution of beta-tubulin. These results show that PP-1 and -2A maintain cytoskeletal organization and that inhibition of this control reduces cytoskeletal organization and increases tumor cell motility.

Inhibition of TPA and 12(S)-HETE-stimulated tumor cell adhesion by prostacyclin and its stable analogs: rationale for their antimetastatic effects

We have investigated the regulatory role of PGI2 and its stable analogs, i.e., iloprost and cicaprost, on 12(S)-HETE- and TPA-enhanced tumor cell integrin expression and adhesion. Walker 256 carcinosarcoma cells express alpha IIb beta 3 integrin receptors, which mediate their adhesion to endothelium, subendothelial matrix and fibronectin. Adhesion is enhanced by treatment with exogenous 12(S)-HETE but not 12(R)-HETE or other lipoxygenase-derived hydroxy fatty acids, as well as by TPA. Both 12(S)-HETE and TPA enhanced alpha IIb beta 3 expression on W256 cells. PGI2 iloprost and cicaprost inhibited both 12(S)-HETE- and TPA-enhanced adhesion to endothelium and subendothelial matrix as well as alpha IIb beta 3 expression on W256 cells. The mechanism responsible for the effect of PGI2 was explored. Prostacyclin treatment of W256 cells resulted in an enhanced production of cAMP in a time- and dose-dependent manner. Pre-treatment of tumor cells with increasing concentrations of adenosine resulted in a dose-dependent decrease in the PGI2 effect on TPA or 12(S)-HETE-enhanced adhesion, suggesting that the PGI2 effect is mediated through PKA. Dibutyryl cAMP also blocked the 12(S)-HETE- or TPA-enhanced adhesion, and adenosine pre-treatment did not result in an inhibition of the dibutyryl cAMP effect. Collectively, our results suggest that the cyclooxygenase metabolite PGI2 can antagonize the lipoxygenase metabolite 12(S)-HETE- and TPA-enhanced alpha IIb beta 3 expression and tumor cell adhesion via activation of adenylate cyclase and elevation of intracellular levels of cAMP.

Implications for cAMP-dependent protein kinase in the maintenance of the interphase state

The cAMP dependent protein kinase (A-kinase) is one of the first and best studied kinases in mammalian cells. There is extensive evidence that A-kinase activity acts antagonistically toward mitotic entry both in oocyte and somatic cells. Firstly, A-kinase seems to directly compromise the activation process of the cdc2 cyclin B mitotic kinase. Secondly, as shown by specific in vivo inhibition of A-kinase using microinjection of a stable form of its inhibitor peptide PKI, A-kinase modulates several key interphase cellular processes including cytoskeletal dynamics, transcription, chromatin structure and nuclear localization. We discuss the potential mechanisms involved in the down regulation of A-kinase activity at the interphase/mitosis transition.

Identification of the major physiologic phosphorylation site of human keratin 18: potential kinases and a role in filament reorganization

There is ample in vitro evidence that phosphorylation of intermediate filaments, including keratins, plays an important role in filament reorganization. In order to gain a better understanding of the function of intermediate filament phosphorylation, we sought to identify the major phosphorylation site of human keratin polypeptide 18 (K18) and study its role in filament assembly or reorganization. We generated a series of K18 ser-->ala mutations at potential phosphorylation sites, followed by expression in insect cells and comparison of the tryptic 32PO4-labeled patterns of the generated constructs. Using this approach, coupled with Edman degradation of the 32PO4-labeled tryptic peptides, and comparison with tryptic peptides analyzed after labeling normal human colonic tissues, we identified ser-52 as the major K18 physiologic phosphorylation site. Ser-52 in K18 is not glycosylated and matches consensus sequences for phosphorylation by CAM kinase, S6 kinase and protein kinase C, and all these kinases can phosphorylate K18 in vitro predominantly at that site. Expression of K18 ser-52-->ala mutant in mammalian cells showed minimal phosphorylation but no distinguishable difference in filament assembly when compared with wild-type K18. In contrast, the ser-52 mutation played a clear but nonexclusive role in filament reorganization, based on analysis of filament alterations in cells treated with okadaic acid or arrested at the G2/M stage of the cell cycle. Our results show that ser-52 is the major physiologic phosphorylation site of human K18 in interphase cells, and that its phosphorylation may play an in vivo role in filament reorganization.

Analysis of cellular phosphoproteins by two-dimensional gel electrophoresis: applications for cell signaling in normal and cancer cells

Two-dimensional (2-D) gel electrophoresis has been used to map proteins from various cell types in an effort to eventually link such maps to the sequencing of the entire human genome. While this analysis indicates the cellular disposition and expression of proteins, another application of 2-D gels, the analysis of phosphoproteins, can provide much information as to the assembly and "wiring" of the signal transduction circuits within cells which appear to be enervated by phosphate exchange. The preparation and separation of 32P-labeled proteins is described, as well as various analytical methods, including: the variety of gel systems available for specialist types of analyses, comparing 33P- and 32P-labeling of proteins, imaging techniques, phosphoamino analysis, phosphopeptide separation, identifying the amino acid groups that are phosphorylated, and the identification of phosphoproteins on 2-D gels by immunoprecipitation, corunning of purified proteins, comparative mapping and microsequencing, and by Western blotting. Examples (in brackets) are given of applications in which 2-D phosphogels can be applied, which offer advantages over other techniques. These include: (i) identifying in vivo substrates for kinases (protein kinase C activated by phorbol myristate acetate), (ii) investigating cytokine signaling pathways (tumor necrosis factor and interleukin-1), (iii) investigating the effects of drugs on signaling pathways (okadaic acid, menadione and cyclooxygenase inhibitors), (iv) characterization of specific phosphoproteins (heat-shock protein Hsp27 and stathmin), (v) comparing normal and transformed cells (MRC-5 human lung fibroblasts and their SV-40-transformed counterparts, MRC-5 SV1 cells), (vi) purifying phosphoproteins, (vii) investigating the relationship of protein phosphorylation to stages in the cell cycle (stathmin), (viii) investigating protein/protein interactions, (ix) mapping in vitro kinase substrates (protein kinase C, protein kinase A, and mitogen activated protein kinase activated protein kinase 2), and (x) locating and identifying cellular phosphatases (Hsp27 phosphatase). It is possible that the mapping of phosphoproteins can be linked to other 2-D gel databases and that information derived from these can be used in the future to better understand the signaling mechanisms of normal and cancerous cells.

Ontogeny of the cytoskeleton during mammalian oogenesis

Mammalian oogenesis is a process which requires a variety of changes in the structure and function of the specialized female germ cell. Evidence suggests that the cytoskeleton may mediate several of these structural and functional changes. In this review we evaluate what is known of cytoskeletal function during oogenesis, with emphasis on specialized cytoskeletal features in mammals. Existing investigations suggest that the oocyte, as a highly specialized cell, contains unique cytoskeletal elements which exhibit functions restricted to the process of early development.

Activation of the protein kinase a signal transduction pathway by granulocyte-macrophage colony-stimulating factor or by genetic manipulation reduces cytoskeletal organization in Lewis lung carcinoma variants

Granulocyte-macrophage colony-stimulating factor (GM-CSF) that is produced by metastatic Lewis lung carcinoma (LLC-LN7) cells functions as an autocrine stimulator of tumor-cell motility through protein kinase A (PKA) signal transduction. This GM-CSF-mediated enhancement of LLC-LN7 cell motility coincides with a reduction in the level of polymerized F-actin. In contrast, non-metastatic LLC-C8 tumor cells, which have a diminished level of PKA signaling, do not produce GM-CSF and do not respond to exogenous GM-CSF, since they remain non-motile and retain a high content of filamentous actin. The capacity of PKA to regulate the cytoskeletal organization of tumor cells was further studied with the use of LLC variants that had been stably transfected to over-express the C alpha subunit of PKA (CEV cells) or to express a mutant cAMP-resistant PKA RI alpha subunit resulting in a defective PKA (REV cells). When compared with wild-type metastatic LLC-LN7 cells, in which the F-actin staining was too diffuse to be clearly visualized microscopically, the PKA-defective REV-LN7 transfectants had an increased level of F-actin. In comparison with the wild-type non-metastatic LLC-C8 cells, which had a high content of F-actin, the CEV-C8 transfectants that over-expressed PKA activity had a reduced level of F-actin. The reduced polymerization of actin in these CEV-C8 transfectants was accompanied by reduced levels of the intermediate filament protein vimentin and a shift in the distribution both of F-actin and of vimentin to the periphery of the cells. These results show reduced cytoskeletal organization in metastatic LLC-LN7 cells as compared with that of non-metastatic LLC-C8 cells, and indicate that elevation of PKA activity, either by autologous GM-CSF or by genetic manipulation, diminishes cytoskeletal organization.

Transient increase in vimentin phosphorylation and vimentin-HSC70 association in 9L rat brain tumor cells experiencing heat-shock

Characteristic changes in vimentin were studied in 9L rat brain tumor cells treated at 45 degrees C. During heat-shock treatment, vimentin molecules were rapidly phosphorylated and reorganized from a filamentous form into a perinuclear higher-order structure that was less extractable by nonionic detergent. These effects were found to be highly transient, peaked at 30 min after the onset of heat-shock treatment, and subsided thereafter. Simultaneously, the solubility of the constitutively expressed heat-shock protein 70 (HSC70) was also temporarily decreased and the kinetics was identical to that of vimentin. The results indicated that HSC70 and vimentin were co-insolubilized during the heat-shock treatment. We propose that the reorganization of the intermediate filaments resulted from enhanced phosphorylation of vimentin leads to the concurrent association of HSC70 to the intermediate filaments. This process may play an essential role in regulating heat-shock genes.

Differences in vimentin distribution in glial cells in culture revealed with an antibody against a phosphorylated epitope

We have previously described that spatial and temporal changes in the organization of vimentin that are correlated with protein kinase C (PKC)-induced phosphorylation of vimentin can be detected with the mouse monoclonal antibody B3 in cultures of amoeboid microglia [Ciesielski-Treska et al. (1991) J. Neurosci. Res. 29, 362-378]. The antibodies were generated to native form of vimentin-containing filaments and antibody B3 reveals a restricted immunostaining of vimentin in glial cells from human, rat and mouse origin. In the present study we show the distribution of epitope B3 analyzed by immunofluorescence within defined populations of rat glial cells. Relatively high immunoreactivity was found in Type 1 astrocytes, Type 2 astrocytes and oligodendrocytes had low immunoreactivity. Although the results suggested that in Type 1 astrocytes the phosphorylated epitope is prominent, its phosphorylation was not found to be cell cycle-dependent, and appeared unrelated to the organizational changes of intermediate filaments associated with the morphological conversion of polygonal to stellate astrocytes. As expected, the immunofluorescence was increased by exposition of astrocyte cultures to an activator of PKC, confirming our previous conclusion that the immunoreactivity of the epitope B3 depends on PKC-mediated phosphorylation. In addition, the finding that the immunofluorescence of vimentin was more homogeneous in quiescent, serum-deprived astrocytes and also in astrocytes exposed to an inhibitor of protein synthesis, cycloheximide, may suggest that phosphorylation of the epitope B3 depends on a protein factor present in fetal calf serum. The immunofluorescence studies on cultures enriched in Type 2 astrocytes and in oligodendrocytes indicate that the epitope B3 is hypophosphorylated in glial cells of this lineage and becomes dephosphorylated after terminal differentiation of oligodendrocytes. These observations suggest that in Type 2 astrocytes and in oligodendrocytes the low level of phosphorylation of vimentin could be related to the down regulation in vimentin expression.

Phosphorylation and disruption of intermediate filament proteins in oligodendrocyte precursor cultures treated with calyculin A

Treatment of primary cultures of oligodendrocyte precursors with calyculin A, a potent inhibitor of protein phosphatases 1 and 2A, caused the phosphorylation of two intermediate filament components, nestin and vimentin. Phosphoamino acid analysis demonstrated that phosphorylation took place mainly on serine and to a lesser extent on threonine residues. In addition, calyculin A treatment caused a shift in the distribution of the two proteins from the Triton-X-100 insoluble fraction to the detergent soluble fraction as demonstrated by immunoblotting. This redistribution, which was evident within 15 min after treatment and was nearly completed by 90 min, was accompanied by a disruption of the intermediate filament network. Thus, both nestin and vimentin retracted from the cytoplasmic processes to form a large perikaryal ring as shown by immunocytochemical analysis. Both morphological and biochemical changes were reversed 2-5 hr after removal of calyculin A from the culture medium.

Movement of the free catalytic subunit of cAMP-dependent protein kinase into and out of the nucleus can be explained by diffusion

The catalytic (C) subunit of cyclic AMP (cAMP) dependent protein kinase (PKA) has previously been shown to enter and exit the nucleus of cells when intracellular cAMP is raised and lowered, respectively. To determine the mechanism of nuclear translocation, fluorescently labeled C subunit was injected into living REF52 fibroblasts either as free C subunit or in the form of holoenzyme (PKA) in which the catalytic and regulatory subunits were labeled with fluorescein and rhodamine, respectively. Quantification of nuclear and cytoplasmic fluorescence intensities revealed that free C subunit nuclear accumulation was most similar to that of macromolecules that diffuse into the nucleus. A glutathione S-transferase-C subunit fusion protein did not enter the nucleus following cytoplasmic microinjection. Puncturing the nuclear membrane did not decrease the nuclear concentration of C subunit, and C subunit entry into the nucleus did not appear to be saturable. Cooling or depleting cells of energy failed to block movement of C subunit into the nucleus. Photobleaching experiments showed that even after reaching equilibrium at high [cAMP], individual molecules of C subunit continued to leave the nucleus at approximately the same rate that they had originally entered. These results indicate that diffusion is sufficient to explain most aspects of C subunit subcellular localization.

Vimentin serves as a phosphate sink during the apparent activation of protein kinases by okadaic acid in mammalian cells

The vimentin contents of four mammalian cell lines originating from rat and human tissues were determined by immunoblotting and scanning densitometry. On per cell volume basis, vimentin content in 9L, KD, and HeLa cells was found to be 206.6, 151.6, and 19.1 ng/microliters, respectively. A431 cells were devoid of vimentin. Protein phosphorylation was augmented by treatment of 600 nM okadaic acid for 1 h in these cells. During the apparent activation of protein kinases, vimentin became hyperphosphorylated and the phosphorylation level of other nonvimentin phosphoproteins was relatively little affected in 9L and KD cells. In contrast, cytokeratins and other nonvimentin proteins were heavily phosphorylated in OA-treated HeLa and A431 cells. Regression analysis indicated that the relative increase in phosphorylation level of nonvimentin phosphoproteins was inversely correlated to the contents of vimentin in the four cell lines [r2 = -0.985]. These observations strongly suggest that vimentin acts as a phosphate sink by which the effects of "excess kinase activity" inflicted by phosphatases inhibition was attenuated.

Protein phosphatases limit tumor motility

Elevators of cAMP, such as prostaglandin E2 (PGE2), activate protein kinase A (PKA) and induce PKA-stimulated motility and metastasis by metastatic Lewis lung carcinoma cells (LLC-LN7). Non-metastatic LLC (LLC-C8) are unresponsive to cAMP elevation even though they are not deficient in the PKA enzymes. To determine whether this PKA unresponsiveness might be due to increased dephosphorylation by serine/threonine protein phosphatases (PP-1/2A) within non-metastatic LLC-C8, the effects of the PP-1/2A inhibitor okadaic acid on the migration and invasion by non-metastatic LLC-C8 cells was measured. Okadaic acid stimulated motility of non-metastatic LLC-C8 cells to a level that was comparable to that of metastatic LLC-LN7 cells. PGE2 further increased the motility of the non-metastatic LLC-C8 cells when okadaic acid was present, although not in the absence of okadaic acid. The stimulation of motility by okadaic acid was diminished when PKA activity was inhibited. Dose-response studies with concentrations of okadaic acid that selectively inhibited PP-2A or both PP-2A and PP-1 showed a progressive increase in migration of non-metastatic LLC-C8 cells, suggesting that both PP-1 and PP-2A limit their motility. By contrast, metastatic LLC-LN7 cells were more motile than were non-metastatic LLC-C8 cells, but this motility was only marginally affected by okadaic acid. Comparisons of the levels of PP-1/2A enzyme activities in the LLC variants showed more activity in non-metastatic LLC-C8 than in metastatic LLC-LN7 cells. The identity of the PP whose activity was increased in the non-metastatic LLC-C8 was assessed by using okadaic acid, which selectively inhibits PP-2A activity at low concentrations and PP-1 and PP-2A at high concentrations, and calyculin A, which inhibits PP-2A at a similar concentration to that affected by okadaic acid but is more potent at inhibiting PP-1. The inhibition of PP activities by okadaic acid and by calyculin A showed a pattern which suggested the presence both of PP-1 and of PP-2A in non-metastatic LLC-C8 cells, but the presence of PP-1 and a reduction in PP-2A in metastatic LLC-LN7 cells. The sum of these data suggests that PKA-stimulated motility is restricted both by PP-1 and by PP-2A in non-metastatic LLC, and that a deficiency in this restriction results in increased migration and invasion.

The roles of the rod end and the tail in vimentin IF assembly and IF network formation

Using mutagenesis, we investigated the importance of two vimentin domains: (a) a highly conserved segment near the carboxy end of the alpha-helical rod, and (b) the tail, with which the rod end is known to interact. As judged by in vitro filament assembly and expression in transiently transfected cells lacking an endogenous vimentin network, the rod-tail interaction is not essential for 10 nm filament structure in vitro or for formation of fibrous arrays in culture. However, when mutated, amino acid residues within the rod and the tail segments can cause perturbations in IF assembly and in IF network formation. Finally, our studies show that the vimentin tail seems to play a role both in thermodynamically stabilizing IF structure in vitro and in establishing proper IF networks in vivo.

Induction of vimentin modification and vimentin-HSP72 association by withangulatin A in 9L rat brain tumor cells

Withangulatin A induced cell rounding up and the morphological alteration resulted from the reorganization of all of the major cytoskeletal components, i.e., vimentin, tubulin, and actin, as revealed by immunofluorescence techniques. When the withangulatin A-treated cells changed to a round-up morphology, vimentin intermediate filaments were found to be collapsed and clustered around the nucleus. The alteration was accompanied by characteristic changes of vimentin molecules, including augmentation of phosphorylation, retardation of electrophoretic mobility, and decrease in detergent extractability. The levels of vimentin phosphorylation were augmented by 2.5- and 1.8-fold in cells incubated with 50 microM withangulatin A for 1 and 3 h, respectively. The electrophoretic mobility of vimentin was partially retarded in cells treated with withangulatin A for 1 h at 10 microM and a completely upshift mobility was observed after 5 h treatment at 50 microM. In addition, vimentin molecules became less extractable by nonident P-40 after the cells were treated with withangulatin A and this effect was dose dependent. The decrease in solubility of vimentin was accompanied by the redistribution of HSP72 into the detergent nonextractable fraction and these two events were well correlated. Our results suggest that withangulatin A induced the modification of vimentin, which resulted in the alteration of cell morphology and redistribution of intracellular HSP72, an event that may play an important role in the induction of heat-shock response.

p70s6k function is essential for G1 progression

An essential step in the pathway by which growth factors trigger cellular proliferation is the induction of high levels of protein synthesis. This appears in part to be controlled by multiple phosphorylation of the ribosomal protein S6 (refs 4, 5). The main kinase responsible, p70s6k (refs 6-8), is activated through the phosphorylation of four sites clustered in a putative autoinhibitory domain, which is mediated by a signalling pathway distinct from those used by other well characterized mitogen-activated serine/threonine kinases (such as p42/p44mapk or p90rsk; refs 10, 11). Here we investigate the role of p70s6k in the mitogenic response. Microinjection of quiescent rat embryo fibroblasts with any of three distinct polyclonal antibodies to p70s6k abolishes serum-induced entry into S phase of the cell cycle. This effect is preceded by almost complete abrogation of the activation of protein synthesis and the expression of an essential immediate early gene product, c-fos. The inhibitory effect on DNA synthesis is also elicited by microinjection of the antibodies late in G1 phase, consistent with the finding that p70s6k activity remains high throughout G1.

Identification of a vimentin-like function associated molecule (FAM) on rat NK cells: evidence for receptor function

Monoclonal antibody (MoAb) 5C6 specifically binds to fish, rat and human NK cells and inhibits cytotoxicity. The molecule recognized by this MoAb is a 50-53-kDa membrane protein on rat leukaemic NK (CRC) cells. In the present study, we have obtained a partial internal amino acid sequence from a purified 42-kDa fragment of the CRC-function associated molecule (FAM). Three tryptic peptide fragments were sequenced and each showed homology to intermediate filament vimentin sequences as deduced from (GenBank) mouse cDNA sequences. Amino acid composition analysis indicated that similar to cytoskeletal vimentin, the FAM contained a high percentage of non-polar amino acids. To further assess the similarities between this protein and vimentin, two commercially available anti-vimentin MoAbs and one anti-vimentin polyclonal antibody were tested for binding and inhibition of NK cytotoxicity. All anti-vimentin MoAbs inhibited killing by rat NWNA cells of appropriate targets. Anti-vimentin MoAb 13.2 bound to 41% of NWNA cells compared with approximately 58% binding for MoAb 5C6. Capping and sequential binding experiments showed that MoAb 5C6 effectively removed, from CRC-cell membranes, the protein recognized by MoAb V9. Sequential addition of these two MoAbs (MoAb 13.2 followed by MoAb V9) to CRC cells did not produce competitive binding. Biochemical and Western blot analysis of the vimentin-like protein obtained from CRC cells indicated that this protein has a molecular weight of 48-50 kDa, with an isoelectric point of pH 6.1-6.3. This protein is cross-reactive by Western blot analysis with anti-vimentin and anti-intermediate filament (IFA) antigen MoAbs but not with anti-desmin or anti-actin MoAbs. The molecular weight heterogeneity (43 versus 48-50 kDa) of the CRC protein was also examined. Western blot analysis of the CRC extract after different in vitro incubation times at 37 degrees C and 4 degrees C demonstrated that the 50-53-kDa 'native' protein degraded to a 42-kDa protein by 24 and 48 h respectively. This degradation was inhibitable by 10 mM EGTA. Evidence is presented which indicates that a vimentin-like protein on transformed rat NK cells may be an antigen binding receptor which initiates target cell lysis.

Role of phosphorylation in keratin and vimentin filament integrity in cultured thyroid epithelial cells

Cytokeratin and vimentin intermediate filaments (IFs) possess relatively stable polymeric properties which can be affected by phosphorylation. The present study, using cultures of thyroid epithelial cells, shows by indirect immunofluorescence that these cells contain both keratin tonofilament and vimentin IF complexes. Immunoblots of Triton X-100 insoluble cytoskeletal fractions show vimentin, and approximately 52 kDa type II and 40/38 kDa type I keratins. Under "basal" conditions, following prelabeling of cells with [32PO4], vimentin is not significantly phosphorylated, while both type II and I keratins are phosphorylated. Treatment of cells for 20 min with 1 mM dbcAMP or 0.4 microM 12-O-tetradecanoyl-phorbol-13-acetate (TPA), to stimulate protein kinase A and C, respectively, has no effect on either the phosphorylation state or cytoplasmic filament integrity of vimentin. However, while dbcAMP also does not affect keratin filaments, TPA increases both type II and I phosphorylation approximately 3-fold, and concomitantly disrupts tonofilament complexes associated with the nucleus, cytoplasm, and desmosomes. TPA-treated cells also show dramatic shape changes and protrusive activity. Tryptic peptide mappings show phosphorylations of at least 6 and approximately 2 additional sites for type II and I keratins, respectively, vs. [32P]-peptides from control cells. Treatment of [32PO4]-labeled cells with 0.4 microM calyculin A to inhibit types 1 and 2A phosphatase activity causes hyperphosphorylation of both vimentin and keratin, disruption of IF complexes, and actomyosin/cell contraction within 20 min. Quantitatively, approximately 50% of the type II/I keratin hyperphosphorylations are at some sites apparently also phosphorylated after TPA treatment. Thus, in these cells, IFs are specifically and differentially affected and regulated by the activity of several kinases.

PKC mediates 12(S)-HETE-induced cytoskeletal rearrangement in B16a melanoma cells

The fatty acid 12(S)-HETE may be a new second messenger capable of activating PKC. In tumor cells 12(S)-HETE stimulates cytoskeleton-dependent cellular responses such as adhesion and spreading. Analysis of 12(S)-HETE effects on B16a melanoma cell cytoskeleton revealed reversible rearrangement of microtubules, microfilaments, the actin-binding proteins, vinculin, myosin heavy (MHC) and light chains (MLC), as well as bundling of vimentin intermediate filaments. The alterations in microfilaments and intermediate filaments occurred very rapidly, i.e., 5 min after exposure of tumor cells to 12(S)-HETE. The 12(S)-HETE-induced cytoskeletal alterations were accompanied by centrifugal organelle-translocation. Interestingly, MLC exhibited clear association with the cytoplasmic organelles. Biochemical analysis of the 12(S)-HETE effect indicated a PKC-mediated reversible hyperphosphorylation of MLC, vimentin, and a 130 kD cytoskeletal-associated protein. Optimal effects were obtained after 5 min treatment with 12(S)-HETE at 0.1 microM concentration. 12(S)-HETE pretreatment induced tumor cell spreading on a fibronectin matrix which required the intactness of all three major cytoskeletal components. The spreading process was dependent upon the activity of PKC. Our data suggest that 12(S)-HETE is a physiological stimulant of PKC. Further, it induces rearrangement of the cytoskeleton of tumor cells in interphase resulting in the stimulation of cytoskeleton-dependent cell activity such as spreading.

Binding of nucleic acids to intermediate filaments of the vimentin type and their effects on filament formation and stability

Guanine-rich polynucleotides such as poly(dG), oligo(dG)12-18 or poly(rG) were shown to exert a strong inhibitory effect on vimentin filament assembly and also to cause disintegration of preformed filaments in vitro. Gold-labeled oligo(dG)25 was preferentially localized at the physical ends of the aggregation and disaggregation products and at sites along filaments with a basic periodicity of 22.7 nm. Similar effects were observed with heat-denatured eukaryotic nuclear DNA or total rRNA, although these nucleic acids could affect filament formation and structure only at ionic strengths lower than physiological. However, whenever filaments were formed or stayed intact, they appeared associated with the nucleic acids. These electron microscopic observations were corroborated by sucrose gradient analysis of complexes obtained from preformed vimentin filaments and radioactively labeled heteroduplexes. Among the duplexes of the DNA type, particularly poly(dG).poly(dC), and, of those of the RNA type, preferentially poly(rA).poly(rU), were carried by the filaments with high efficiency into the pellet fraction. Single-stranded 18S and 28S rRNA interacted only weakly with vimentin filaments. Nevertheless, in a mechanically undisturbed environment, vimentin filaments could be densely decorated with intact 40S and 60S ribosomal subunits as revealed by electron microscopy. These results indicate that, in contrast to single-stranded nucleic acids with their compact random coil configuration, double-stranded nucleic acids with their elongated and flexible shape have the capability to stably interact with the helically arranged, surface-exposed amino-terminal polypeptide chains of vimentin filaments. Such interactions might be of physiological relevance in regard to the transport and positioning of nucleic acids and nucleoprotein particles in the various compartments of eukaryotic cells. Conversely, nucleic acids might be capable of affecting the cytoplasmic organization of vimentin filament networks through their filament-destabilizing potentials.

Host cell factors controlling vimentin organization in the Xenopus oocyte

To study vimentin filament organization in vivo we injected Xenopus oocytes, which have no significant vimentin system of their own, with in vitro-synthesized RNAs encoding Xenopus vimentins. Exogenous vimentins were localized primarily to the cytoplasmic surface of the nucleus and to the subplasma membrane "cortex." In the cortex of the animal hemisphere, wild-type vimentin forms punctate structures and short filaments. In contrast, long anastomosing vimentin filaments are formed in the vegetal hemisphere cortex. This asymmetry in the organization of exogenous vimentin is similar to that of the endogenous keratin system (Klymkowsky, M. W., L. A. Maynell, and A. G. Polson. 1987. Development (Camb.). 100:543-557), which suggests that the same cellular factors are responsible for both. Before germinal vesicle breakdown, in the initial stage of oocyte maturation, large vimentin and keratin filament bundles appear in the animal hemisphere. As maturation proceeds, keratin filaments fragment into soluble oligomers (Klymkowsky, M. W., L. A. Maynell, and C. Nislow. 1991. J. Cell Biol. 114:787-797), while vimentin filaments remain intact and vimentin is hyperphosphorylated. To examine the role of MPF kinase in the M-phase reorganization of vimentin we deleted the conserved proline of vimentin's single MPF-kinase site; this mutation had no apparent effect on the prophase or M-phase behavior of vimentin. In contrast, deletion of amino acids 19-68 or 18-61 of the NH2-terminal "head" domain produced proteins that formed extended filaments in the animal hemisphere of the prophase oocyte. We suggest that the animal hemisphere cortex of the prophase oocyte contains a factor that actively suppresses the formation of extended vimentin filaments through a direct interaction with vimentin's head domain. During maturation this "suppressor of extended filaments" appears to be inactivated, leading to the formation of an extended vimentin filament system.

Fatty acid modulation of tumor cell-platelet-vessel wall interaction

Prostaglandins and other eicosanoids have been studied extensively in their physical, biochemical, biophysical and pharmacological aspects. However, studies on their role in tumor progression, especially metastases are relatively recent. Following a brief overview of the history of discovery and metabolism of eicosanoids and other fatty acids, we discuss the functions of these fatty acids (with emphasis on prostacyclin, thromboxane A2, 12-hydroxyeicosatetraenoic acid and 13-hydroxyoctadecadienoic acid) in cell transformation, tumor promotion and particularly in tumor cell metastasis. The relation between these monohydroxy fatty acids and tumor cell metastasis is discussed from three different perspectives, i.e., their effects on tumor cells, on platelets and on endothelial cells. The mechanism of these effects are then addressed at cell adhesion molecule, motility, protease, cell cytoskeleton, protein kinase and eicosanoid receptor levels. Finally, regulation of three key enzymes which generate eicosanoids (phospholipase, prostaglandin endoperoxide synthase and lipoxygenase) is explored.

Continuous growth of vimentin filaments in mouse fibroblasts

We have investigated the dynamics of intermediate filament assembly in vivo by following the fate of heterologous chicken vimentin subunits expressed under the control of an inducible promoter in transfected mouse fibroblasts. Using RNase protection, metabolic protein pulse-chase and immunofluorescence microscopy, we have examined the fate of newly assembled subunits under physiological conditions in situ. Following induction and subsequent removal of inducer, chicken vimentin mRNA had a half-life of approximately 6 h while both chicken and mouse vimentin protein polymer had long half-lives--roughly equivalent to the cell generation time. Moreover, following deinduction, chicken vimentin immunolocalization progressed from a continuous (8-10 h chase) to a discontinuous (&gt; or = 20 h chase) pattern. The continuous chicken vimentin staining reflects the uniform incorporation of chicken vimentin throughout the endogenous mouse vimentin network while the discontinuous or punctate chicken vimentin staining represents short interspersed segments of assembled chicken vimentin superimposed on the endogenous polymer. This punctate staining pattern of chicken vimentin was present throughout the entire array of intermediate filaments, with no bias toward the perinuclear region. These results are consistent with a continuous growth model of intermediate filament assembly, wherein subunit addition occurs at discrete sites located throughout the cytoskeleton.

Differential phosphorylation of CK8 and CK18 by 12-O-tetradecanoyl-phorbol-13-acetate in primary cultures of mouse hepatocytes

The phosphorylation of cytokeratin was investigated in primary cultures of hepatocytes. The two hepatocyte cytokeratins CK8 and CK18 (55,000 and 49,000 M(r) respectively) were phosphorylated, CK8 being more phosphorylated than CK18. Treatment of the hepatocytes with 150 nM 12-O-tetradecanoyl-phorbol-13-acetate (TPA) an activator of protein kinase C induced a transient increase in the level of phosphorylation of CK8 but not CK18. This effect was maximal after 15 min of TPA treatment and was maintained for up to 3 h. After 22 h of treatment with TPA, which down-regulates protein kinase C, CK8 phosphorylation was returned to the basal level. Further addition of TPA to the 22-h treated cells did not cause an increase in CK8 phosphorylation. Indirect immunofluorescence microscopy with a monoclonal antibody to CK8 indicated that while the addition of TPA induced the formation of granular cytokeratin aggregates in some hepatocytes, in most hepatocytes no major changes in the intermediate filament network were observed. Staining for actin showed that actin microfilaments were rapidly reorganized after the treatment and a loss of stress fibres were observed. We propose that CK8 is an in vivo substrate for protein kinase C and that the specific phosphorylation of CK8 plays a role in protein kinase C signal transduction.

Intermediate filaments: not so tough after all

The dynamic properties of cellular protein polymers such as microtubules and microfilaments depend to a large extent on the cell's capacity to modify rapidly the exchange rate between polymerized and unpolymerized pools of subunits. Until quite recently the dynamic nature of intermediate filaments was underestimated because of their biochemical stability in vitro and a paucity of studies on their characteristics in vivo. However, the recent studies described in this review show that the karyoskeletal and cytoskeletal structures that assemble from many intermediate filament proteins possess the properties expected of dynamic protein polymer networks.

Reversible hyperphosphorylation and reorganization of vimentin intermediate filaments by okadaic acid in 9L rat brain tumor cells

Okadaic acid (OA), a protein phosphatase inhibitor, was found to induce hyperphosphorylation and reorganization of vimentin intermediate filaments in 9L rat brain tumor cells. The process was dose dependent. Vimentin phosphorylation was initially enhanced by 400 nM OA in 30 min and reached maximal level (about 26-fold) when cells were treated with 400 nM OA for 90 min. Upon removal of OA, dephosphorylation of the hyperphosphorylated vimentin was observed and the levels of phosphorylation returned to that of the controls after the cells recovered under normal growing conditions for 11 h. The phosphorylation and dephosphorylation of vimentin induced by OA concomitantly resulted in reversible reorganization of vimentin filaments and alteration of cell morphology. Cells rounded up as they were entering mitosis in the presence of OA and returned to normal appearance after 11 h of recovery. Immuno-staining with anti-vimentin antibody revealed that vimentin filaments were disassembled and clustered around the nucleus when the cells were treated with OA but subsequently returned to the filamentous states when OA was removed. Two-dimensional electrophoresis analysis further revealed that hyperphosphorylation of vimentin generated at least seven isoforms having different isoelectric points. Furthermore, the enhanced vimentin phosphorylation was accompanied by changes in the detergent-solubility of the protein. In untreated cells, the detergent-soluble and -insoluble vimentins were of equal amounts but the solubility could be increased when vimentins were hyperphosphorylated in the presence of OA. Taken together, the results indicated that OA could be involved in reversible hyperphosphorylation and reorganization of vimentin intermediate filaments, which may play an important role in the structure-function regulation of cytoskeleton in the cell.

Changes in the cytoskeleton of 3T3 fibroblasts induced by the phosphatase inhibitor, calyculin-A

Addition of the protein phosphatase inhibitor, calyculin-A, to 3T3 fibroblasts causes a marked change in cell morphology. Initially the cells become rounded, develop surface blebs and then detach from the substratum. In the detached cells an unusual ball-like structure is observed. This study focuses on the cytoskeleton during these calyculin-A-induced morphological changes. Stress fibres disappear as the cells begin to round and aggregates of actin are formed towards the apical surface of the cell. These aggregates condense, in the detached cells, to form the ball structure of approximately 3 microns diameter. Between the ball and the nucleus are cables of intermediate filaments that appear to be attached to the surface of the ball and to the nuclear lamina. Using a procedure designed for the isolation of nuclei the nucleus-ball complex can be obtained. Analysis of the nucleus-ball preparation by immunofluorescence and electron microscopy demonstrate that the ball contains actin and that intermediate filaments are located between the ball and the nucleus. In this preparation, the intermediate filaments also appear to attach to the surfaces of the ball and the nucleus. Electrophoretic analysis of the nucleus-ball preparation indicates that, in addition to actin, a major component of the ball is myosin. It is suggested that the formation of the ball is caused by an actin-myosin-based contractile process, initiated by the phosphorylation of myosin. The aggregation of the actomyosin draws together the intermediate filaments into the area between the ball and nucleus. This hypothesis requires that vimentin binds both to the nucleus and to some component of the ball.

Increased assembly of cytoskeletal proteins associated with the transformation of human liver cells into fibroblast-like cells

A topoisomerase II inhibitor, novobiocin, and a deacetylase inhibitor, butyrate, synergistically transformed human liver cells into fibroblast-like cells. This morphological change was associated with an increased production of procollagen type III peptide and a simultaneous assembly of actin, tubulin, vimentin and cytokeratin. Novobiocin and butyrate had no marked effect on the phosphorylation state of cytokeratin proteins, but synergistically enhanced [3H]acetate uptake. From these results, it can be speculated that protein acetylation plays an important role in inducing the assembly of cytoskeletal proteins and the morphological transformation of human liver cells.

Purification and characterization of a vimentin-specific protease in mouse myeloid leukemia cells. Regulation during differentiation and identity with cathepsin G

Strong vimentin-degrading activity was found in a mouse myelomonocytic leukemic cell line, M1. When M1 cells were induced to differentiate into macrophage-like cells, this degrading activity decreased, while expression of the vimentin gene increased as reported previously [Tsuru, A., Nakamura, N., Takayama, E., Suzuki, Y., Hirayoshi, K. and Nagata, K. (1990) J. Cell Biol. 110, 1655-1664]. This activity was not due to calpain, which was reported to degrade vimentin, because it was independent of the presence or absence of Ca2+. This activity was revealed to be strongly associated with membranes by differential-centrifugation experiments. To identify this protease, purification of the degradation enzyme was performed. A membrane fraction was prepared and extracted with a buffer containing Triton X-100, then subjected to column chromatography using carboxymethyl-Sepharose and heparin-Sepharose. Quantitative analysis using the purified protease revealed that the specificity of this protease was more than 1000-fold higher for vimentin than for bovine serum albumin, ovalbumin and actin. Four protein bands expressing the activity were finally identified by SDS/PAGE. Amino-terminal sequences of these four proteins were identical, suggesting lower-molecular-mass proteins were degradative products. Furthermore, it was revealed that the sequence had the highest similarity with that of human cathepsin G. This result was consistent with the cathpsin-G-like properties of the purified protease, such as the optimum pH and the specificities for inhibitors. The purified protease degraded a synthetic substrate for cathespin G, succinyl-alanyl-alanyl-prolyl-phenylalanyl-p-nitroanilide, with a comparable specific activity to human cathespin G and was specifically detected with anti-(human cathepsin G) serum in immunoblot analysis. The purified protease thus belongs to the 'cathepsin G family', and perhaps is a mouse homologue of human cathepsin G.

Heat-resistant variants of the Chinese hamster ovary cell: alteration of cellular structure and expression of vimentin

Three heat-resistant mutant cell lines (78-1, 78-2, 78-3) were previously selected from Chinese hamster ovary cells. In this study, we investigated whether the differences in intrinsic thermal sensitivity result from alteration of stress protein levels or cellular structural changes. Although there was no significant difference in the levels of stress proteins, i.e., constitutive HSP70 in wild type and three heat-resistant mutant strains, there were marked differences in the amounts of vimentin among the cell lines. Two-dimensional gel electrophoresis and Western blot showed a 2.3-2.9-fold increase in the level of vimentin in the mutant cells under normal growth conditions. Northern blot also revealed higher amounts of vimentin mRNA in the mutant cells. Electron microscopy and immunofluorescence suggest that increased amounts of the vimentin-containing intermediate filaments are correlated with the heat-resistant phenotypes.

Stimulation of the metastatic properties of Lewis-lung-carcinoma cells by autologous granulocyte-macrophage colony-stimulating factor

Using both polymerase-chain-reaction analysis and the soft-agar colony-forming unit assay, granulocyte-macrophage colony-stimulating factor (GM-CSF) was shown to be expressed by cloned metastatic Lewis-lung-carcinoma (LLC-LN7) cells but not by non-metastatic LLC-C8 cells. Furthermore, the metastatic LLC-LN7 cells were shown to respond both to autologous GM-CSF and to exogenous recombinant GM-CSF (rGM-CSF). In the presence of neutralizing anti-GM-CSF antibodies, the metastatic LLC cells became less able to migrate or to adhere and invade through a reconstituted basement membrane. Moreover, the addition of rGM-CSF further enhanced the capacity of the metastatic LLC cells to adhere to the reconstituted basement membrane. This stimulation of metastatic properties of the LLC cells by either autologous or exogenous GM-CSF was associated with enhanced endogenous protein phosphorylation. Two proteins of approximately Mr 45,000 and Mr 64,000 were the dominant target proteins to be phosphorylated by the presence of GM-CSF. These results suggest that autologous GM-CSF may function as an autocrine stimulator of the metastatic properties of metastatic LLC cells.

Intermediate filament dynamics

The view of intermediate filaments as static cytoskeletal elements is changing. Studies of exogenous intermediate filament proteins, either microinjected or expressed from transfected genes, have demonstrated that a continuous incorporation of subunits into the polymerized filaments is taking place. This incorporation appears to be required for maintaining normal cytoplasmic networks of intermediate filaments. At the post-translational level, phosphorylation is an important factor in regulating dynamic aspects of intermediate filament organization and structure.

Intermediate filaments in Sertoli cells

Using immunohistochemical techniques both at light and electron microscopic levels, the arrangement and distribution of intermediate filaments in Sertoli cells of normal testis (in rat and human), during pre- and postnatal development (in rabbit, rat, and mouse) and under experimental and pathological conditions (human, rat), have been studied and related to the pertinent literature. Intermediate filaments are centered around the nucleus, where they apparently terminate in the nuclear envelope providing a perinuclear stable core area. From this area they radiate to the plasma membranes; apically often a close association with microtubules is seen. Basally, direct contacts of the filaments with focal adhesions occur, while the relationship to the different junctions of Sertoli cells is only incompletely elucidated. In the rat (not in human) a group of filaments is closely associated with the ectoplasmic specializations surrounding the head of elongating spermatids. Both in rat and human, changes in cell shape during the spermatogenic cycle are associated with a redistribution of intermediate filaments. As inferred from in vitro studies reported in the literature, these changes are at least partly hormone-dependent (vimentin phosphorylation subsequent to FSH stimulation) and influenced by local factors (basal lamina, germ cells). Intermediate filaments, therefore, are suggested to be involved in the hormone-dependent mechanical integration of exogenous and endogenous cell shaping forces. They permit a cycle-dependent compartmentation of the Sertoli cell into a perinuclear stable zone and a peripheral trafficking zone with fluctuating shape. The latter is important with respect to the germ cell-supporting surface of the cell which seems to limit the spermatogenetic potential of the male gonad.

Cyclin A is required for the onset of DNA replication in mammalian fibroblasts

Cyclin A protein is synthesized and localized into the nucleus at the onset of S phase in nontransformed mammalian fibroblasts. Inhibition of cyclin A synthesis or activity through microinjection of plasmids encoding antisense cyclin A cDNA or affinity-purified anti-cyclin A antibodies during G1 phase was shown to abolish the nuclear staining for cyclin A in plasmid-injected cells, and both procedures led to inhibition of DNA synthesis. No similar effect was observed with injection of other antisense vectors including antisense cyclin B, and reinjection of purified human cyclin A protein into cyclin A antisense-injected cells effectively relieved this inhibition of DNA synthesis. Taken together, these data suggest that cyclin A plays a major role in the control of DNA replication in mammalian cells.

Regulation of protein kinase A activation and prostaglandin E2-stimulated migration of Lewis lung carcinoma clones

Lewis lung carcinoma (LLC) clones were used in in vitro models for dissemination to identify mechanisms regulating the stimulation of metastatic LLC-LN7 migration by prostaglandin E2 (PGE2) or forskolin plus 3-isobutyl-I-methylxanthine (IBMX), and the lack of responsiveness to generated cAMP in non-metastatic LLC-C8 cells. The regulatory subunits of protein kinase A (PKA) from LLC-LN7 cells bound more 8-N3-32P-cAMP, even though production of regulatory subunits was equal to that in LLC-C8 cells. Protein kinase C (PKC) differentially regulated PKA activation in the LLC variants. PKC activation inhibited PGE2-stimulated migration by LLC-LN7 cells. Inhibition of PKC with staurosporine stimulated LLC-LN7 cell migration to a level comparable with that induced by PGE2. However, PGE2 did not further stimulate the migration of staurosporine-treated cells. The PGE2 or staurosporine stimulation of LLC-LN7 cell migration was dependent on PKA activation. The effects that modulation of PKA and PKC had on LLC-LN7 cell migration paralleled the effects on endogenous protein phosphorylation. LLC-LN7 cell autophosphorylation was stimulated to a similar degree by PGE2, forskolin plus IMBX, staurosporine, or the combination of staurosporine and forskolin plus IBMX. In contrast, neither migration nor autophosphorylation was stimulated in non-metastatic LLC-C8 cells by cAMP elevation or by PKC inhibition. Autophosphorylation, although not migration, of LLC-C8 cells was stimulated by forskolin plus IBMX when PKC activity was inhibited. These results suggest that the increased PKA response of metastatic LLC-LN7 cells is contributed by an increased binding of cAMP by the PKA regulatory subunits and a reduced level of regulation by PKC.

Cytokeratin phosphorylation, cytokeratin filament severing and the solubilization of the maternal mRNA Vg1

During meiotic maturation, the cortical cytokeratin filament system of the Xenopus oocyte disappears (Klymkowsky, M. W., and L. A. Maynell. 1989. Dev. Biol. 134:479). Here we demonstrate that this disappearance results from the severing of cytokeratin filaments into a heterogenous population of oligomers, with S- values ranging from 12S and greater. Cytokeratin filament severing correlates with the hyperphosphorylation of the type II cytokeratin of the oocyte. Both the severing of cytokeratin filaments and cytokeratin hyperphosphorylation are reversed by treatment with cycloheximide. These data suggest that fragmentation of cytokeratin filaments is controlled, at least in part, by the phosphorylation of the type II cytokeratin, and that the cytokeratin kinase activity responsible is biosynthetically labile. Cytokeratin filaments have been suggested to anchor the maternal mRNA Vg1 to the vegetal cortex of the oocyte (Pondel, M., and M. L. King. 1988. Proc. Natl. Acad. Sci. USA. 85:7216). By injecting fractions containing active maturation promoting factor or a purified, mutant cyclin protein, we find that the bulk of the Vg1 mRNA in the oocyte can be solubilized under conditions that block the fragmentation of cytokeratin filaments, and that the fragmentation of cytokeratin filaments itself leads to the solubilization of only a minor fraction of the Vg1 mRNA. Thus, at best, cytokeratin filaments directly anchor only a minor fraction of the Vg1 mRNA in the oocyte. Moreover, factors distinct from maturation promoting factor appear to be required for the complete solubilization of Vg1 mRNA during oocyte maturation.