Actin stress fiber disruption and tropomyosin isoform switching in normal thyroid epithelial cells stimulated by thyrotropin and phorbol esters

Persistent cAMP-signals triggered by internalized G-protein-coupled receptors

Real-time monitoring of G-protein-coupled receptor (GPCR) signaling in native cells suggests that the receptor for thyroid stimulating hormone remains active after internalization, challenging the current model for GPCR signaling.

G-protein–coupled receptors (GPCRs) are generally thought to signal to second messengers like cyclic AMP (cAMP) from the cell surface and to become internalized upon repeated or prolonged stimulation. Once internalized, they are supposed to stop signaling to second messengers but may trigger nonclassical signals such as mitogen-activated protein kinase (MAPK) activation. Here, we show that a GPCR continues to stimulate cAMP production in a sustained manner after internalization. We generated transgenic mice with ubiquitous expression of a fluorescent sensor for cAMP and studied cAMP responses to thyroid-stimulating hormone (TSH) in native, 3-D thyroid follicles isolated from these mice. TSH stimulation caused internalization of the TSH receptors into a pre-Golgi compartment in close association with G-protein α_s-subunits and adenylyl cyclase III. Receptors internalized together with TSH and produced downstream cellular responses that were distinct from those triggered by cell surface receptors. These data suggest that classical paradigms of GPCR signaling may need revision, as they indicate that cAMP signaling by GPCRs may occur both at the cell surface and from intracellular sites, but with different consequences for the cell.

Cells respond to many environmental cues through the activity of cell surface receptor proteins, which sense these cues and convey that information to signaling molecules inside the cell. G-protein–coupled receptors (GPCRs) form the largest eukaryotic family of plasma membrane receptors. They convert the information provided by extracellular stimuli into intracellular second messengers, like cyclic AMP (cAMP). After prolonged stimulation, they are internalized inside cells, an event that to date has been thought to terminate the production of second messengers. Though many of the key steps of GPCR signaling are known in detail, precisely how signaling and termination actually occur in time and space (i.e., in subcellular compartments or microdomains) is still largely unexplored. To observe GPCR signaling in living cells, we generated mice expressing a fluorescent sensor that allows monitoring the intracellular levels of cAMP with a microscope. We utilized this system to study, directly in native thyroid follicles, the signal sent by the receptor for thyroid-stimulating hormone (TSH). Our findings indicate that TSH receptors are internalized rapidly after activation but continue to stimulate cAMP production inside cells and that this sustained, cAMP production is apparently required for localized activation of downstream components. These data challenge the current model of the GPCR-cAMP pathway by suggesting the existence of previously unrecognized intracellular site(s) for cAMP generation and of differential signaling outcomes as a result of intracellular GPCR signaling. Such intracellular site(s) may provide specialized signaling platforms, thus contributing to the spatiotemporal regulation of cAMP production and to signaling specificity within the GPCR family.

Tropomyosin-based regulation of the actin cytoskeleton in time and space

Tropomyosins are rodlike coiled coil dimers that form continuous polymers along the major groove of most actin filaments. In striated muscle, tropomyosin regulates the actin-myosin interaction and, hence, contraction of muscle. Tropomyosin also contributes to most, if not all, functions of the actin cytoskeleton, and its role is essential for the viability of a wide range of organisms. The ability of tropomyosin to contribute to the many functions of the actin cytoskeleton is related to the temporal and spatial regulation of expression of tropomyosin isoforms. Qualitative and quantitative changes in tropomyosin isoform expression accompany morphogenesis in a range of cell types. The isoforms are segregated to different intracellular pools of actin filaments and confer different properties to these filaments. Mutations in tropomyosins are directly involved in cardiac and skeletal muscle diseases. Alterations in tropomyosin expression directly contribute to the growth and spread of cancer. The functional specificity of tropomyosins is related to the collaborative interactions of the isoforms with different actin binding proteins such as cofilin, gelsolin, Arp 2/3, myosin, caldesmon, and tropomodulin. It is proposed that local changes in signaling activity may be sufficient to drive the assembly of isoform-specific complexes at different intracellular sites.

Cyclic adenosine 3',5'-monophosphate (cAMP)-dependent protein kinases, but not exchange proteins directly activated by cAMP (Epac), mediate thyrotropin/cAMP-dependent regulation of thyroid cells

TSH, mainly acting through cAMP, is the principal physiological regulator of thyroid gland function, differentiation expression, and cell proliferation. Both cAMP-dependent protein kinases [protein kinase A (PKA)] and the guanine-nucleotide-exchange factors for Rap proteins, exchange proteins directly activated by cAMP (Epac) 1 and Epac2, are known to mediate a broad range of effects of cAMP in various cell systems. In the present study, we found a high expression of Epac1 in dog thyrocytes, which was further increased in response to TSH stimulation. Epac1 was localized in the perinuclear region. Epac2 showed little or no expression. The TSH-induced activation of Rap1 was presumably mediated by Epac1 because it was mimicked by the Epac-selective cAMP analog (8-p-chloro-phenyl-thio-2'-O-methyl-cAMP) and not by PKA-selective cAMP analogs. Surprisingly, in view of the high Epac1 expression and its TSH responsiveness, all the cAMP-dependent functions of TSH in cultures or tissue incubations of dog thyroid, including acute stimulation of thyroid hormone secretion, H(2)O(2) generation, actin cytoskeleton reorganization, p70(S6K1) activity, delayed stimulation of differentiation expression, and mitogenesis, were induced only by PKA-selective cAMP analogs. The Epac activator 8-p-chloro-phenyl-thio-2'-O-methyl-cAMP, used alone or combined with PKA-selective cAMP analogs, had no measurable effect on any of these TSH targets. Therefore, PKA activation seems to mediate all the recognized cAMP-dependent effects of TSH and is thus presumably responsible for the pathological consequences of its deregulation. The role of Epac1 and TSH-stimulated Rap1 activation in thyrocytes is still elusive.

Differential involvement of the actin cytoskeleton in differentiation and mitogenesis of thyroid cells: inactivation of Rho proteins contributes to cyclic adenosine monophosphate-dependent gene expression but prevents mitogenesis

In thyroid epithelial cells, TSH via cAMP induces a rounding up of the cells associated with actin stress fiber disruption, expression of differentiation genes and cell cycle progression. Here we have evaluated the role of small G proteins of the Rho family and their impact on the actin cytoskeleton in these different processes in primary cultures of canine thyrocytes. TSH and forskolin, but not growth factors, rapidly inactivated RhoA, Rac1, and Cdc42, as assayed by detection of GTP-bound forms. Using toxins that inactivate Rho proteins (toxin B, C3 exoenzyme) or activate them [cytotoxic necrotizing factor 1 (CNF1)], in comparison with disruption of the actin cytoskeleton by dihydrocytochalasin B (DCB) or latrunculin, two unexpected conclusions were reached: 1) inactivation of Rho proteins by cAMP, by disorganizing actin microfilaments and inducing cell retraction, could be necessary and sufficient to mediate at least part of the cAMP-dependent induction of thyroglobulin and thyroid oxidases, but only partly necessary for the induction of Na(+)/I(-) symporter and thyroperoxidase; 2) as indicated by the effect of their inhibition by toxin B and C3, some residual activity of Rho proteins could be required for the induction by cAMP-dependent or -independent mitogenic cascades of DNA synthesis and retinoblastoma protein (pRb) phosphorylation, through mechanisms targeting the activity, but not the stimulated assembly, of cyclin D3-cyclin-dependent kinase 4 complexes. However, at variance with current concepts mostly derived from fibroblast models, DNA synthesis induction and cyclin D3-cyclin-dependent kinase 4 activation were resistant to actin depolymerization by dihydrocytochalasin B in canine thyrocytes, which provides a first such example in a normal adherent cell.

Effects of thyroid-stimulating hormone on the replicative cycle of vesicular stomatitis virus in primary cultured sheep thyroid cells

Sheep thyroid cells in primary culture are highly sensitive to thyroid stimulating hormone (TSH). We infected thyroid cells with vesicular stomatitis virus (VSV) in the course of studies on cell polarity, and we found that TSH augmented the speed of the replicative cycle of VSV but did not affect the final yield of the virus. Three hours post-infection, at a multiplicity of infection of 10, the virus was detected in the cell layer of the cultures incubated with TSH but not in those without TSH. Five hours post-infection, there was a 100-fold increase in the medium in the yield of VSV and a 60-fold increase in the cell-associated virus in the TSH-treated cells compared with the cells without TSH. We found that the early stages of infection were accelerated by TSH. This effect appears to be due, at least in part, to increased processing in the lysosomes, thus allowing deposition of the transcriptionally-active nucleocapsid into the cytoplasm. These studies show that TSH is critically involved in the infectivity of VSV and that by manipulating cell culture conditions, an increased rate of virus production can be achieved.

Tropomyosin isoforms in nonmuscle cells

Vertebrate nonmuscle cells, such as human and rat fibroblasts, express multiple isoforms of tropomyosin, which are generated from four different genes and a combination of alternative promoter activities and alternative splicing. The amino acid variability among these isoforms is primarily restricted to three alternatively spliced exon regions; an amino-terminal region, an internal exon, and a carboxyl-terminal exon. Recent evidence reveals that these variable exon regions encode amino acid sequences that may dictate isoform-specific functions. The differential expression of tropomyosin isoforms found in cell transformation and cell differentiation, as well as the differential localization of tropomyosin isoforms in some types of culture cells and developing neurons suggest a differential isoform function in vivo. Tropomyosin in striated muscle works together with the troponin complex to regulate muscle contraction in a Ca(2+)-dependent fashion. Both in vitro and in vivo evidence suggest that multiple isoforms of tropomyosin in nonmuscle cells may be required for regulating actin filament stability, intracellular granule movement, cell shape determination, and cytokinesis. Tropomyosin-binding proteins such as caldesmon, tropomodulin, and other unidentified proteins may be required for some of these functions. Strong evidence for the distinct functions carried out by different tropomyosin isoforms has been generated from genetic analysis of yeast and Drosophila tropomyosin mutants.

Suppression of syntheses of high molecular weight nonmuscle tropomyosins in macrophages

In mouse fibroblasts, at least five TM isoforms are identified and they can be grouped into the high (TM1, TM2, and TM3) and low molecular weight TM isoforms (TM4 and TM5). Suppression of one of the high molecular weight tropomyosin (TM) isoforms in nonmuscle cells is implicated to be one of the causes for disorganization of actin microfilament bundles and subsequent changes in cell motility and cell shape. In this study, we studied the expression of tropomyosin isoforms in macrophages that exhibit high motility and ability to change cell shape. Two-dimensional gel electrophoresis followed by Western blot analysis using polyclonal anti-TM antiserum revealed that the high molecular weight TM isoforms were lacking in both resident and activated mouse peritoneal macrophages. Analyses of newly synthesized TM isoforms, Northern blot analyses using isoform-specific cDNA probes, and immunostaining with monoclonal anti-TM antibody that recognizes only the high molecular weight TM isoforms also demonstrated that the syntheses of the high molecular weight TM isoforms (TM1, TM2, and TM3) were completely suppressed, whereas the low molecular weight TM isoforms (TM4 and TM5) were expressed in macrophages. These results indicate that macrophages intrinsically lack the high molecular weight TM isoforms. In order to obtain information about cellular localization of the low molecular weight TM isoforms in macrophages, they were immunostained with polyclonal anti-TM antiserum that recognizes both the high and low molecular weight TM isoforms. The results showed that the low molecular weight TM isoforms were co-localized with F-actin in punctate and short fibrous structures. In addition, we performed in situ hybridization analysis to examine localizations of the TM mRNAs in fibroblasts and macrophages. The results showed that TM mRNAs were localized throughout the cytoplasm.

Coordinated expression of five tropomyosin isoforms and beta-actin in astrocytes treated with dibutyryl cAMP and cytochalasin D

Cytochalasin D and dBcAMP cause cultured astrocytes to change from flat cells to retracted process-bearing cells. F-actin was present throughout cells stimulated with dBcAMP for 16 h, whereas cytochalasin D caused F-actin to form massive aggregates at the tips of the cell processes. The two drugs differently regulated the expression of both beta-actin and tropomyosin genes in astrocytes cultured in the presence or absence of serum: dBcAMP caused down-regulation and cytochalasin D caused up-regulation. Northern blot analyses indicated that: (1) serum deprivation halved the concentration of all tropomyosin transcripts (TM-1, TM-2, TM-4, TMBr-1, TMBr-2). Serum induced TM-4 via transcriptional activation, independent of protein synthesis, (2) dBcAMP induced down-regulation of beta-actin (-50%) and tropomyosin transcripts (-35 to 52%) even in the presence of serum. The concentration of profilin mRNA decreased in dBcAMP-reactive astrocytes (-46%). The decrease in beta-actin mRNA concentration was not blocked by cycloheximide, whereas down-regulation of tropomyosin transcripts was completely reversed when protein synthesis was inhibited, and (3) cytochalasin D induced an increase in the concentration of tropomyosin transcripts (+69 to 185%) which was cumulative with serum stimulation. Cytochalasin D induction of both beta-actin and TM-4 operated through transcriptional activation, independent of protein synthesis. The production of all tropomyosin transcripts examined here were strictly coordinated with beta-actin expression in serum-, dBcAMP- and cytochalasin D-treated astrocytes. This indicates that the differential expression of tropomyosin isoforms occurring during astrocyte maturation is due to more complex regulation than that involved in serum- or cAMP-stimulated astrocytes.

General inhibition by transforming growth factor beta 1 of thyrotropin and cAMP responses in human thyroid cells in primary culture

Transforming growth factor beta 1 (TGF beta 1) mRNA has previously been identified in human thyroid cells and this agent has been shown to inhibit DNA synthesis in thyroid cells of some other species. In normal human thyroid cells in primary culture, TGF beta 1 inhibited inconstantly the low basal DNA synthesis and strongly the stimulation of DNA synthesis by epidermal growth factor (EGF) and serum, and by thyroid-stimulating hormone (TSH) acting through cAMP. This inhibition, by TGF beta 1, of the TSH and cAMP-dependent DNA synthesis was associated with an inhibition of PCNA (proliferating cell nuclear antigen) synthesis. TGF beta 1 almost completely abolished the cAMP induced stimulation of iodide uptake and thyroperoxidase synthesis. It thus, like EGF, also acts as a dedifferentiating agent. Investigation of the pattern of protein synthesis by two-dimensional gel electrophoresis revealed that while TGF beta 1, by itself, increased the synthesis of only one protein, a tropomyosin isoform, it inhibited most of the effects of cAMP on protein synthesis (35 out of 45 cAMP-regulated proteins were affected). It also reversed the effect of cAMP on the morphology of the thyrocytes. The fact that TGF beta 1 did not affect the increase in cAMP provoked by TSH in human thyroid cells while inhibiting most of the effects of dibutyryl cAMP in these cells suggests an action at a step distal to cAMP generation.(ABSTRACT TRUNCATED AT 250 WORDS)

Phagocytosis induced by thyrotropin in cultured thyroid cells is associated with myosin light chain dephosphorylation and stress fiber disruption

The actin/myosin II cytoskeleton and its role in phagocytosis were examined in primary cultures of dog thyroid cells. Two (19 and 21 kD) phosphorylated light chains of myosin (P-MLC) were identified by two-dimensional gel electrophoresis of antimyosin immunoprecipitates, and were associated with the Triton X-100 insoluble, F-actin cytoskeletal fraction. Analyses of Triton-insoluble and soluble 32PO4-prelabeled protein fractions indicated that TSH (via cAMP) or TPA treatment of intact cells decreases the MLC phosphorylation state. Phosphoamino acid and tryptic peptide analyses of 32P-MLCs from basal cells showed phosphorylation primarily at threonine and serine residues; most of the [32P] appeared associated with a peptide containing sites typically phosphorylated by MLC kinase. Even in the presence of the agents which induced dephosphorylation, the phosphatase inhibitor, calyculin A, caused a severalfold increase in MLC phosphorylation at several distinct serine and threonine sites which was also associated with actomyosin and cell contraction. Phosphorylation of cell homogenate proteins or the cytoskeletal fraction with [gamma-32P]ATP indicated that Ca2+, EGTA, or trifluoperazine (TFP) has little effect on the phosphorylation of MLC. Both fluorescent phalloidin and antimyosin staining of cells showed distinct dorsal and ventral stress fiber complexes which were disrupted within 30 min by TSH and cAMP; TPA appeared to cause disruption of dorsal, and rearrangement of ventral complexes. Concomitant with MLC dephosphorylation and stress fiber disruption, TSH/cAMP, but not TPA, induced dorsal phagocytosis of latex beads. While stimulation of either A or C-kinase disrupts dorsal stress fibers and rearranges actomyosin, another event(s) mediated by A-kinase appears necessary for phagocytic activity.

Intermediate filaments in normal thyrocytes: modulation of vimentin expression in primary cultures

In dog thyrocyte primary cultures, the antagonistic effects of thyrotropin (TSH) and epidermal growth factor (EGF) on differentiation expression were accompagnied by distinct long-term morphological changes: TSH-treated cells showed an epitheloid morphology; EGF reversibly induced a fusiform shape. Using indirect immunofluorescence microscopy and two-dimensional gel electrophoresis, we studied the modifications in the distribution and synthesis of the intermediate filament proteins of the cytoskeleton in response to TSH and EGF. These factors had little effect on the expression of cytokeratins 8 and 18, which were expressed in 98% of cells. However, TSH induced a profound redistribution of cytokeratins (and actin) with the appearance of a marked staining of cell junctions. Vimentin was coexpressed with cytokeratins in about 40% of cells from normal thyroid follicles freshly isolated by collagenase. During culture, immunostained vimentin network progressively developed in 90% of control and EGF-treated cells simultaneously with vimentin synthesis. In contrast, only 20% of TSH-treated cells reacted with vimentin antibody and we observed a marked decrease in vimentin synthesis in response to TSH. Therefore, vimentin synthesis, which should occur in at least some normal thyroid follicles in vivo, was inhibited in vitro by TSH which promotes differentiation expression. However, EGF-treated cells thereafter cultured with TSH regained an epitheloid morphology and differentiation in spite of the persistency of a complete network of vimentin.

Epidermal growth factor and phorbol ester actions on the TSH induced down regulation of the isoenzyme I (PKA I) of cyclic AMP-dependent protein kinases in dog thyroid cell primary cultures

In dog thyroid cell primary cultures the prolonged presence (up to 4-6 days) of TSH induced down regulation of the isoenzyme I (PKA I) of cAMP-dependent protein kinases. In the simultaneous presence of TSH and EGF this down regulation of PKA I was maintained, although it was slightly smaller than in assays without EGF. In contrast, the simultaneous presence of TPA, totally inhibited the TSH induced down regulation of PKA I. These results partly explain the previously observed additivity of TSH and EGF, and the non-additivity of TSH and TPA actions on cell proliferation in these cells.

Activation of the cyclic AMP cascade as an oncogenic mechanism: the thyroid example

Three cascades activate thyroid cell proliferation: the EGF-protein tyrosine kinase pathway, the phorbol ester-protein kinase C pathway and the thyrotropin-cyclic AMP pathway. While the first 2 cascades converge early, they remain distinct from the cyclic AMP cascade until very late in G1. The cyclic AMP cascade is characterized by an early and transient expression of c-myc, which may explain why it induces proliferation and differentiation expression. Constitutive activation of this cascade causes growth and hyperfunction, ie, hyperfunctioning adenomas. The various possible defects that could lead to such a constitutive activation are discussed.

Regulation of tropomyosin expression in transformed granulosa cell lines with steroidogenic ability

The expression of the different tropomyosin isoforms was analyzed in primary granulosa cell cultures and in established granulosa cell lines cotransfected with SV40 and Ha-ras DNA which retain a high steroidogenic response to cAMP stimulation. In contrast to normal cells which greatly reduce the expression of all tropomyosin isoforms during development of steroidogenic ability, in the doubly transformed cells only the synthesis of the high molecular weight isoforms nos 2 and 3 was decreased. The expression of isoforms 1 and 5 was elevated in the cotransfected lines and that of tropomyosin 1 was further enhanced by cAMP stimulation. The increased synthesis of tropomyosins 1 and 5 is unique to SV40 transformation, since it was observed also in cells transfected with SV40 DNA alone. These cells displayed a well organized microfilament system, but have lost the ability to differentiate. The reduced expression of tropomyosins 2 and 3 and a poorly organized microfilament system appear to be a dominant feature of both the highly differentiated normal- and transformed-granulosa cells. It is suggested that the switches in tropomyosin isoform expression during development of the steroidogenic phenotype and in cell transformation may account for necessary changes in microfilament organization which accompany these cellular processes.

Control of protein synthesis by thyrotropin and epidermal growth factor in human thyrocytes: role of morphological changes

The effect of thyrotropin (TSH) and epidermal growth factor (EGF) on the synthesis of proteins has been studied using two-dimensional gel electrophoresis in primary cultures of thyroid cells developing as a monolayer or that remained associated as dense aggregates. (1) A 4-day treatment of monolayer cells by TSH or dibutyryl cAMP enhanced the synthesis of 26 proteins and decreased that of 19 others. (2) The synthesis of 29 proteins was similarly modified by TSH and dibutyryl cAMP in both types of culture organizations. Both agents stimulated the synthesis of thyroperoxidase and of proliferating cell nuclear antigen (PCNA)/cyclin and decreased that of actin and of a high Mr isoform of tropomyosin. (3) TSH induced the retraction of monolayer cells. Its effect on the synthesis of many proteins was mimicked by culturing unstimulated cells as dense aggregates instead of monolayers which similarly affected cell morphology. (4) EGF alone had no effect on protein synthesis in monolayer cells but it inhibited both the morphological changes induced by TSH and dibutyryl cAMP and the effect of these agents on the synthesis of 23 proteins including thyroperoxidase.

IN CONCLUSION

(1) TSH and cAMP induce both proliferation and the expression of differentiation in thyroid cells while EGF has a small mitogenic effect but a marked inhibitory action on differentiation expression; (2) many TSH (cAMP) and EGF effects on the pattern of protein synthesis might be related to morphological changes; (3) the expression of the differentiation marker thyroperoxidase and of the mitogenic marker PCNA/cyclin appears independent of cell configuration and morphology.

Phorbol myristate acetate inhibits HeLa 229 invasion by Bordetella pertussis and other invasive bacterial pathogens

The microfilament inhibitors cytochalasins B and D have been traditionally used to indirectly evaluate the requirement for actin in the uptake of invasive bacterial pathogens by nonprofessional phagocytes. Through their effects on microfilaments, both cytochalasins also impart profound alterations in cellular morphology and surface topology, which likely interfere with adherence. Alterations affecting adherence would complicate interpretation of the effect of cytochalasins on entry alone. As an alternative to cytochalasins, the effect of the tumor promoter phorbol myristate acetate (PMA) was examined for its effects on uptake of several invasive bacterial pathogens by HeLa 229 cells. In this communication, PMA was shown to induce a similar change in HeLa cell actin distribution, but, in contrast to cytochalasins B and D, PMA had no significant effect on gross cell morphology. The modified actin distribution was shown to reduce internalization of Bordetella pertussis, Yersinia pseudotuberculosis, Shigella flexneri, and Salmonella hadar in a dose-dependent manner at concentrations ranging from 1 to 1,000 ng/ml. The magnitude of reduction at a PMA concentration of 1,000 ng/ml was greater than the reduction elicited by cytochalasin B at 2.5 micrograms/ml but was less than that elicited by cytochalasin D at 2.5 micrograms/ml. Mezerein, a functional analog of PMA, caused a similar dose-dependent reduction in uptake of B. pertussis, whereas an inactive analog of PMA, alpha-4-phorbol-12,13-didecanoate was without effect on invasion. Binding studies further reveal that pretreatment of HeLa cells with PMA or mezerein did not significantly impair the ability of B. pertussis to adhere, in contrast to cytochalasins B and D, which caused a marked reduction in adherence.

Transient expression of the transformed phenotype stimulated by 12-O-tetradecanoyl phorbol-13-acetate

The objective of the present work was to determine whether a short-term perturbation of cells by 12-O-tetradecanoyl phorbol-13-acetate (TPA) treatment caused shape changes identical to those found in oncogenic transformation. A cell line derived from the rat respiratory tract epithelium, 1000 W, was used, in which shape changes had been identified previously as the cells underwent transformation during long-term growth in vitro. These changes corresponded to a steeper rise of the cell from the substrate and a smoothing of the surface contours throughout the periphery of the cell. The phenotype was measured by maximum likelihood estimation, based on the values of several geometrical shape descriptors. With continuous TPA treatment, the cells adopted a transformed phenotype by 2 h. The effect was maximal by 5 h but began to decline by 10 h. Shape change in the opposite direction was stimulated by treatment with the protein kinase C inhibitor staurosporine, and its effects were counteracted if the cells were simultaneously exposed to TPA. No appreciable metabolism of [20-3H]TPA occurred until 24 h after treatment. Enumerating the shape descriptors whose values composed the transformed phenotype indicated that the TPA-stimulated changes were qualitatively similar to those accompanying oncogenic transformation. The subsequent alterations, however, involved few of the variables that composed the transformed phenotype and therefore did not represent a true reversal of the change. Changes observed up to 5 h were not dependent on new RNA synthesis but required continued protein synthesis.

Localization of urokinase-type plasminogen activator receptor on U937 cells: phorbol ester PMA induces heterogeneity

The binding of human urokinase-type plasminogen activator (u-PA) to the surface of the human monocytic cell line U937 was studied by immunological detection of bound u-PA or binding of biotinylated diisopropyl fluorophosphate-inactivated human u-PA visualized by light or electron microscopy. Untreated U937 cells showed a characteristic binding pattern, with the majority of the u-PA bound to the microvillar-containing protruding pole of the cells. After treatment with the phorbol ester PMA, the resulting adherent cell population was very heterogeneous with respect to both cellular morphology and u-PA binding. The bound u-PA was distributed on both the dorsal and the substrate side of the cells, and the patches of bound u-PA could not be correlated to any typical membrane conformations or cell-cell or cell-substratum contacts. When a monoclonal antibody directed against the amino-terminal fragment (ATF) of u-PA was used, the results were identical regardless of whether intact u-PA or ATF was used for binding to the cells. In contrast, when a monoclonal antibody recognizing the non-receptor-binding protease domain of u-PA was used, bound ATF showed no staining, while bound intact u-PA was stained as efficiently as above. The alteration of u-PA receptor distribution following treatment with PMA could be related to the changes in glycosylation and ligand affinity of the purified u-PA receptor previously described following PMA treatment of U937 cells.

Use of two-dimensional gel electrophoresis and autoradiography as a tool in cell biology: the example of the thyroid and the liver

Different applications of two-dimensional gel electrophoresis and the research strategies that this methodology allows, with examples drawn from our own work on thyroid and liver cells, are described.