Enhanced sensitivity to apoptosis in Ras-transformed thyroid cells

Ras mutations occur at high frequency in thyroid cancer. In vitro, the effects of Ras in thyroid cells are pleiotropic in that expression of activated Ras has been reported to stimulate proliferation and apoptosis. An understanding of the factors that contribute to the survival versus demise of Ras-transformed cells is essential to our understanding of the contribution of Ras to thyroid neoplasia and other cancers. Constitutive expression of oncogenic H-Ras sensitized Wistar rat thyroid (WRT) cells to apoptosis stimulated by multiple insults. When deprived of matrix attachment, Ras-transformed cells perished by apoptotic cell death at a high frequency. In contrast, parental cells were more resistant to suspension-induced cell death. Ras effects on anchorage-independent cell death were reproduced by a mutant protein that signals selectively to Raf-1, but not by mutant Ras that preferentially binds to RalGDS. Expression of a Ras mutant that selectively activates PI3K resulted in substantial protection from detachment-induced cell death. MAPK activity was increased in adherent Ras12V- and Ras12V35S-expressing cells, but abolished upon detachment. Interestingly, impaired MAPK activity was sufficient to stimulate apoptosis in adherent Ras-transformed cells, but not in parental cells. Treatment with a PI3K inhibitor also stimulated apoptosis selectively in Ras-transformed cells. These results demonstrate that constitutive expression of activated Ras elicits differential effects on the survival of thyroid cells. Moreover, Ras expression results in a greater dependence of thyroid cells on MAPK and PI3K activity for their survival.

Coordinated regulation of Rap1 and thyroid differentiation by cyclic AMP and protein kinase A

Originally identified as an antagonist of Ras action, Rap1 exhibits many Ras-independent effects, including a role in signaling pathways initiated by cyclic AMP (cAMP). Since cAMP is a critical mediator of the effects of thyrotropin (TSH) on cell proliferation and differentiation, we examined the regulation of Rap1 by TSH in a continuous line of rat thyroid-like cells. Both cAMP and protein kinase A (PKA) contribute to the regulation of Rap1 activity and signaling by TSH. TSH activates Rap1 through a cAMP-mediated and PKA-independent mechanism. TSH phosphorylates Rap1 in a PKA-dependent manner. Interference with PKA activity blocked phosphorylation but not the activation of Rap1. Rather, PKA inhibitors prolonged Rap1 activation, as did expression of a Rap1A mutant lacking a PKA phosphorylation site. These results indicate that PKA elicits negative feedback regulation on cAMP-stimulated Rap1 activity in some cells. The dual regulation of Rap1 by cAMP and PKA extends to downstream effectors. The ability of TSH to stimulate Akt phosphorylation was markedly enhanced by the expression of activated Rap1A and was repressed in cells expressing a putative dominant-negative Rap1A mutant. Although the expression of activated Rap1A was sufficient to stimulate wortmannin-sensitive Akt phosphorylation, TSH further increased Akt phosphorylation in a phosphatidylinositol 3-kinase- and PKA-dependent manner. The ability of TSH to phosphorylate Akt was impaired in cells expressing a Rap1A mutant that could be activated but not phosphorylated. These findings indicate that dual signals, Rap1 activation and phosphorylation, contribute to TSH-stimulated Akt phosphorylation. Rap1 plays an essential role in cAMP-regulated differentiation. TSH effects on thyroid-specific gene expression, but not its effects on proliferation, were markedly enhanced in cells expressing activated Rap1A and repressed in cells expressing a dominant-negative Rap1A mutant. These findings reveal complex regulation of Rap1 by cAMP including PKA-independent activation and PKA-dependent negative feedback regulation. Both signals appear to be required for TSH signaling to Akt.

Cyclic AMP activates Ras

In addition to protein kinase A (PKA), cAMP regulates the activity of cAMP-gated channels and Rap1-specific guanine nucleotide exchange factors. We tested the hypothesis that the targets of cAMP might also include regulators of the Ras protooncogene. In rat thyroid cells, thyrotropin (TSH) stimulates proliferation through a cAMP-mediated pathway that requires Ras activity. Interference with Ras impairs DNA synthesis stimulated by TSH as well as cAMP elevating agents and analogs, demonstrating that the requirement for Ras lies down-stream of cAMP. Although cAMP stimulates proliferation, microinjection of the purified PKA catalytic subunit failed to do so, suggesting that factors in addition to PKA are required for cAMP-stimulated cell cycle progression. When added to thyroid cells expressing human Ha-Ras, TSH rapidly and markedly increased the proportion of GTP-bound Ras. Ras activity was increased within 1 min of TSH addition, maximal at 5-15 min, and declined to basal levels 30-60 min after hormone treatment. Cyclic AMP elevating agents elicited similar effects on Ras, indicating that TSH activates Ras through a cAMP-mediated pathway. Although cAMP-mediated, Ras activation by TSH and cAMP was independent of PKA activity. Moreover, cAMP-stimulated Ras activation was not impaired by tyrosine kinase inhibitors. These results indicate that cAMP activates targets in addition to PKA in thyroid cells, and that these targets may include regulators of Ras. The ability of cAMP elevating agents to activate Ras in addition to PKA may explain the inability of the PKA catalytic subunit to stimulate DNA synthesis in thyroid cells.

Cadherin repertoire determines partner-specific gap junctional communication during melanoma progression

Reduced gap junction activity has long been implicated in tumorigenesis. To elucidate the potential role of intercellular communication in melanoma development, we examined gap junctional capability of melanocytic cells from various stages of tumor progression in coculture models using dye transfer assays. Normal melanocytes coupled with keratinocytes by gap junctional formation, whereas melanoma cells did not. Instead, melanoma cells communicated among themselves and with fibroblasts. This switch in communication partners coincided with a shift from E-cadherin to N-cadherin expression during melanoma development. Forced expression of E-cadherin by adenoviral gene transfer in N-cadherin-expressing melanoma cells restored gap junctional compatibility with keratinocytes. Our data suggest that (1) melanocyte transformation is associated with loss of the pre-existing gap junctional activity with keratinocytes but a concomitant gain of communication with a newly juxtaposed cell type, the fibroblasts, (2) the specificity of gap junctional formation during melanoma development is determined by the cadherin profile on the melanocytic cells and (3) the overall gap junctional activity of melanocytic cells is not reduced with transformation.

Ras signaling through PI3K confers hormone-independent proliferation that is compatible with differentiation

Hormones are specialized mitogens that stimulate proliferation in their differentiated target cells. Thyrotropin (TSH), the physiologic regulator of thyroid cells, stimulates cAMP-mediated proliferation and thyroid-specific gene expression. The mitogenic effects of TSH require Ras, therefore Ras activation should be compatible with the maintenance of thyroid differentiation. However, expression of activated Ras extinguishes the differentiated phenotype of thyroid cells. One explanation for this apparent paradox is the selective utilization of Ras effector pathways. We tested the hypothesis that Ras signaling through PI3K mediates the mitogenic effects of TSH in cells which retain their differentiated character. Expression of a Ras effector mutant (RasV12S35) that signals preferentially through Raf-1, although sufficient to confer TSH-independent proliferation, abolished hormone-regulated expression of thyroglobulin and the sodium/iodide symporter. In contrast, expression of a Ras mutant (RasV12C40) that binds selectively to PI3K conferred TSH-independent proliferation without marked effects on thyroid-specific gene expression. Unlike the inhibitory effects of TSH on the proliferation of RasV12S35-expressing cells, TSH enhanced RasV12C40-stimulated proliferation by further increasing the activity of p70s6k, an important mediator of the mitogenic effects of TSH and RasV12C40. These results demonstrate that channeling Ras-dependent signals to PI3K confers TSH with the ability to stimulate proliferation in differentiated cells. Oncogene (2000) 19, 924 - 932.

Differential effects of acute and chronic exposure to interferon-gamma on cyclic adenosine 3',5'-monophosphate response element-regulated gene expression

TSH stimulates proliferation and maintains differentiated function in thyroid follicular cells. The mitogenic activity and the stimulatory effects of TSH on thyroid-specific gene expression are impaired by interferon-gamma (IFNgamma); however, the mechanisms for these effects have not been elucidated in detail. We examined the effects of IFNgamma on acute responses to TSH in rat thyroid cells. IFNgamma did not impair TSH-stimulated p70/p85 ribosomal protein S6 kinase (p70/p85s6k) activity or cAMP response element (CRE)-regulated gene expression, although it inhibited DNA synthesis and thyroglobulin expression, effects measured over a more prolonged time course than those on kinase activity and reporter gene expression. Unexpectedly, when cells were chronically exposed to IFNgamma, CRE-lacZ promoter activity was decreased, whereas other cAMP-mediated signals, such as p70/p85s6k activity and CRE-binding protein phosphorylation, were unaffected. Activating protein-1-regulated promoters were also impaired by IFNgamma treatment, but with kinetics that differed from those of CRE-regulated promoters. Neither acute nor chronic treatment with interleukin-1beta impaired cAMP signaling, indicating that the effects of IFNgamma are specific. These studies identify CRE- and activating protein-1-regulated promoters as targets of IFNgamma in thyroid cells and fibroblasts. IFNgamma-mediated inhibition of these promoters, in addition to those containing thyroid-specific transcription factor-1-binding sites, may contribute to the profound effects of IFNgamma on thyroid cells.

Atypical protein kinase C-zeta stimulates thyrotropin-independent proliferation in rat thyroid cells

Several reports have indicated that protein kinase C (PKC) is an important regulator of proliferation in thyroid cells. Unlike TSH, the mitogenic effects of phorbol esters are accompanied by de-differentiation. The role of individual PKC isoforms in thyroid cell proliferation and differentiation has not been examined. Recent studies have implicated the atypical PKCzeta, a phorbol ester-unresponsive isozyme, in cell proliferation, death, and survival. We overexpressed PKCzeta in Wistar rat thyroid (WRT) cells and determined that PKCzeta conferred TSH-independent DNA synthesis and cell proliferation. Cells overexpressing PKCzeta show higher levels of phosphorylated p42/p44 MAPK compared with vector-transfected cells. Experiments using a luciferase reporter for Elk-1 revealed that PKCzeta overexpressing cells exhibit higher basal Elk-1 transcriptional activity than vector-transfected control cells. Interestingly, stimulation of Elk-1 transcriptional activity by MEK1, a p42/p44 MAPK kinase, was significantly enhanced in cells overexpressing PKCzeta. Strikingly, TSH retained the ability to stimulate Tg expression in cells expressing PKCzeta. These results suggest that PKCzeta stimulates TSH-independent mitogenesis through a p42/p44 MAPK-dependent pathway. Unlike overexpression of Ras or phorbol ester treatment, PKC overexpression does not impair thyroglobulin (Tg) expression.

Protein kinase A-dependent and -independent signaling pathways contribute to cyclic AMP-stimulated proliferation

The effects of cyclic AMP (cAMP) on cell proliferation are cell type specific. Although the growth-inhibitory effects of cAMP have been well studied, much less is known regarding how cAMP stimulates proliferation. We report that cAMP stimulates proliferation through both protein kinase A (PKA)-dependent and PKA-independent signaling pathways and that phosphatidylinositol 3-kinase (PI3K) is required for cAMP-stimulated mitogenesis. In cells where cAMP is a mitogen, cAMP-elevating agents stimulate membrane ruffling, Akt phosphorylation, and p70 ribosomal S6 protein kinase (p70s6k) activity. cAMP effects on ruffle formation and Akt were PKA independent but sensitive to wortmannin. In contrast, cAMP-stimulated p70s6k activity was repressed by PKA inhibitors but not by wortmannin or microinjection of the N-terminal SH2 domain of the p85 regulatory subunit of PI3K, indicating that p70s6k and Akt can be regulated independently. Microinjection of highly specific inhibitors of PI3K or Rac1, or treatment with the p70s6k inhibitor rapamycin, impaired cAMP-stimulated DNA synthesis, demonstrating that PKA-dependent and -independent pathways contribute to cAMP-mediated mitogenesis. Direct elevation of PI3K activity through microinjection of an antibody that stimulates PI3K activity or stable expression of membrane-localized p110 was sufficient to confer hormone-independent DNA synthesis when accompanied by elevations in p70s6k activity. These findings indicate that multiple pathways contribute to cAMP-stimulated mitogenesis, only some of which are PKA dependent. Furthermore, they demonstrate that the ability of cAMP to stimulate both p70s6k- and PI3K-dependent pathways is an important facet of cAMP-regulated cell cycle progression.

Prostaglandin F2alpha (PGF2alpha) and the isoprostane, 8, 12-iso-isoprostane F2alpha-III, induce cardiomyocyte hypertrophy. Differential activation of downstream signaling pathways

Prostaglandin receptors may be activated by their cognate ligand or by free radical catalyzed isoprostanes, products of arachidonic acid peroxidation. For example, prostaglandin F2alpha (PGF2alpha) causes hypertrophy of neonatal rat ventricular myocytes, via the PGF2alpha receptor (FP). However, the FP may also be activated by the isoprostane, 8,12-iso-iPF2alpha-III (Kunapuli, P., Lawson, J. A., Rokach, J., and FitzGerald, G. A. (1997) J. Biol. Chem. 272, 27147-27154). Both ligands induce myocyte hypertrophy with overlapping potencies. Interestingly, the hypertrophic effects of these two agonists on cardiomyocytes are additive. Furthermore, the preference of these two agonists for activation of intracellular signal transduction pathways differs in several respects. Thus, PGF2alpha and 8,12-iso-iPF2alpha-III stimulate inositol phosphate formation with EC50 values of 50 +/- 12 nM and 3.5 +/- 0.6 microM, respectively. Moreover, PGF2alpha causes a robust activation ( approximately 50-fold) of Erk2, whereas 8,12-iso-iPF2alpha-III has no effect. Similarly, PGF2alpha causes translocation of cytosolic phospholipase A2 and also results in a 7-fold increment in the formation of 6-keto-PGF1alpha, whereas 8,12-iso-iPF2alpha-III exerts no effect on this pathway. On the other hand, both agonists are equally potent in activating JNK1 and c-Jun, whereas neither activates the p38 kinase. Both PGF2alpha and 8,12-iso-iPF2alpha-III activate the p70S6 kinase (p70(S6K)), but not Akt, downstream of phosphatidylinositol-3-kinase (PI3K). However, both wortmannin, a PI3K inhibitor, and rapamycin, an inhibitor of p70(S6K) activity, inhibit 8,12-iso-iPF2alpha-III -induced myocyte hypertrophy, with IC50 values of 60 +/- 12 and 3 +/- 1.7 nM, respectively, whereas neither compound abrogates the PGF2alpha-mediated response. Thus, both PGF2alpha and 8,12-iso-iPF2alpha-III induce myocyte hypertrophy via discrete signaling pathways. Although both agonists signal via the JNK pathway to initiate changes in c-Jun-dependent gene transcription, PGF2alpha preferentially activates the MEK-Erk2- cytosolic phospholipase A2 pathway. In contrast, the PI3K-p70(S6K) pathway appears to be essential for 8,12-iso-iPF2alpha-III-induced myocyte hypertrophy.

Differential effects of protein kinase A on Ras effector pathways

Ras mutants with the ability to interact with different effectors have played a critical role in the identification of Ras-dependent signaling pathways. We used two mutants, RasS35 and RasG37, which differ in their ability to bind Raf-1, to examine Ras-dependent signaling in thyroid epithelial cells. Wistar rat thyroid cells are dependent upon thyrotropin (TSH) for growth. Although TSH-stimulated mitogenesis requires Ras, TSH activates protein kinase A (PKA) and downregulates signaling through Raf and the mitogen-activated protein kinase (MAPK) cascade. Cells expressing RasS35, a mutant which binds Raf, or RasG37, a mutant which binds RalGDS, exhibited TSH-independent proliferation. RasS35 stimulated morphological transformation and anchorage-independent growth. RasG37 stimulated proliferation but not transformation as measured by these indices. TSH exerted markedly different effects on the Ras mutants and transiently repressed MAPK phosphorylation in RasS35-expressing cells. In contrast, TSH stimulated MAPK phosphorylation and growth in cells expressing RasG37. The Ras mutants, in turn, exerted differential effects on TSH signaling. RasS35 abolished TSH-stimulated changes in cell morphology and thyroglobulin expression, while RasG37 had no effect on these activities. Together, the data indicate that cross talk between Ras and PKA discriminates between distinct Ras effector pathways.

Differential effects of cyclic adenosine 3',5'-monophosphate on p70 ribosomal S6 kinase

cAMP exerts differential effects on mitogenic signaling pathways. In many cells, cAMP inhibits growth factor-stimulated MAPK activity and proliferation. In others, cAMP promotes growth. TSH stimulates proliferation through elevations in cAMP in thyroid follicular cells. This mitogenic pathway is dependent upon both protein kinase A and Ras, but not upon Raf-1, mitogen-activated protein kinase kinase, or mitogen-activated protein kinase. We report that TSH, acting through cAMP, activates pp70s6k and that this activity is required for TSH-stimulated DNA synthesis. A similar role for pp70s6k in cAMP-mediated mitogenesis was observed in secondary rat Schwann cells and in Swiss3T3 fibroblasts, two additional cell types that respond to cAMP with growth. In contrast, cAMP elevation did not activate pp70s6k in NIH3T3 or REF52 fibroblasts, cells in which cAMP fails to stimulate proliferation. Together, these results suggest that pp70s6k plays an important and general role in cAMP-mediated proliferation.

RalGDS functions in Ras- and cAMP-mediated growth stimulation

Thyroid-stimulating hormone stimulates proliferation through both the cAMP-dependent protein kinase and Ras but not through Raf-1 and mitogen-activated and extracellular signal-related kinase kinase. We now report that thyroid-stimulating hormone represses mitogen-activated protein kinase activity and that microinjection of an effector domain mutant Ha-Ras protein, Ras(12V,37G), defective in Raf-1 binding and mitogen-activated protein kinase activation, stimulates DNA synthesis in quiescent and thyroid-stimulating hormone-treated thyrocytes. A yeast two-hybrid screen identified RalGDS as a Ras(12V,37G) binding protein and therefore a potential effector of Ras in these cells. Associations between Ras and RalGDS were observed in extracts prepared from thyroid cells. Microinjection of a mutant RalA(28N) protein thought to sequester RalGDS family members reduced DNA synthesis stimulated by Ras as well as cAMP-mediated DNA synthesis in two cell lines which respond to cAMP with mitogenesis. These results support the idea that RalGDS may be an effector of Ras in cAMP-mediated growth stimulation.

Inhibition of Raf-1 signaling by a monoclonal antibody, which interferes with Raf-1 activation and with Mek substrate binding

Raf-1 is a serine/threonine specific kinase that integrates signaling by a large number of mitogens to elicit a transcriptional response in the nucleus. Activated Raf-1 phosphorylates and activates MAPK/ERK kinase Mek), thus initiating the Mek--> MAP kinase cascade, which ultimately results in the phosphorylation and activation of transcription factors by MAP kinase. Here we have characterized the mechanism by which monoclonal antibody URP26K, which binds to an epitope in the Raf-1 kinase domain, inhibits intracellular signal transduction. This antibody preferentially immunoprecipitated the underphosphorylated, non-activated form of Raf-1 from quiescent cells. Baculovirus-expressed Raf-1 immunoprecipitated with URP26K was largely refractory to phosphorylation and activation mediated by protein kinase C (PKC)alpha or the tyrosine kinase Lck. In addition, URP26K reduced the binding of Raf-1 to its substrate Mek in vitro, but did not disturb the association of Raf-1 with Ras. Microinjection of URP26K into Rat-1 cells blocked DNA synthesis initiated by serum, insulin and various purified growth factors, but it did not block DNA synthesis initiated by v-ras. Microinjected URP26K also impaired the expression of stably transfected beta-galactosidase reporter genes regulated by minimal promoter elements. These results demonstrate, (i) that the URP26K monoclonal antibody inhibits Raf-1 by preventing activating Raf-1 phosphorylation and/or association with its substrate Mek, (ii) that inhibition of Raf-1 by URP26K does not interfere with Ras-induced DNA synthesis. In contrast to dominant negative Raf-1 mutants, which also block Ras signaling by binding to the Ras effector domain, antibody mediated Raf-1 inhibition thus reveals a branchpoint of mitogenic signaling at the level of Ras.

Amphibian transcription factor IIIA proteins contain a sequence element functionally equivalent to the nuclear export signal of human immunodeficiency virus type 1 Rev

The human immunodeficiency virus type 1 (HIV-1) Rev protein is required for nuclear export of late HIV-1 mRNAs. This function is dependent on the mutationally defined Rev activation domain, which also forms a potent nuclear export signal. Transcription factor IIIA (TFIIIA) binds to 5S rRNA transcripts and this interaction has been proposed to play a role in the efficient nuclear export of 5S rRNA in amphibian oocytes. Here it is reported that amphibian TFIIIA proteins contain a sequence element with homology to the Rev activation domain that effectively substitutes for this domain in inducing the nuclear export of late HIV-1 mRNAs. It is further demonstrated that this TFIIIA sequence element functions as a protein nuclear export signal in both human cells and frog oocytes. Thus, this shared protein motif may play an analogous role in mediating the nuclear export of both late HIV-1 RNAs and 5S rRNA transcripts.

Nuclear transport of human immunodeficiency virus type 1, visna virus, and equine infectious anemia virus Rev proteins: identification of a family of transferable nuclear export signals

The human immunodeficiency virus type 1 Rev trans activator binds directly to unspliced viral mRNA in the nucleus and activates its transport to the cytoplasm. In additon to the sequences that confer RNA binding and nuclear localization, Rev has a carboxy-terminal region, the activation domain, whose integrity is essential for biological activity. Because it has been established that Rev constitutively exits and reenters the nucleus and that the activation domain is required for nuclear exit, it has been proposed that Rev's activation domain is a nuclear export signal (NES). Here, we used microinjection-based assays to demonstrate that the activation domain of human immunodeficiency virus type 1 Rev imparts rapid nuclear export after its transfer to heterologous substrates. NES- mediated export is specific, as it is sensitive both to inactivation by missense mutation and to selective inhibition by an excess of the wild-type, but not mutant, activation domain peptide. Examination of the Rev trans activators of two nonprimate lentiviruses, visna virus and equine infectious anemia virus, revealed that their activation domains are also potent NESs. Taken together, these data demonstrate that nuclear export can be determined by positively acting peptide motifs, namely, NESs, and suggest that Rev proteins activate viral RNA transport by providing export ribonucleoproteins with specific information that targets them to the cytoplasm.

Ras inhibits thyroglobulin expression but not cyclic adenosine monophosphate-mediated signaling in Wistar rat thyrocytes

We previously reported that microinjection of purified Ras protein stimulated DNA synthesis in quiescent Wistar rat thyrocytes and that TSH (TSH)-stimulated DNA synthesis was Ras-dependent. In contrast to these results, microinjection of cellular or oncogenic Ras significantly reduced TSH-stimulated thyroglobulin (Tg) expression, a marker of thyrocyte differentiation. Microinjection of a dominant inhibitory Ras mutant had no effect on TSH-stimulated Tg expression. As the Tg promoter is cAMP-responsive and Ras was previously reported to interfere with entry of catalytic (C) subunit of the cAMP-dependent protein kinase into the nucleus, experiments were performed to assess the effects of Ras on cAMP-mediated signaling. Microinjection of either cellular or oncogenic Ras had no effect on TSH-stimulated entry of C subunit into the nucleus. Consistent with these data, Ras did not reduce TSH-stimulated cAMP response element binding protein phosphorylation, or cAMP response element-regulated gene expression. These results demonstrate that Ras exerts differential effects on TSH signaling; Ras increases TSH-stimulated DNA synthesis and decreases TSH-induced Tg expression. Moreover, the mechanism through which Ras induces Tg expression lies distal to entry of C subunit into the nucleus, cAMP response element binding protein phosphorylation, and cAMP response element-regulated gene expression.

Identification of a signal for rapid export of proteins from the nucleus

Active nuclear import of protein is controlled by nuclear localization signals (NLSs), but nuclear export is not understood well. Nuclear trafficking of the catalytic (C) subunit of cAMP-dependent protein kinase (cAPK) is critical for regulation of gene expression. The heat-stable inhibitor (PKl) of cAPK contains a nuclear export signal (NES) that triggers rapid, active net extrusion of the C-PKl complex from the nucleus. This NES (residues 35-49), fused or conjugated to heterologous proteins, was sufficient for rapid nuclear export. Hydrophobic residues were critical. The NES is a slightly weaker signal than the SV40 NLS. A sequence containing only residues 37-46, LALKLAGLDI, is also sufficient for nuclear export. This is an example of a protein-based NES having no obvious association with RNA. A similar sequence, LQLPPLERLTL, from Rev, an RNA-binding protein of HIV-1, also is an NES.

Signal transduction pathways leading to insulin-induced early gene induction

We examined the signal transduction pathway leading to insulin stimulation of two immediate early genes, c-fos, and the early growth response gene, Egr-1. In Rat 1 fibroblasts overexpressing normal human insulin receptors (HIRc-B), insulin and IGF-I rapidly and transiently induced the expression of both c-fos and Egr-1 mRNA with maximum accumulation at 30 min, declining to basal levels at 120 min. Insulin (100 ng/mL) increased c-fos and Egr-1 mRNA expression 10-fold (EC50 = 20 ng/mL), whereas IGF-I (100 ng/mL) and serum (20%) led to a 3- and 11.5-fold increase, respectively. Insulin-stimulated c-fos protein expression was maximal at 1 h postinduction and undetectable at 4 h. The effects of insulin and IGF-I on both c-fos mRNA and protein expression were absent in Rat 1 fibroblasts expressing tyrosine kinase-defective human insulin receptors (A/K1018). In cells expressing insulin receptors in which the two C-terminal tyrosines are mutated to phenylalanine (Y/F2 cells), the insulin stimulated increase in Egr-1 and c-fos mRNA was comparable to that of HIRc cells, whereas, in cells expressing C-terminal truncated receptors (delta CT cells), the insulin induced increase in Egr-1 mRNA was normal, but the c-fos mRNA response was severely blunted. As expected, the insulin effect to increase ras GTP formation and MAP kinase activity was negligible in A/K1018 cells but normal, or supernormal, in Y/F2 cells. Importantly, stimulation of ras GTP was increased in delta CT cells, whereas stimulation of MAP kinase activity was almost absent.(ABSTRACT TRUNCATED AT 250 WORDS)

Nitric oxide and cGMP analogs activate transcription from AP-1-responsive promoters in mammalian cells

Nitric oxide (NO) increases cytosolic guanylate cyclase activity and thereby activates the cGMP signal transduction pathway. The cAMP and Ca2+/phospholipid signal transduction pathways activate transcription factors that bind to the cAMP response element (CRE) and phorbol ester response element (TRE), respectively. Little is known about transcriptional regulation of gene expression by NO/cGMP. In transient and stable transfection experiments and in microinjection studies we found that three different NO-releasing agents and two membrane-permeable cGMP analogs activated TRE-regulated but not CRE-regulated reporter genes in rodent fibroblast and epithelial cell lines. Activation of TRE-regulated genes by NO-releasing agents and cGMP analogs appeared to be mediated by the AP-1 (Jun/Fos) transcription factor complex because we observed increased DNA binding of AP-1 and increased junB and c-fos mRNA in cells treated with these agents. The mechanism of gene activation by NO/cGMP was distinct from that used by phorbol esters and cAMP because it was not associated with c-jun mRNA induction and was not observed with CRE-containing promoters.

The expression and intracellular distribution of the heat-stable protein kinase inhibitor is cell cycle regulated

The heat-stable protein kinase inhibitor (PKI) is a potent and specific inhibitor of the catalytic (C) subunit of the cAMP-dependent protein kinase. We report the isolation of a polyclonal antibody raised to purified recombinant PKI alpha. Using this antibody, the intracellular distribution of endogenous PKI alpha was assessed by immunostaining. The PKI alpha expression and intracellular distribution varied as a function of cell cycle progression. PKI alpha expression appeared low in serum-starved cells and in cells in G1 and increased as cells progressed through S phase. Its distribution became increasingly nuclear as cells entered G2/M. Nuclear levels of PKI alpha remained high through cell division and decreased again as cells reentered G1. The cell cycle regulated expression and nuclear distribution suggests a specific role for PKI alpha in the nucleus during the G2/M phases of the cell cycle. Consistent with this, microinjection of PKI alpha antibody into serum-starved cells prevented their subsequent cell cycle progression. Similarly, overexpression of C subunit in cells arrested at the G1/S boundary prevented their subsequent division. Together these results support the idea that PKI alpha plays an important role in the inhibition of nuclear C subunit activity required for cell cycle progression, although a determination of the relative amounts of endogenous nuclear PKI and C-subunit will be required to substantiate this hypothesis.

Heat-stable inhibitors of cAMP-dependent protein kinase carry a nuclear export signal

The heat-stable inhibitor of cAMP-dependent protein kinase (PKI) was shown previously to export the kinase catalytic subunit (C) from the nucleus (Fantozzi, D. A., Harootunian, A. T., Wen, W., Taylor, S. S., Feramisco, J.R., Tsien, R. Y., and Meinkoth, J. L. (1994) J. Biol. Chem. 269, 2676-2686), in addition to its ability to inhibit kinase activity. In this study, the mechanism of PKI export is investigated. The injection of a C-PKI complex containing both labeled PKI and C-subunit revealed that both proteins exit the nucleus in unison. A fusion protein of C-subunit with glutathione S-transferase (GST) (140 kDa) cannot transverse the nuclear membrane in either direction, but can be exported from the nucleus when complexed with PKI, supporting the presence of a nuclear export signal (NES) in the C-PKI complex. Fusions of PKI alpha with GST (70 kDa) or PKI beta 1 with maltose-binding protein (MBP) (50 kDa) remain effective at exporting complexes with C-subunit. The export of C-PKI is also sensitive to temperature and energy depletion. Taken together, these results demonstrate that export is both energy- and temperature-dependent, but size-independent, consistent with an active signal-mediated export process. GST-PKI exits from the nucleus even in the absence of C-subunit, indicating that the NES resides entirely on PKI, but suggesting that fusion of PKI to GST leads to a conformational change that mimics the exposure of the NES caused by the binding of C. Since both PKI alpha and PKI beta 1 can export C-subunit, the predicted export signal is likely to reside on the residues conserved between PKI alpha and PKI beta 1.

Human regulatory subunit RI beta of cAMP-dependent protein kinases: expression, holoenzyme formation and microinjection into living cells

The human regulatory subunit RI beta of cAMP-dependent protein kinases was expressed in Escherichia coli as a fusion protein with glutathione S-transferase. Purification was performed by affinity chromatography on glutathione-agarose beads after cleavage with thrombin. The human recombinant RI beta protein migrated at 55 kDa on SDS-PAGE and displayed immunoreactivity with an anti-human RI beta antiserum. Furthermore, the purified recombinant RI beta protein was shown to exist as a dimer that was able to form holoenzyme with the catalytic subunit C alpha. The rate of RI beta 2C alpha 2 holoenzyme formation was faster in the presence than in the absence of MgATP. The kinase activity measured before and after adding cAMP to the holoenzyme showed that the presence of cAMP resulted in holoenzyme dissociation and release of active C alpha-subunit, due to cAMP binding to RI beta. Compared to a RI alpha 2C alpha 2 holoenzyme, the RI beta 2C alpha 2 holoenzyme exhibited a more than twofold higher sensitivity to cAMP. The subcellular localization of RI beta was analyzed in quiescent REF-52 fibroblasts and Wistar rat thyroid (WRT) cells after microinjection of fluorescently labeled proteins into the cytoplasm. A cytoplasmic distribution was observed when free RI beta was injected, whereas free C alpha injected into the cytoplasm appeared in the nucleus. When holoenzymes with labeled RI beta and unlabeled C alpha, or unlabeled RI beta and labeled C alpha, were injected, unstimulated cells showed fluorescence in the cytoplasm of both cell types. REF-52 cells stimulated with 8-bromo-cAMP (8-Br-cAMP) and WRT cells treated with thyrotropin (TSH) showed fluorescence mainly in the cytoplasm when RI beta was the labeled subunit of the in vivo dissociated holoenzyme. In contrast, nuclear fluorescence was evident from the release and translocation of labeled C alpha from the holoenzyme complex after stimulation with 8-Br-cAMP or TSH.

Divergent patterns of immediate early gene expression in response to thyroid-stimulating hormone and insulin-like growth factor I in Wistar rat thyrocytes

The rapid and transient induction of immediate early gene expression accompanies growth factor stimulation. TSH and insulin-like growth factor I (IGF-I) are important regulators of the thyroid follicular cell and stimulate both proliferation and differentiation. The signaling pathways induced by TSH and IGF-I are at least partially distinct. TSH uses cAMP as a second messenger, whereas the IGF-I receptor possesses protein tyrosine kinase activity. Although both agents stimulate DNA synthesis and proliferation in Wistar rat thyroid cells, they induce dramatically different patterns of immediate early gene expression. IGF-I stimulates the expression of c-fos, c-jun, junB, and egr1. In contrast, TSH stimulates c-fos and junB but not egr1 expression. TSH inhibits basal levels of c-jun expression in quiescent cells and represses serum and IGF-I-stimulated c-jun, c-fos, and egr1 expression. Consistent with these results, TSH represses serum- and phorbol ester-stimulated AP-1 activity. Although TSH and IGF-I individually stimulate DNA synthesis in thyroid cells, they exert opposing effects on the expression of some immediate early genes, including c-jun and egr1.

A Rab1 mutant affecting guanine nucleotide exchange promotes disassembly of the Golgi apparatus

The Golgi apparatus is a dynamic organelle whose structure is sensitive to vesicular traffic and to cell cycle control. We have examined the potential role for rab1a, a GTPase previously associated with ER to Golgi and intra-Golgi transport, in the formation and maintenance of Golgi structure. Bacterially expressed, recombinant rab1a protein was microinjected into rat embryonic fibroblasts, followed by analysis of Golgi morphology by fluorescence and electron microscopy. Three recombinant proteins were tested: wild-type rab, mutant rab1a(S25N), a constitutively GDP-bound form (Nuoffer, C., H. W. Davidson, J. Matteson, J. Meinkoth, and W. E. Balch, 1994. J. Cell Biol. 125: 225-237), and mutant rab1a(N124I) defective in guanine nucleotide binding. Microinjection of wild-type rab1a protein or a variety of negative controls (injection buffer alone or activated ras protein) did not affect the appearance of the Golgi, as visualized by immunofluorescence of alpha-mannosidase II (Man II), used as a Golgi marker. In contrast, microinjection of the mutant forms promoted the disassembly of the Golgi stacks into dispersed vesicular structures visualized by immunofluorescence. When S25N-injected cells were analyzed by EM after immunoperoxidase labeling, Man II was found in isolated ministacks and large vesicular elements that were often surrounded by numerous smaller unlabeled vesicles resembling carrier vesicles. Golgi disassembly caused by rab1a mutants differs from BFA-induced disruption, since beta-COP remains membrane associated, and Man II does not redistribute to the ER. BFA can still cause these residual Golgi elements to fuse and disperse, albeit at a slower rate. Moreover, BFA recovery is incomplete in the presence of rab1 mutants or GTP gamma S. We conclude that GTP exchange and hydrolysis by GTPases, specifically rab1a, are required to form and maintain normal Golgi stacks. The similarity of Golgi disassembly seen with rab1a mutants to that occurring during mitosis, may point to a molecular basis involving rab1a for fragmentation of the Golgi apparatus during cell division.

Insulin and insulin-like growth factor-I signal transduction requires p21ras

We have investigated the role of cellular p21ras protein in insulin and insulin-like growth factor-I (IGF-I) signaling pathways. Insulin stimulation increased Ras-GTP formation in Rat-1 fibroblasts overexpressing normal human insulin receptors (HIRc-B), far greater than in parental Rat-1 fibroblasts, indicating that competent insulin receptors mediate this response. Cellular microinjection of a dominant-negative mutant p21ras protein (N17 ras) or anti-p21ras monoclonal antibody (Y13-259) into HIRc-B cells reduced insulin- and IGF-I-stimulated DNA synthesis by 75-90%. Insulin-induced c-fos protein expression was also inhibited by 74%. Microinjection of oncogenic p21ras (T-24 ras) into HIRc-B cells activated the mitogenic pathway, and coinjection of N17 ras and T-24 ras showed that oncogenic p21ras rescued the cells from the N17 ras blockade. This later finding indicates that T-24 ras acts downstream of N17 ras. In conclusion, 1) microinjection of a dominant interferring ras mutant into quiescent cells abrogated subsequent insulin and IGF-I mitogenic signaling; 2) oncogenic ras protein rescued cells from the N17 ras blockade, indicating that T24 ras action is downstream of the site of N17 inhibition; and 3) p21ras is an intermediate signaling molecule in the insulin/IGF-I signal transduction pathway and is required for gene expression and DNA synthesis.

Thermostable inhibitor of cAMP-dependent protein kinase enhances the rate of export of the kinase catalytic subunit from the nucleus

The catalytic (C) subunit of cAMP-dependent protein kinase is inhibited by the regulatory (R) subunit and by a thermostable inhibitor (PKI). Both inhibitors also affect the intracellular distribution of the C subunit. Whether injected into the cytoplasm or into the nucleus, free C subunit can enter and exit the nucleus freely. After 30 min its distribution is identical and is independent of the initial site of injection. In contrast, when C is injected into the cytoplasm complexed with R or PKI, the complexes are restricted to the cytoplasm (1-3). However, unlike the R subunit, which is restricted to the cytoplasm like the holoenzyme, free PKI enters the nucleus rapidly following its injection into the cytoplasm. When holoenzyme is injected directly into the nucleus, it cannot exit and return to the cytoplasm. In contrast, nuclear injection of a C.PKI complex results in the rapid exit of the C subunit from the nucleus. In equilibrated cells previously injected with the C subunit, subsequent cytoplasmic injection of either PKI or type 1 R depletes the nucleus of C although PKI does so faster, consistent with its ability to enter the nucleus. Both inhibitors block the cAMP response element-regulated gene expression. Hence PKI may serve as a nuclear scavenger of C providing a mechanism not only for inhibition but also for subcellular localization in the presence of cAMP by restricting the access of the C subunit to the nucleus.

Signal transduction through the cAMP-dependent protein kinase

Temporal cellular events responsible for hormonal activation of responses mediated by the cAMP-dependent protein kinase (PKA) have been studied in living cells. By selectively perturbing molecular function of Gs, the catalytic subunit of PKA (C), or the nuclear factor CREB, in cells through microinjection of inhibitory agents specific for these molecules or activated forms of these molecules, we have obtained evidence for a requirement for the function of each of these molecules in the hormonal stimulation of cAMP-regulated genes. Moreover, by introducing fluorescently labeled PKA subunits into these cells as molecular tracers, or by immunofluorescence of C subunit, we have observed biological translocation of C subunit from the cytoplasm to the nucleus during transcriptional activation and a quenching of this by the inhibitor molecule, PKI. The implications of these cellular and molecular events in the signal transduction of hormonal responses are discussed.

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.

Inhibition of thyrotropin-stimulated DNA synthesis by microinjection of inhibitors of cellular Ras and cyclic AMP-dependent protein kinase

Microinjection of a dominant interfering mutant of Ras (N17 Ras) caused a significant reduction in thyrotropin (thyroid-stimulating hormone [TSH])-stimulated DNA synthesis in rat thyroid cells. A similar reduction was observed following injection of the heat-stable protein kinase inhibitor of the cyclic AMP-dependent protein kinase. Coinjection of both inhibitors almost completely abolished TSH-induced DNA synthesis. In contrast to TSH, overexpression of cellular Ras protein did not stimulate the expression of a cyclic AMP response element-regulated reporter gene. Similarly, injection of N17 Ras had no effect on TSH-stimulated reporter gene expression. Moreover, overexpression of cellular Ras protein stimulated similar levels of DNA synthesis in the presence or absence of the heat-stable protein kinase inhibitor. Together, these results suggest that in Wistar rat thyroid cells, a full mitogenic response to TSH requires both Ras and cyclic APK-dependent protein kinase.

Protein phosphatase 2A potentiates activity of promoters containing AP-1-binding elements

The involvement of serine/threonine protein phosphatases in signaling pathways which modulate the activity of the transcription factor AP-1 was examined. Purified protein phosphatase types 1 (PP1) and 2A (PP2A) were microinjected into cell lines containing stably transfected lacZ marker genes under the control of an enhancer recognized by AP-1. Microinjection of PP2A potentiated serum-stimulated beta-galactosidase expression from the AP-1-regulated promoter. Similarly, transient expression of the PP2A catalytic subunit with c-Jun resulted in a synergistic transactivation of an AP-1-regulated reporter gene. PP2A, but not PP1, potentiated serum-induced c-Jun expression, which has been previously shown to be autoregulated by AP-1 itself. Consistent with these results, PP2A dephosphorylated c-Jun on negative regulatory sites in vitro, suggesting one possible direct mechanism for the effects of PP2A on AP-1 activity. Microinjection of PP2A had no effect on cyclic AMP (cAMP)-induced expression of a reporter gene containing a cAMP-regulated promoter, while PP1 injection abolished cAMP-induced gene expression. Taken together, these results suggest a specific role for PP2A in signal transduction pathways that regulate AP-1 activity and c-Jun expression.

Effect of the thermostable protein kinase inhibitor on intracellular localization of the catalytic subunit of cAMP-dependent protein kinase

cAMP-dependent protein kinase mediates a variety of cellular responses in most eukaryotic cells. Many of these responses are cytoplasmic, whereas others appear to require nuclear localization of the catalytic subunit. In order to understand further the molecular basis for subcellular localization of the catalytic subunit, the effect of the heat stable protein kinase inhibitor (PKI) was investigated. The subcellular localization of the catalytic (C) subunit was determined both in the presence and absence of PKI, by microinjecting fluorescently labeled C subunit into single living cells. When injected alone, a significant fraction of the dissociated C subunit localized to the nucleus. When coin-injected with an excess of PKI, little of the C subunit localized to the nucleus, suggesting that accumulation of catalytic subunit in the nucleus requires either enzymatic activity or a nuclear localization signal. Inactivation of the catalytic subunit in vitro by treatment with N-ethylmaleimide did not prevent localization in the nucleus, indicating that enzymatic activity was not a prerequisite for nuclear localization. In an effort to search for a specific signal that might mediate nuclear localization, a complex of the catalytic subunit with a 20-residue inhibitory peptide derived from PKI (PKI(5-24)) was microinjected. In contrast to intact PKI, the peptide was not sufficient to block nuclear accumulation. In the presence of PKI(5-24), the C subunit localized to the nucleus in a fashion analogous to that of dissociated, active C subunit despite evidence of no catalytic activity in situ. Thus, nuclear localization of the C subunit appears to be independent of enzymatic activity but most likely dependent upon a signal. The signal is apparently masked by both the regulatory subunit and PKI but not by the inhibitory peptide.

Conjugated bile acid uptake by Xenopus laevis oocytes induced by microinjection with ileal poly A+ mRNA

Apical membranes of ileal enterocytes contain the major Na+/bile acid cotransporter activity in mammals. Microinjection of guinea pig ileal mucosal Poly A+ mRNA (25 ng) into Xenopus oocytes resulted in 22,23-3H-cholyltaurine uptake at day 3 after injection (453 fmol/oocyte-hr), while control viral mRNA (25 ng) gave an uptake rate of 133 fmol/oocyte-hr. The transport rate increased in direct relationship to the concentration of injected mRNA, cholyltaurine, or Na+ in the incubation media. Uptake of cholyltaurine using rabbit ileal mucosal Poly A+ mRNA was 3891 fmole/oocyte-hr compared to rabbit jejunal-mucosa Poly A+ mRNA (control) injections inducing 728 fmol/oocyte-hr. Such expression of the ileal Na+/bile acid cotransporter may facilitate cloning of this key mammalian gene.

Inhibition of thyrotropin-induced DNA synthesis in thyroid follicular cells by microinjection of an antibody to the stimulatory G protein of adenylate cyclase, Gs

Thyrotropin (TSH) is an important regulator of thyroid follicular cells. While its role in the maintenance of differentiated functions is undisputed, its role as a mitogen is less clear. TSH induces DNA synthesis and cell proliferation in some cells, while in others, TSH is mitogenic only in the presence of additional growth factors such as insulin-like growth factor-1. TSH causes elevations in intracellular cAMP and is thought to utilize this second messenger system in its mitogenic action. We studied TSH as a mitogen in Wistar rat thyroid cells (WRT) (Brandi, M. L., Rotella, C. M., Mavilia, C., Franceschelli, F., Tanini, A., and Toccafondi, R. (1987) Mol. Cell. Endocrinol. 54, 91-103) and examined the role of the guanine nucleotide binding protein, Gs, in its mitogenic action. WRT cells synthesized DNA in response to TSH and elevations in cAMP. In addition, TSH caused a rapid stimulation of an indicator gene whose expression is regulated by cAMP response elements. Following microinjection of an inhibitory polyclonal antibody raised against the Gs protein, both TSH-induced changes in gene expression and DNA synthesis were significantly reduced. These results demonstrate that virtually all of the mitogenic action of TSH is transduced through the Gs protein in WRT cells, presumably through the regulation of adenylate cyclase. Whether all or only part of TSH action is mediated by cAMP and the cAMP-dependent protein kinase remains to be determined.

Inhibition of DNA synthesis in living cells by microinjection of Gi2 antibodies

Heterotrimeric guanine nucleotide binding proteins function in the coupling of a diverse span of cell surface receptors to a variety of intracellular signaling pathways, some of which stimulate cellular proliferation. With the recent discovery that mutated forms of G proteins are present in specific tumors, there has been an increased interest in the determination of the role of specific subtypes of G proteins in the regulation of cellular growth. We have attempted to determine which subtypes of G proteins are directly involved in serum-stimulated DNA synthesis through microinjection of inhibitory antibodies into living cells. Inhibitory rabbit polyclonal antibodies directed against specific Gi alpha subunits were introduced into living Balb/c 3T3 fibroblasts by microinjection, and the effect upon serum-stimulated DNA synthesis was examined. Results of these experiments indicate that Gi2 plays a direct role in serum-stimulated DNA synthesis in living cells and suggest that G proteins may function in a variety of mitogenic signaling pathways initiated by serum growth factors.

TSH, IGF-1 and activated ras protein induce DNA synthesis in cultured thyroid cells

TSH, IGF-1 and cellular ras genes have been proposed to function in thyroid cell transformation. Using cultured follicular cells, we demonstrate that TSH, IGF-1 and microinjected activated ras protein individually stimulate DNA synthesis. TSH-stimulated pathways include Gs at the plasma membrane and cyclic AMP response elements in the nucleus. The pathways and nuclear targets of IGF-1 and ras action appear at least partially distinct from those used by TSH.

Induction of a cyclic AMP-responsive gene in living cells requires the nuclear factor CREB

We constructed cell lines containing various enhancer elements cloned upstream from a marker gene. By microinjection of specific antibodies directed against transcriptional activator proteins into these cell lines, we have developed a functional assay for factors which regulate the activity of target promoters. Here we show that microinjection of a highly specific antibody to the cyclic AMP enhancer element-binding (CREB) protein diminishes gene expression in response to cyclic AMP in living fibroblasts.

Dynamics of the distribution of cyclic AMP-dependent protein kinase in living cells

The intracellular distribution of regulatory molecules may provide a mechanism for controlling gene expression. The subcellular location of cAMP-dependent protein kinase was analyzed in living cells by microinjection of regulatory and catalytic subunits labeled with fluorescein. Following microinjection, type I holoenzyme was found in the cytoplasm and remained there for up to 4 hr. Upon dissociation of holoenzyme with 8-bromo-cAMP, free catalytic subunit appeared in the nucleus while regulatory subunit remained in the cytoplasm. Similarly, purified catalytic subunit was transported to the nucleus in the absence of elevated intracellular cAMP following its introduction into the cytoplasm. Translocation to the nucleus was apparent within 10 min and persisted for at least 2 hr. In contrast, purified regulatory subunit, like holoenzyme, was maintained in the cytoplasm. These results suggest that one function of the type I regulatory subunit is to serve as a cytoplasmic anchor, sequestering the catalytic subunit in the cytoplasm until holoenzyme dissociates in response to increased cAMP.

Characterization of an episome produced in hamster cells that amplify a transfected CAD gene at high frequency: functional evidence for a mammalian replication origin

In a previous study (G. M. Wahl, B. Robert de Saint Vincent, and M. L. De Rose, Nature (London) 307:516-520, 1984), we used gene transfer of a CAD cosmid to demonstrate that gene position profoundly affects amplification frequency. One transformant, T5, amplified the donated CAD genes at a frequency at least 100-fold higher than did the other transformants analyzed. The CAD genes in T5 and two drug-resistant derivatives were chromosomally located. In this report, we show that a subclone of T5 gives rise to an extrachromosomal molecule (CAD episome) containing the donated CAD genes. Gel electrophoresis indicated that the CAD episome is approximately 250 to 300 kilobase pairs, and a variety of methods showed that it is a covalently closed circle. We show that the CAD episome replicates semiconservatively and approximately once per cell cycle. Since the CAD cosmid, which comprises most of the CAD episome, does not replicate autonomously when transfected into cells, our results indicate that either the process which generated the episome resulted in a cellular origin of DNA replication being linked to the CAD sequences or specific rearrangements within the episome generated a functional origin. The implications of these results for mechanisms of gene amplification and the genesis of minute chromosomes are discussed.

New RNA species is produced by alternate polyadenylation following rearrangement associated with CAD gene amplification

Mammalian cells selected to resist N-(phosphonacetyl)-L-aspartate (PALA) contain amplified copies of the CAD gene. While a single 7.9-kb mRNA species is detected in PALA-sensitive and most PALA-resistant cell lines, two RNA species (7.9 and 10.2 kb) are detected in two related drug-resistant mutants presumably derived from the same parental cell. In this report we show that the 10.2-kb RNA is produced as a direct consequence of a sequence rearrangement adjacent to the 3' end of the CAD gene in these cell lines. A CAD gene containing the sequence rearrangement was cloned from one of these lines and found to produce both RNA species when transfected into CAD-deficient cells. DNA sequencing and S1 analysis demonstrate that the 10.2-kb RNA is produced by alternative polyadenylation rather than by alternative splicing. Sequence analysis also reveals that several consensus poly(A) addition signals (AATAAA) were brought into close proximity to the CAD gene by virtue of the rearrangement. While sequences adjacent to each of the polyadenylation signals contain additional features postulated to be important for the selection of the site of poly(A) addition, S1 mapping analysis indicates that only one of the polyadenylation signals is used. A comparison of all of these sites suggests that multiple sequence motifs are required to form a functional polyadenylation and cleavage signal.