The serum response factor nuclear localization signal: general implications for cyclic AMP-dependent protein kinase activity in control of nuclear translocation

The transcription factor serum response factor stimulates axon regeneration through cytoplasmic localization and cofilin interaction

Axonal injury generates growth inert retraction bulbs with dynamic cytoskeletal properties that are severely compromised. Conversion of "frozen" retraction bulbs into actively progressing growth cones is a major aim in axon regeneration. Here we report that murine serum response factor (SRF), a gene regulator linked to the actin cytoskeleton, modulates growth cone actin dynamics during axon regeneration. In regeneration-competent facial motoneurons, Srf deletion inhibited axonal regeneration. In wild-type mice after nerve injury, SRF translocated from the nucleus to the cytoplasm, suggesting a cytoplasmic SRF function in axonal regeneration. Indeed, adenoviral overexpression of cytoplasmic SRF (SRF-ΔNLS-GFP) stimulated axonal sprouting and facial nerve regeneration in vivo. In primary central and peripheral neurons, SRF-ΔNLS-GFP stimulated neurite outgrowth, branch formation, and growth cone morphology. Furthermore, we uncovered a link between SRF and the actin-severing factor cofilin during axonal regeneration in vivo. Facial nerve axotomy increased the total cofilin abundance and also nuclear localization of phosphorylated cofilin in a subpopulation of lesioned motoneurons. This cytoplasmic-to-nucleus translocation of P-cofilin upon axotomy was reduced in motoneurons expressing SRF-ΔNLS-GFP. Finally, we demonstrate that cytoplasmic SRF and cofilin formed a reciprocal regulatory unit. Overexpression of cytoplasmic SRF reduced cofilin phosphorylation and vice versa: overexpression of cofilin inhibited SRF phosphorylation. Therefore, a regulatory loop consisting of SRF and cofilin might take part in reactivating actin dynamics in growth-inert retraction bulbs and facilitating axon regeneration.

Nuclear import of serum response factor in airway smooth muscle

We have previously shown that the transcription-promoting activity of serum response factor (SRF) is partially regulated by its extranuclear redistribution. In this study, we examined the cellular mechanisms that facilitate SRF nuclear entry in canine tracheal smooth muscle cells. We used in vitro pull-down assays to determine which karyopherin proteins bound SRF and found that SRF binds KPNA1 and KPNB1 through its nuclear localization sequence. Immunoprecipitation studies also demonstrated direct SRF-KPNA1 interaction in HEK293 cells. Import assays demonstrated that KPNA1 and KPNB1 together were sufficient to mediate rapid nuclear import of SRF-GFP. Our studies also suggest that SRF is able to gain nuclear entry through an auxiliary, nuclear localization sequence-independent mechanism.

A hitchhiker's guide to the MADS world of plants

Plant life critically depends on the function of MADS-box genes encoding MADS-domain transcription factors, which are present to a limited extent in nearly all major eukaryotic groups, but constitute a large gene family in land plants. There are two types of MADS-box genes, termed type I and type II, and in plants these groups are distinguished by exon-intron and domain structure, rates of evolution, developmental function and degree of functional redundancy. The type I genes are further subdivided into three groups - Mα, Mβ and Mγ - while the type II genes are subdivided into the MIKC^C and MIKC* groups. The functional diversification of MIKC^C genes is closely linked to the origin of developmental and morphological novelties in the sporophytic (usually diploid) generation of seed plants, most spectacularly the floral organs and fruits of angiosperms. Functional studies suggest different specializations for the different classes of genes; whereas type I genes may preferentially contribute to female gametophyte, embryo and seed development and MIKC*-group genes to male gametophyte development, the MIKC^C-group genes became essential for diverse aspects of sporophyte development. Beyond the usual transcriptional regulation, including feedback and feed-forward loops, various specialized mechanisms have evolved to control the expression of MADS-box genes, such as epigenetic control and regulation by small RNAs. In future, more data from genome projects and reverse genetic studies will allow us to understand the birth, functional diversification and death of members of this dynamic and important family of transcription factors in much more detail.

PKA-dependent phosphorylation of serum response factor inhibits smooth muscle-specific gene expression

OBJECTIVE

Our goal was to identify phosphorylation sites that regulate serum response factor (SRF) activity to gain a better understanding of the signaling mechanisms that regulate SRF's involvement in smooth muscle cell (SMC)-specific and early response gene expression.

METHODS AND RESULTS

By screening phosphorylation-deficient and mimetic mutations in SRF(-/-) embryonic stem cells, we identified T159 as a phosphorylation site that significantly inhibits SMC-specific gene expression in an embryonic stem cell model of SMC differentiation. This residue conforms to a highly conserved consensus cAMP-dependent protein kinase (PKA) site, and in vitro and in vivo labeling studies demonstrated that it was phosphorylated by PKA. Results from gel shift and chromatin immunoprecipitation assays demonstrated that T159 phosphorylation inhibited SRF binding to SMC-specific CArG elements. Interestingly, the myocardin factors could at least partially rescue the effects of the T159D mutation under some conditions, but this response was promoter specific. Finally, PKA signaling had much less of an effect on c-fos promoter activity and SRF binding to the c-fos CArG.

CONCLUSIONS

Our results indicate that phosphorylation of SRF by PKA inhibits SMC-specific transcription suggesting a novel signaling mechanism for the control of SMC phenotype.

Critical role of serum response factor in pulmonary myofibroblast differentiation induced by TGF-beta

Transforming growth factor-beta (TGF-beta) is a cytokine implicated in wound healing and in the pathogenesis of pulmonary fibrosis. TGF-beta stimulates myofibroblast differentiation characterized by expression of contractile smooth muscle (SM)-specific proteins such as SM-alpha-actin. In the present study, we examined the role of serum response factor (SRF) in the mechanism of TGF-beta-induced pulmonary myofibroblast differentiation of human lung fibroblasts (HLF). TGF-beta stimulated SM-alpha-actin expression in HLF, which paralleled with a profound induction of SRF expression and activity. Inhibition of SRF by the pharmacologic SRF inhibitor (CCG-1423), or via adenovirus-mediated transduction of SRF short hairpin RNA (shSRF), blocked the expression of both SRF and SM-alpha-actin in response to TGF-beta without affecting Smad-mediated signaling of TGF-beta. However, forced expression of SRF on its own did not promote SM-alpha-actin expression, whereas expression of the constitutively transactivated SRF fusion protein (SRF-VP16) was sufficient to induce SM-alpha-actin expression, suggesting that both expression and transactivation of SRF are important. Activation of protein kinase A (PKA) by forskolin or iloprost resulted in a significant inhibition of SM-alpha-actin expression induced by TGF-beta, and this was associated with inhibition of both SRF expression and activity, but not of Smad-mediated gene transcription. In summary, this is the first direct demonstration that TGF-beta-induced pulmonary myofibroblast differentiation is mediated by SRF, and that inhibition of myofibroblast differentiation by PKA occurs through down-regulation of SRF expression levels and SRF activity, independent of Smad signaling.

Cell-specific nuclear import of plasmid DNA in smooth muscle requires tissue-specific transcription factors and DNA sequences

Two shortcomings of nonviral gene therapy are a lack of tissue-specific targeting of vectors and low levels of gene transfer. Our laboratory has begun to address these limitations by designing plasmids that enter the nucleus of specific cell types in the absence of cell division, thereby enhancing expression in a controlled manner. We have shown that a 176 bp portion of the smooth muscle gamma-actin (SMGA) promoter can mediate plasmid nuclear import specifically in smooth muscle cells (SMCs). Here, we demonstrate that the binding sites for serum response factor (SRF) and NKX3-1/3-2 within this DNA nuclear targeting sequence (DTS) are required for plasmid nuclear import. Knockdown of these factors with siRNA abrogates plasmid nuclear import, indicating that they are necessary cofactors. In addition, coinjection of recombinant SRF and Nkx3.2 with the vector in TC7 epithelial cells rescues import. Finally, we show that the SRF nuclear localization sequence (NLS) is required for vector nuclear import. We propose that SRF and NKX3-1/3-2 bind the SMGA DTS in the cytoplasm, thus coating the plasmid with NLSs that mediate translocation across the nuclear pore complex. This discovery could aid in the development of more efficient nonviral vectors for gene transfer to SMCs.

Cross-species sequence analysis reveals multiple charged residue-rich domains that regulate nuclear/cytoplasmic partitioning and membrane localization of a kinase anchoring protein 12 (SSeCKS/Gravin)

A kinase anchoring proteins (AKAPs) assemble and compartmentalize multiprotein signaling complexes at discrete subcellular locales and thus confer specificity to transduction cascades using ubiquitous signaling enzymes, such as protein kinase A. Intrinsic targeting domains in each AKAP determine the subcellular localization of these complexes and, along with protein-protein interaction domains, form the core of AKAP function. As a foundational step toward elucidating the relationship between location and function, we have used cross-species sequence analysis and deletion mapping to facilitate the identification of the targeting determinants of AKAP12 (also known as SSeCKS or Gravin). Three charged residue-rich regions were identified that regulate two aspects of AKAP12 localization, nuclear/cytoplasmic partitioning and perinuclear/cell periphery targeting. Using deletion mapping and green fluorescent protein chimeras, we uncovered a heretofore unrecognized nuclear localization potential. Five nuclear localization signals, including a novel class of this type of signal termed X2-NLS, are found in the central region of AKAP12 and are important for nuclear targeting. However, this nuclear localization is suppressed by the negatively charged C terminus that mediates nuclear exclusion. In this condition, the distribution of AKAP12 is regulated by an N-terminal targeting domain that simultaneously directs perinuclear and peripheral AKAP12 localization. Three basic residue-rich regions in the N-terminal targeting region have similarity to the MARCKS proteins and were found to control AKAP12 localization to ganglioside-rich regions at the cell periphery. Our data suggest that AKAP12 localization is regulated by a hierarchy of targeting domains and that the localization of AKAP12-assembled signaling complexes may be dynamically regulated.

Protein kinase A inhibits lysophosphatidic acid induction of serum response factor via alterations in the actin cytoskeleton

Lysophosphatidic acid (LPA; 1-acyl-2-hydroxy-sn-glycero-3-phosphate) is a lipid growth factor that stimulates the proliferation of ovarian cancer cells. Recent studies indicate that elevation of cellular cAMP levels inhibits ovarian epithelial cancer cell growth. In this study, we investigated the effects of elevating cellular cAMP levels on LPA stimulation of OVCAR-3 ovarian cancer cell growth and on LPA stimulation of the serum response factor (SRF) transcription factor. Treatment of OVCAR-3 cells with forskolin and isobutylmethylxanthine (IBMX; 3-Isobutyl-1-methylxanthine) inhibited LPA stimulation of growth. LPA stimulation of SRF-mediated transcription was also inhibited in OVCAR-3 cells that were incubated with forskolin, dibutyryl cyclic AMP (db-cAMP), or paired cAMP analogues (N(6)-mono-tert-butylcarbamoyladenosine-3', 5'-cyclic monophosphate [6-MBC-cAMP] and Sp-5,6-DCl-BIMPS), which selectively activate type II protein kinase A. In contrast, incubation with a cAMP analogue (8-(4-chloro-phenylthio)-2'-O-methyadenosine-3',5'-cyclic monophosphate [8CPT-2Me-cAMP]) that specifically activates the cAMP inducible Rap1 exchange factor, Epac, did not inhibit SRF. Similar results were obtained when HepG2 hepatoma cells, which do not express endogenous LPA receptors, were transfected with a single LPA receptor (LPA(1)). We observed that treatment of OVCAR-3 cells with forskolin greatly reduced both F-actin staining and focal adhesion labeling with anti-paxillin antibodies. Treatment of OVCAR-3 cells with the F-actin stabilizing compound, jasplakinolide, prevented the protein kinase A (PKA)-mediated inhibition of SRF. These results suggest that PKA inhibits LPA stimulation of SRF by promoting the dissolution of F-actin and that this is likely to contribute to the cAMP-mediated inhibition of ovarian cancer cell growth.

Androgen receptor activation by G(s) signaling in prostate cancer cells

The androgen receptor (AR) is activated in prostate cancer patients undergoing androgen ablative therapy and mediates growth of androgen-insensitive prostate cancer cells, suggesting it is activated by nonandrogenic factors. We demonstrate that activated alpha subunit of heterotrimeric guanine nucleotide-binding G(s) protein activates the AR in prostate cancer cells and also synergizes with low concentration of androgen to more fully activate the AR. The G alpha(s) activates protein kinase A, which is required for the nuclear partition and activation of AR. These data suggest a role for G alpha(s) and PKA in the transactivation of AR in prostate cancer cells under the environment of reduced androgen levels.

Identification of a novel serum response factor cofactor in cardiac gene regulation

The transcription factor serum response factor (SRF) plays an important role in the regulation of a variety of cardiac genes during development and during adult aging. A novel SRF cofactor, herein called p49/STRAP, for SRF-dependent transcription regulation-associated protein, was recently identified in our laboratory. This protein interacted mainly with the transcriptional activation domain of the SRF protein and was found to bind to SRF or to the complex of SRF and another cofactor, such as myocardin or Nkx2.5. The expression of p49/STRAP affected the promoter activity of SRF target genes in a non-uniform manner. For example, p49 activated MLC2v and cardiac actin promoters when it was co-transfected with SRF, but it repressed atrial natriuretic factor promoter activity, which was strongly induced by myocardin. The p49/STRAP mRNA was observed to be highly expressed in fetal, adult, and senescent human hearts, and also in hearts of young adult and old mice, suggesting that p49/STRAP may be an important SRF cofactor in the transcriptional regulation of mammalian cardiac muscle genes throughout the life span.

Platelet-derived growth factor-BB-mediated activation of Akt suppresses smooth muscle-specific gene expression through inhibition of mitogen-activated protein kinase and redistribution of serum response factor

Platelet-derived growth factor (PDGF) inhibits expression of smooth muscle (SM) genes in vascular smooth muscle cells and blocks induction by arginine vasopressin (AVP). We have previously demonstrated that suppression of SM-alpha-actin by PDGF-BB is mediated in part through a Ras-dependent pathway. This study examined the role of phosphatidylinositol 3-kinase (PI3K)y and its downstream effector, Akt, in regulating SM gene expression. PDGF caused a rapid sustained activation of Akt, whereas AVP caused only a small transient increase. PDGF selectively caused a sustained stimulation of p85/p110 alpha PI3K. In contrast, p85/110 beta PI3K activity was not altered by either PDGF or AVP, whereas both agents caused a delayed activation of Class IB p101/110 gamma PI3K. Expression of a gain-of-function PI3K or myristoylated Akt (myr-Akt) mimicked the inhibitory effect of PDGF on SM-alpha-actin and SM22 alpha expression. Pretreatment with LY 294002 reversed the inhibitory effect of PDGF. Expression of myr-Akt selectively inhibited AVP-induced activation of c-Jun N-terminal kinase and p38 mitogen-activated protein kinases, which we have shown are critical for induction of these genes. Nuclear extracts from PDGF-stimulated or myr-Akt expressing cells showed reduced serum response factor binding to SM-specific CArG elements. This was associated with appearance of serum response factor in the cytoplasm. These data indicate that activation of p85/p110 alpha/Akt mediates suppression of SM gene expression by PDGF.

Serum response factor function and dysfunction in smooth muscle

Tight control of smooth muscle cell (SM) proliferation, differentiation, and apoptosis requires a balance between signaling and transcriptional events. Recent developments in vascular research revealed that serum response factor (SRF) function is important for the regulation of each of these processes. The cloning and characterization of several SM specific genes and the discovery that SRF is central for their expression fueled studies aimed at understanding the role of molecular partners including co-activators and co-repressors. Perturbations of pathways involving SRF are associated with abnormalities in the myogenic program and aberrant phenotypic consequences. Surprisingly, studies on airway SM have remained an underrepresented area of investigation. Our laboratory described a novel regulatory mechanism of SRF function in airway myocytes by modulation of its subcellular localization. This review summarizes current knowledge on the structure and function of this essential transcription factor as well different modes of regulating SRF expression and activity that are becoming key players in directing SM function in health and disease.

Protein kinase A phosphorylation modulates transport of the polypyrimidine tract-binding protein

The heterogeneous nuclear ribonucleoprotein particle (hnRNP) proteins play important roles in mRNA processing in eukaryotes, but little is known about how they are regulated by cellular signaling pathways. The polypyrimidine-tract binding protein (PTB, or hnRNP I) is an important regulator of alternative pre-mRNA splicing, of viral RNA translation, and of mRNA localization. Here we show that the nucleo-cytoplasmic transport of PTB is regulated by the 3',5'-cAMP-dependent protein kinase (PKA). PKA directly phosphorylates PTB on conserved Ser-16, and PKA activation in PC12 cells induces Ser-16 phosphorylation. PTB carrying a Ser-16 to alanine mutation accumulates normally in the nucleus. However, export of this mutant protein from the nucleus is greatly reduced in heterokaryon shuttling assays. Conversely, hyperphosphorylation of PTB by coexpression with the catalytic subunit of PKA results in the accumulation of PTB in the cytoplasm. This accumulation is again specifically blocked by the S16A mutation. Similarly, in Xenopus oocytes, the phospho-Ser-16-PTB is restricted to the cytoplasm, whereas the non-Ser-16-phosphorylated PTB is nuclear. Thus, direct PKA phosphorylation of PTB at Ser-16 modulates the nucleo-cytoplasmic distribution of PTB. This phosphorylation likely plays a role in the cytoplasmic function of PTB.

The RhoA/Rho kinase pathway regulates nuclear localization of serum response factor

RhoA and its downstream target Rho kinase regulate serum response factor (SRF)-dependent skeletal and smooth muscle gene expression. We previously reported that long-term serum deprivation reduces transcription of smooth muscle contractile apparatus encoding genes, by redistributing SRF out of the nucleus. Because serum components stimulate RhoA activity, these observations suggest the hypothesis that the RhoA/Rho kinase pathway regulates SRF-dependent smooth muscle gene transcription in part by controlling SRF subcellular localization. Our present results support this hypothesis: cotransfection of cultured airway myocytes with a plasmid expressing constitutively active RhoAV14 selectively enhanced transcription from the SM22 and smooth muscle myosin heavy chain promoters and from a purely SRF-dependent promoter, but had no effect on transcription from the MSV-LTR promoter or from an AP2-dependent promoter. Conversely, inhibition of the RhoA/Rho kinase pathway by cotransfection with a plasmid expressing dominant negative RhoAN19, by cotransfection with a plasmid expressing Clostridial C3 toxin, or by incubation with the Rho kinase inhibitor, Y-27632, all selectively reduced SRF-dependent smooth muscle promoter activity. Furthermore, treatment with Y-27632 selectively reduced binding of SRF from nuclear extracts to its consensus DNA target, selectively reduced nuclear SRF protein content, and partially redistributed SRF from nucleus to cytoplasm, as revealed by quantitative immunocytochemistry. Treatment of cultured airway myocytes with latrunculin B, which reduces actin polymerization, also caused partial redistribution of SRF into the cytoplasm. Together, these results demonstrate for the first time that the RhoA/Rho kinase pathway controls smooth muscle gene transcription in differentiated smooth muscle cells, in part by regulating the subcellular localization of SRF. It is conceivable that the RhoA/Rho kinase pathway influences SRF localization through its effect on actin polymerization dynamics.

Inhibition of the cardiac alpha-actin gene in embryonic cardiac myocytes by dominant-negative serum response factor

Serum response factor (SRF), a transcription factor ubiquitously involved in the processes of cellular proliferation and differentiation, has been implicated in cardiac and skeletal muscle development because of its strong expression in embryonic muscle lineages, and its necessity for the transcription of transiently transfected muscle genes that contain SRF binding sites. This study was designed to ascertain whether SRF is required for the expression of an endogenous SRF-dependent gene during differentiation of early embryonic cardiac myocytes by introducing a dominant-negative SRF construct via retroviral delivery. Although no effect on overt cellular differentiation was detected, semi-quantitative RT-PCR revealed that expression of the SRF-dependent gene cardiac alpha-actin was inhibited, whereas expression of the non-SRF-dependent genes glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and cardiac troponin-C was unaffected. No effect on myocyte proliferation was detected. Curiously, immunohistochemical localization of SRF protein suggested that whereas endogenous SRF was homogeneously dispersed throughout the cytoplasm and nucleus, the dominant-negative SRF protein was concentrated in the nucleus. These results extend previous findings using transiently transfected genes to the endogenous level, indicating that SRF is required for the full expression of muscle genes that contain SRF binding sites during cardiac myocyte differentiation.

The proximal serum response element in the Egr-1 promoter mediates response to thrombin in primary human endothelial cells

Thrombin signaling in endothelial cells provides an important link between coagulation and inflammation. We report here that thrombin induces endogenous Egr-1 mRNA and Egr-1 promoter activity in primary human endothelial cells by approximately 6-fold and 3-fold, respectively. In transient transfection assays, deletion of the 3' cluster of serum response elements (SREs), but not the 5' cluster of SREs, resulted in a loss of thrombin response. When coupled to a heterologous core promoter, a region spanning the 3' SRE cluster contained information for thrombin response, whereas a region spanning the 5' SRE cluster had no such effect. A point mutation of the most proximal SRE (SRE-1), but not of the proximal Ets motif or upstream SREs, abrogated the response to thrombin. In electrophoretic mobility shift assays, nuclear extracts from thrombin-treated cells displayed increased binding of total and phosphorylated serum response factor (SRF) to SRE-1. Thrombin-mediated induction of Egr-1 was blocked by inhibitors of MEK1/2, but not by inhibitors of protein kinase C, phosphatidylinositol 3-kinase, or p38 mitogen-activated protein kinase (MAPK). Taken together, these data suggest that thrombin induces Egr-1 expression in endothelial cells by a MAPK-dependent mechanism that involves an interaction between SRF and SRE-1.

Protein kinase A balances the growth factor-induced Ras/ERK signaling

Protein kinase A (PKA) has been proposed to regulate the signal transduction through the Ras/extracellular-regulated kinase (ERK) pathway. Here we demonstrate that when the PKA activity was inhibited prior to growth factor stimulus the signal flow through the Ras/ERK pathway was significantly increased. Furthermore, the data indicated that this PKA-mediated regulation was simultaneously targeted to the upstream kinase Raf-1 and to the ERK-specific phosphatase mitogen-activated protein kinase phosphatase-1 (MKP-1). Moreover, our data suggested that the level of PKA activity determined the transcription rate of mkp-1 gene, whereas the Ras/ERK signal was required to protect the MKP-1 protein against degradation. These results point to a tight regulatory relationship between PKA and the growth factor signaling, and further suggest an important role for basal PKA activity in such regulation. We propose that PKA adjusts the activity of the Ras/ERK pathway and maintains it within a physiologically appropriate level.

HOXB4 homeodomain protein is expressed in developing epidermis and skin disorders and modulates keratinocyte proliferation

The HOX homeodomain proteins are fundamental regulators of organ and tissue development, where they are thought to function as transcription factors, and HOX gene expression has been associated with numerous types of cancers. Previous studies have demonstrated that enforced expression of the HOXB4 protein transforms cultured fibroblasts and leads to a selective expansion of the hematopoietic stem cell pool, suggesting that this protein might play a role in cellular proliferation. In support of this concept, we now show that enforced expression of HOXB4 in human neonatal keratinocytes results in increased cellular proliferation and colony formation as well as decreased expression of the alpha-2-integrin and CD44 cell surface adhesion molecules. We previously have reported HOXB4 gene expression in the basal and suprabasal layers of developing human skin and now show extensive HOXB4 mRNA in psoriatic skin and basal cell carcinoma. In fetal human skin HOXB4 protein expression was both nuclear and cytoplasmic within epidermal basal cells and in hair follicle inner and outer root sheath cells, whereas strong nuclear signals were observed in the bulge region. In adult skin, HOXB4 protein expression was both nuclear and cytoplasmic, but was predominantly localized to the intermediate and differentiated cell layers. In contrast to the striking gradient patterns of HOX gene and protein expression previously described in developing spinal cord and limb, HOXB4 protein was uniformly detected in all regions of the fetal and adult skin. Although little HOXB4 signal localized to proliferative cell layers, as marked by proliferating cell nuclear antigen (PCNA) staining, in normal adult epidermis, nuclear HOXB4 protein expression substantially overlapped with PCNA-positive cell in a series of samples of hyperproliferative skin. Taken together, these data suggest that nuclear HOXB4 protein may play a role in the regulation of cellular proliferation/adhesion in developing fetal human epidermis and in hyperproliferation conditions, including cancers, in adult epidermis. Published 2002 Wiley-Liss, Inc.

High RhoA activity maintains the undifferentiated mesenchymal cell phenotype, whereas RhoA down-regulation by laminin-2 induces smooth muscle myogenesis

Round embryonic mesenchymal cells have the potential to differentiate into smooth muscle (SM) cells upon spreading/elongation (Yang, Y., K.C. Palmer, N. Relan, C. Diglio, and L. Schuger. 1998. Development. 125:2621-2629; Yang, Y., N.K. Relan, D.A. Przywara, and L. Schuger. 1999. Development. 126:3027-3033; Yang, Y., S. Beqaj, P. Kemp, I. Ariel, and L. Schuger. 2000. J. Clin. Invest. 106:1321-1330). In the developing lung, this process is stimulated by peribronchial accumulation of laminin (LN)-2 (Relan, N.K., Y. Yang, S. Beqaj, J.H. Miner, and L. Schuger. 1999. J. Cell Biol. 147:1341-1350). Here we show that LN-2 stimulates bronchial myogenesis by down-regulating RhoA activity. Immunohistochemistry, immunoblotting, and reverse transcriptase-PCR indicated that RhoA, a small GTPase signaling protein, is abundant in undifferentiated embryonic mesenchymal cells and that its levels decrease along with SM myogenesis. Functional studies using agonists and antagonists of RhoA activation and dominant positive and negative plasmid constructs demonstrated that high RhoA activity was required to maintain the round undifferentiated mesenchymal cell phenotype. This was in part achieved by restricting the localization of the myogenic transcription factor serum response factor (SRF) mostly to the mesenchymal cell cytoplasm. Upon spreading on LN-2 but not on other main components of the extracellular matrix, the activity and level of RhoA decreased rapidly, resulting in translocation of SRF to the nucleus. Both cell elongation and SRF translocation were prevented by overexpression of dominant positive RhoA. Once the cells underwent SM differentiation, up-regulation of RhoA activity induced rather than inhibited SM gene expression. Therefore, our studies suggest a novel mechanism whereby LN-2 and RhoA modulate SM myogenesis.

Early postnatal cardiac changes and premature death in transgenic mice overexpressing a mutant form of serum response factor

Serum response factor (SRF) is a key regulator of a number of extracellular signal-regulated genes important for cell growth and differentiation. A form of the SRF gene with a double mutation (dmSRF) was generated. This mutation reduced the binding activity of SRF protein to the serum response element and reduced the capability of SRF to activate the atrial natriuretic factor promoter that contains the serum response element. Cardiac-specific overexpression of dmSRF attenuated the total SRF binding activity and resulted in remarkable morphologic changes in the heart of the transgenic mice. These mice had dilated atrial and ventricular chambers, and their ventricular wall thicknesses were only 1/2 to 1/3 the thickness of that of nontransgenic mice. Also these mice had smaller cardiac myocytes and had less myofibrils in their myocytes relative to nontransgenic mice. Altered gene expression and slight interstitial fibrosis were observed in the myocardium of the transgenic mice. All the transgenic mice died within the first 12 days after birth, because of the early onset of severe, dilated cardiomyopathy. These results indicate that dmSRF overexpression in the heart apparently alters cardiac gene expression and blocks normal postnatal cardiac growth and development.

Large isoform of hepatitis delta antigen activates serum response factor-associated transcription

Hepatitis delta virus infection sometimes causes severe and fulminant hepatitis as a coinfection or superinfection along with the hepatitis B virus. To elucidate the underlying mechanism of injury caused by hepatitis delta virus, we examined whether two isoforms of the hepatitis delta antigen (HDAg) had any effect on five well defined intracellular signal transduction pathways: serum response factor (SRF)-, serum response element (SRE)-, nuclear factor kappaB-, activator protein 1-, and cyclic AMP response element-dependent pathways. Reporter assays revealed that large HDAg (LHDAg) activated the SRF- and SRE-dependent pathways. In contrast, small HDAg (SHDAg) did not activate any of five pathways. LHDAg enhanced the transcriptional ability of SRF without changing its DNA binding affinity in an electrophoretic mobility shift assay. In addition, LHDAg activated a rat SM22alpha promoter containing SRF binding site and a human c-fos promoter containing SRE. In conclusion, LHDAg, but not SHDAg, enhances SRF-associated transcriptions. Despite structural similarities between the two HDAgs, there are significant differences in their effects on intracellular signal transduction pathways. These results may provide clues that will aid in the clarification of functional differences between LHDAg and SHDAg and the pathogenesis of delta hepatitis.

Physiological control of smooth muscle-specific gene expression through regulated nuclear translocation of serum response factor

Prolonged serum deprivation induces a structurally and functionally contractile phenotype in about 1/6 of cultured airway myocytes, which exhibit morphological elongation and accumulate abundant contractile apparatus-associated proteins. We tested the hypothesis that transcriptional activation of genes encoding these proteins accounts for their accumulation during this phenotypic transition by measuring the transcriptional activities of the murine SM22 and human smooth muscle myosin heavy chain promoters during transient transfection in subconfluent, serum fed or 7 day serum-deprived cultured canine tracheal smooth muscle cells. Contrary to our expectation, SM22 and smooth muscle myosin heavy chain promoter activities (but not viral murine sarcoma virus-long terminal repeat promoter activity) were decreased in long term serum-deprived myocytes by at least 8-fold. Because serum response factor (SRF) is a required transcriptional activator of these and other smooth muscle-specific promoters, we evaluated the expression and function of SRF in subconfluent and long term serum-deprived cells. Whole cell SRF mRNA and protein were maintained at high levels in serum-deprived myocytes, but SRF transcription-promoting activity, nuclear SRF binding to consensus CArG sequences, and nuclear SRF protein were reduced. Furthermore, immunocytochemistry revealed extranuclear redistribution of SRF in serum-deprived myocytes; nuclear localization of SRF was restored after serum refeeding. These results uncover a novel mechanism for physiological control of smooth muscle-specific gene expression through extranuclear redistribution of SRF and consequent down-regulation of its transcription-promoting activity.

Involvement of protein kinase A in fibroblast growth factor-2-activated transcription

Polypeptide growth factors activate common signal transduction pathways, yet they can induce transcription of different target genes. The mechanisms that control this specificity are not completely understood. Recently, we have described a fibroblast growth factor (FGF)-inducible response element, FiRE, on the syndecan-1 gene. In NIH 3T3 cells, the FiRE is activated by FGF-2 but not by several other growth factors, such as platelet-derived growth factor or epidermal growth factor, suggesting that FGF-2 activates signaling pathways that diverge from pathways activated by other growth factors. In this paper, we report that the activation of FiRE by FGF-2 requires protein kinase A (PKA) in NIH 3T3 cells. The PKA-specific inhibitor H-89 (N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide) blocked the FGF-2-induced activation of FiRE, the transcription of the syndecan-1 gene, and cell proliferation. Also, expression of a dominant-negative form of PKA inhibited the FGF-2-induced FiRE activation and the transcription of the syndecan-1 gene. The binding of activator protein-1 transcription-factor complexes, required for the activation of FiRE, was blocked by inhibition of PKA activity before FGF-2 treatment. In accordance with the growth factor specificity of FiRE, the activity of PKA was stimulated by FGF-2 but not by platelet-derived growth factor or epidermal growth factor. Furthermore, a portion of the PKA catalytic subunit pool was translocated to the nucleus by FGF-2. Noticeably, the total cellular cAMP concentration was not affected by FGF-2 stimulus. We propose that the FGF-2-selective transcriptional activation through FiRE is caused by the ability of FGF-2 to control PKA activity.

Cell-specific nuclear import of plasmid DNA

One factor limiting the success of non-viral gene therapy vectors is the relative inability to target genes specifically to a desired cell type. To address this limitation, we have begun to develop cell-specific vectors whose specificity is at the level of the nuclear import of the plasmid DNA. We have recently shown that nuclear import of plasmid DNA is a sequence-specific event, requiring the SV40 enhancer, a region known to bind to a number of general transcription factors (Dean DA, Exp Cell Res 1997; 230: 293). From these studies we developed a model whereby transcription factor(s) bind to the DNA in the cytoplasm to create a protein-DNA complex that can enter the nucleus using the protein import machinery. Our model predicts that by using DNA elements containing binding sites for transcription factors expressed in unique cell types, we should be able to create plasmids that target to the nucleus in a cell-specific manner. Using the promoter from the smooth muscle gamma actin (SMGA) gene whose expression is limited to smooth muscle cells, we have created a series of reporter plasmids that are expressed selectively in smooth muscle cells. Moreover, when injected into the cytoplasm, plasmids containing portions of the SMGA promoter localize to the nucleus of smooth muscle cells, but remain cytoplasmic in fibroblasts and CV1 cells. In contrast, a similar plasmid carrying the SV40 enhancer is transported into the nuclei of all cell types tested. Nuclear import of the SMGA promoter-containing plasmids could be achieved when the smooth muscle specific transcription factor SRF was expressed in stably transfected CV1 cells, supporting our model for the nuclear import of plasmids. Finally, these nuclear targeting sequences were also able to promote increased gene expression in liposome- and polycation-transfected non-dividing cells in a cell-specific manner, similar to their nuclear import activity. These results provide proof of principle for the development of cell-specific non-viral vectors for any desired cell type.

MAPKAP kinase 2 phosphorylates serum response factor in vitro and in vivo

Several growth factor- and calcium-regulated kinases such as pp90(rsk) or CaM kinase IV can phosphorylate the transcription factor serum response factor (SRF) at serine 103 (Ser-103). However, it is unknown whether stress-regulated kinases can also phosphorylate SRF. We show that treatment of cells with anisomycin, arsenite, sodium fluoride, or tetrafluoroaluminate induces phosphorylation of SRF at Ser-103 in both HeLa and NIH3T3 cells. This phosphorylation is dependent on the kinase p38/SAPK2 and correlates with the activation of MAPKAP kinase 2 (MK2). MK2 phosphorylates SRF in vitro at Ser-103 with similar efficiency as the small heat shock protein Hsp25 and significantly better than CREB. Comparison of wild type murine fibroblasts with those derived from MK2-deficient mice (Mk(-/-)) reveals MK2 as the major SRF kinase induced by arsenite. These results demonstrate that SRF is targeted by several signal transduction pathways within cells and establishes SRF as a nuclear target for MAPKAP kinase 2.

Up-regulation of nuclear protein import by nuclear localization signal sequences in living cells

Using an in vivo assay system, nuclear import ability in individual cells was determined by examining the nuclear import rate. It was found that when a small (not excess) amount of SV40 T-NLS peptides was co-injected, the nuclear import rate of SV40 T-NLS-containing substrates apparently increased. This up-regulation was reproduced by the co-injection of peptides containing bipartite type NLS of CBP80, but not mutated non-functional NLS peptides, which suggests that these phenomena are specific for functional NLSs. It was further shown that although, in growth-arrested cells, the nuclear import rate was down-regulated compared to growing cells, the elevation of the functional import rate by co-injected NLS peptides reached the same level as in proliferating cells. This up-regulation was abolished by the addition of a protein kinase inhibitor, staurosporine. These results suggest that although potential nuclear import ability does not vary in each cell, the rate of nuclear import may be controlled by the amount of karyophilic proteins, which need to be carried into the nucleus from the cytoplasm, possibly via an NLS-dependent phosphorylation reaction.

Inducible and constitutive transcription factors in the mammalian nervous system: control of gene expression by Jun, Fos and Krox, and CREB/ATF proteins

This article reviews findings up to the end of 1997 about the inducible transcription factors (ITFs) c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, Fra-2, Krox-20 (Egr-2) and Krox-24 (NGFI-A, Egr-1, Zif268); and the constitutive transcription factors (CTFs) CREB, CREM, ATF-2 and SRF as they pertain to gene expression in the mammalian nervous system. In the first part we consider basic facts about the expression and activity of these transcription factors: the organization of the encoding genes and their promoters, the second messenger cascades converging on their regulatory promoter sites, the control of their transcription, the binding to dimeric partners and to specific DNA sequences, their trans-activation potential, and their posttranslational modifications. In the second part we describe the expression and possible roles of these transcription factors in neural tissue: in the quiescent brain, during pre- and postnatal development, following sensory stimulation, nerve transection (axotomy), neurodegeneration and apoptosis, hypoxia-ischemia, generalized and limbic seizures, long-term potentiation and learning, drug dependence and withdrawal, and following stimulation by neurotransmitters, hormones and neurotrophins. We also describe their expression and possible roles in glial cells. Finally, we discuss the relevance of their expression for nervous system functioning under normal and patho-physiological conditions.

cAMP-dependent protein kinase phosphorylates and activates nuclear Ca2+-ATPase

A Ca2+-pump ATPase, similar to that in the endoplasmic reticulum, has been located on the outer membrane of rat liver nuclei. The effect of cAMP-dependent protein kinase (PKA) on nuclear Ca2+-ATPase (NCA) was studied by using purified rat liver nuclei. Treatment of isolated nuclei with the catalytic unit of PKA resulted in the phosphorylation of a 105-kDa band that was recognized by antibodies specific for sarcoplasmic reticulum Ca2+-ATPase type 2b. Partial purification and immunoblotting confirmed that the 105-kDa protein band phosphorylated by PKA is NCA. The stoichiometry of phosphorylation was 0.76 mol of phosphate incorporated/mol of partially purified enzyme. Measurement of ATP-dependent 45Ca2+ uptake into purified nuclei showed that PKA phosphorylation enhanced the Ca2+-pumping activity of NCA. We show that PKA phosphorylation of Ca2+-ATPase enhances the transport of 10-kDa fluorescent-labeled dextrans across the nuclear envelope. The findings reported in this paper are consistent with the notion that the crosstalk between the cAMP/PKA- and Ca2+-dependent signaling pathways identified at the cytoplasmic level extends to the nucleus. Furthermore, these data support a function for crosstalk in the regulation of calcium-dependent transport across the nuclear envelope.

Synergistic increase in c-fos expression by simultaneous activation of the ras/raf/map kinase- and protein kinase A signaling pathways is mediated by the c-fos AP-1 and SRE sites

Expression of the c-fos proto-oncogene is induced by numerous stimuli some of which are transmitted through the Ras/Raf/MAP kinase or the cAMP-dependent protein kinase (PKA) pathways. The effect of cell-specific interactions between these pathways on c-fos expression was investigated by exposing quiescent NIH3T3 cells to serum, forskolin, or a combination. Co-stimulation with serum and forskolin resulted in a more than additive increase in c-fos transcription. Synergistic increase in c-fos promoter activity was also observed in transient transfection studies after co-stimulation with serum plus forskolin or co-transfection with c-Raf and PKA expression plasmids. Analysis of the cAMP signaling pathway revealed that the synergy was neither due to an increase in PKA activity nor to Ser-133 phosphorylation/activation of CREB. The activation status of the MAP kinases ERK1 and ERK2 in co-treated cells was comparable to that in serum-treated cells. Co-stimulation with forskolin did not alter the phosphorylation state of Elk-1 compared to serum-induced phosphorylation of Elk-1. Deletion of c-fos promoter elements previously shown to be important for regulation of c-fos expression in response to mitogens indicates a role for SRE and FAP-1 elements.

Cationic lipid-based gene delivery systems: pharmaceutical perspectives

Gene delivery systems are designed to control the location of administered therapeutic genes within a patient's body. Successful in vivo gene transfer may require (i) the condensation of plasmid and its protection from nuclease degradation, (ii) cellular interaction and internalization of condensed plasmid, (iii) escape of plasmid from endosomes (if endocytosis is involved), and (iv) plasmid entry into cell nuclei. Expression plasmids encoding a therapeutic protein can be, for instance, complexed with cationic liposomes or micelles in order to achieve effective in vivo gene transfer. A thorough knowledge of pharmaceutics and drug delivery, bio-engineering, as well as cell and molecular biology is required to design optimal systems for gene therapy. This mini-review provides a critical discussion on cationic lipid-based gene delivery systems and their possible uses as pharmaceuticals.

Phosphorylation of Srp1p, the yeast nuclear localization signal receptor, in vitro and in vivo

Srp1p, the protein encoded by SRP1 of the yeast Saccharomyces cerevisiae, is a yeast nuclear localization signal (NLS) receptor protein. We have previously reported isolation of a protein kinase from yeast extracts that phosphorylates Srp1p complexed with NLS peptides/proteins. From partial amino acid sequences of the four subunits of the purified kinase, we have now identified this protein kinase to be identical to yeast casein kinase II (CKII). It was previously thought that autophosphorylation of the 36 kDa subunit of the yeast enzyme was stimulated by the substrate, GST-Srp1p. However, with the use of a more refined system, no stimulation of autophosphorylation of the 36 kDa subunit of yeast CKII was observed. Biochemical and mutational analyses localized the in vitro phosphorylation site of Srp1p by CKII to serine 67. It was shown that, in the absence of NLS peptides/proteins, phosphorylation of the intact Srp1p protein is very weak, but deletion of the C-terminal end causes great stimulation of phosphorylation without NLS peptides/proteins. Thus, the CKII phosphorylation site is apparently masked in the intact protein structure by the presence of a C-terminal region, probably between amino acids 403 and 516. Binding of NLS peptides/proteins most likely causes a change in protein conformation, exposing the CKII phosphorylation site. Mutational alterations of serine 67, the CKII phosphorylation site, to valine (S67V) and aspartic acid (S67D) were not found to cause any significant deleterious effects on cell growth. Analysis of in vivo phosphorylation showed that at least 30% of the wild type Srp1p molecules are phosphorylated in growing cells, and that the phosphorylation is mostly at the serine 67 CKII site. The ability of Srp1p purified from E coli and treated with calf intestinal phosphatase to bind a SV40 T-antigen NLS peptide was compared with that of Srp1p which was almost fully phosphorylated by CKII. No significant difference was observed. It appears that NLS binding does not require any phosphorylation of Srp1p, either by CKII or by some other protein kinase.

Nuclear localization of the Arabidopsis APETALA3 and PISTILLATA homeotic gene products depends on their simultaneous expression

The Arabidopsis APETALA3 (AP3) and PISTILLATA (PI) proteins are thought to act as transcription factors and are required for specifying floral organ identities. To define the nuclear localization signals within these proteins, we generated translational fusions of the coding regions of AP3 and PI to the bacterial uidA gene that encodes beta-glucuronidase (GUS). Transient transformation assays of either the AP3-GUS or PI-GUS fusion protein alone resulted in cytoplasmic localization of GUS activity. However, coexpression of AP3-GUS with PI, or PI-GUS with AP3, resulted in nuclear localization of GUS activity. Stable transformation with these fusion proteins in Arabidopsis showed similar results. The nuclear colocalization signals in AP3 and PI were mapped to the amino-terminal regions of each protein. These observations suggest that the interaction of the AP3 and PI gene products results in the formation of a protein complex that generates or exposes a colocalization signal required to translocate the resulting complex into the nucleus. The colocalization phenomenon that we have described represents a novel mechanism to coordinate the functions of transcription factors within the nucleus.

Stimulation of nuclear import by simian virus 40-transformed cell extracts is dependent on protein kinase activity

We previously reported that both the nuclear import rate of large karyophilic gold particles and the functional size of the pores are significantly greater in simian virus 40-transformed fibroblasts (the SV-T2 cell line) than in nontransformed BALB/c 3T3 cells. In this study, we found that cytosolic fractions obtained from SV-T2 cultures can increase nuclear transport capacity (both import rate and pore size) when microinjected into BALB/c 3T3 cells. The transport-enhancing function of the extracts can be abolished by the protein kinase inhibitors staurosporine and K252a as well as 5'-p-fluorosulfonylbenzoyladenosine and protein phosphatase 2A, which, although less specific, also interfere with kinase activity. Increases in transport capacity of the same magnitude as that produced by the SV-T2 extracts were obtained by microinjecting protein kinase A or C or recombinant mitogen-activated protein kinase. These data provide further support for the interpretation that the enhancer is a protein kinase. From experiments performed with specific kinase inhibitor peptides, it appears likely that protein kinase C is the active factor in the SV-T2 cytosolic fractions; however, this will require further verification. It was also determined, by using gold particles coated with bovine serum albumin conjugated to synthetic nuclear localization signal peptides that lacked phosphorylation sites, that the enhancer affects the transport machinery rather than the activity of the nuclear localization signals.

Direct selection of cDNAs using whole chromosomes

We have developed a method for direct selection of cDNAs using whole chromosomes as target DNA. Double-strand cDNAs were synthesized from human fetal brain polyadenylated mRNAs. Flow-sorted chromosomes 17 and 19 were amplified by degenerate oligonucleotide primed polymerase chain reaction (DOP-PCR) and used to capture ds cDNAs by an improved magnetic bead capture protocol. To demonstrate the capabilities of this method, the selected cDNAs were used as probes in FISH experiments. The selected cDNA populations specifically painted chromosomes 17 or 19 on metaphase spreads. These results demonstrate that it is possible to do chromosome painting using cDNA probes and that this method is a means to rapidly select expressed sequences encoded by any portion of the genome.

RSK3 encodes a novel pp90rsk isoform with a unique N-terminal sequence: growth factor-stimulated kinase function and nuclear translocation

A novel pp90rsk Ser/Thr kinase (referred to as RSK3) was cloned from a human cDNA library. The RSK3 cDNA encodes a predicted 733-amino-acid protein with a unique N-terminal region containing a putative nuclear localization signal. RSK3 mRNA was widely expressed (but was predominant in lung and skeletal muscle). By using fluorescence in situ hybridization, the human RSK3 gene was localized to band q27 of chromosome 6. Hemagglutinin epitope-tagged RSK3 was expressed in transiently transfected COS cells. Growth factors, serum, and phorbol ester stimulated autophosphorylation of recombinant RSK3 and its kinase activity toward several protein substrates known to be phosphorylated by RSKs. However, the relative substrate specificity of RSK3 differed from that reported for other isoforms. RSK3 also phosphorylated potential nuclear target proteins including c-Fos and histones. Furthermore, although RSK3 was inactivated by protein phosphatase 2A in vitro, the enzyme was not activated by ERK2/mitogen-activated protein (MAP) kinase. In contrast, the kinase activity of another epitope-tagged RSK isoform (RSK-1) was significantly increased by in vitro incubation with ERK2/MAP kinase. Finally, we used affinity-purified RSK3 antibodies to demonstrate by immunofluorescence that endogenous RSK3 undergoes serum-stimulated nuclear translocation in cultured HeLa cells. These results provide evidence that RSK3 is a third distinct isoform of pp90rsk which translocates to the cell nucleus, phosphorylates potential nuclear targets, and may have a unique upstream activator. RSK3 may therefore subserve a discrete physiologic role(s) that differs from those of the other two known mammalian RSK isoforms.

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.