Role of megakaryoblastic acute leukemia-1 in ERK1/2-dependent stimulation of serum response factor-driven transcription by BDNF or increased synaptic activity

Serum response factor (SRF)-mediated transcription contributes to developmental and adult brain plasticity. Therefore, we investigated the role of a newly identified SRF coactivator, MKL1, in the regulation of SRF-driven transcription in rat forebrain neurons. MKL1 expression was found in newborn rat cortical or hippocampal neurons in culture as well as in adult rat forebrain. Immunostaining demonstrated constitutive nuclear localization of MKL1 in the CA1 region of the hippocampus, in the deep layers of the neocortex, and in cultured neurons. Overexpression of MKL1 in primary cortical neurons elevated SRF-driven transcription and enhanced its stimulation by BDNF. In addition, inhibition of endogenous MKL1 by overexpression of a dominant-negative MKL1 mutant or by small interfering RNA reduced BDNF activation of SRF-driven transcription. In neurons, endogenous MKL1 was associated with SRF-regulated chromatin regions of several endogenous genes including c-fos, JunB, Srf, and Cyr61. BDNF activation of MKL1/SRF-driven transcription was dependent on the extracellular signal-regulated kinase 1/2 (ERK1/2) pathway, which also led to MKL1 phosphorylation. Finally, synaptic activity stimulation of SRF-driven transcription was reduced by inhibition of endogenous MKL1. Conversely, synaptic activity enhanced transcription by overexpressed MKL1. MKL1 regulation by synaptic activity was mediated through the NMDA receptor-activated ERK1/2. These results suggest that neuronal MKL1 contributes to SRF-regulated gene expression induced by BDNF or synaptic activity. In addition, MKL1 appears as a novel mediator of the signaling between ERK1/2 and SRF. Moreover, MKL1 is a likely regulator of SRF-driven transcription programs that underlie neuronal plasticity.

Serum response factor is a critical requirement for VEGF signaling in endothelial cells and VEGF-induced angiogenesis

Angiogenesis, new capillary blood vessel formation, is essential for embryonic development, wound healing, and cancer growth. Vascular endothelial growth factor (VEGF) induces angiogenesis by activating endothelial cell migration and proliferation. Serum response factor (SRF) is a transcription factor important for embryonic development and activation of immediate early gene expression. The roles of SRF in endothelial cell biology and angiogenesis have not been explored. Here we demonstrate that SRF is a downstream mediator of VEGF signaling in endothelial cells and a critical requirement for VEGF-induced angiogenesis. Knockdown of SRF protein levels in human and rat endothelial cells abolished VEGF-induced in vitro angiogenesis, impaired endothelial cell migration and proliferation, and inhibited VEGF-induced actin polymerization and immediate early gene expression. Injection of SRF antisense expression plasmid into gastric ulcers in rats significantly inhibited in vivo angiogenesis in granulation tissue. Mechanistically, this study also revealed that VEGF promotes SRF expression and nuclear translocation and increases SRF binding activity to DNA in endothelial cells through both Rho-actin and MEK-ERK dependent signaling pathways. These findings have potential therapeutic implications, e.g., local anti-SRF treatment may inhibit angiogenesis crucial for tumor growth.

Inhibitory cardiac transcription factor, SRF-N, is generated by caspase 3 cleavage in human heart failure and attenuated by ventricular unloading

BACKGROUND

Knowledge about molecular mechanisms leading to heart failure is still limited, but reduced gene activities and modest activation of caspase 3 are hallmarks of end-stage heart failure. We postulated that serum response factor (SRF), a central cardiac transcription factor, might be a cleavage target for modest activated caspase 3, and this cleavage of SRF may play a dominant inhibitory role in propelling hearts toward failure.

METHODS AND RESULTS

We examined SRF protein levels from cardiac samples taken at the time of transplantation in 13 patients with end-stage heart failure and 7 normal hearts. Full-length SRF was markedly reduced and processed into 55- and 32-kDa subfragments in all failing hearts. SRF was intact in normal samples. In contrast, the hearts of 10 patients with left ventricular assist devices showed minimal SRF fragmentation. Specific antibodies to N- and C-terminal SRF sequences and site-directed mutagenesis revealed 2 alternative caspase 3 cleavage sites, so that 2 fragments were detected of each containing either the N- or C-terminal SRF. Expression of SRF-N, the 32-kDa fragment, in myogenic cells inhibited the transcriptional activity of alpha-actin gene promoters by 50% to 60%, which suggests that truncated SRF functioned as a dominant-negative transcription factor.

CONCLUSIONS

Caspase 3 activation in heart failure sequentially cleaved SRF and generated a dominant-negative transcription factor, which may explain the depression of cardiac-specific genes. Moreover, caspase 3 activation may be reversible in the failing heart with ventricular unloading.

A novel role for serum response factor in neuronal survival

Recent studies indicate that neuroprotection afforded by brain-derived neurotrophic factor (BDNF) is mediated by extracellular signal-regulated kinase (ERK) and phosphatidylinositol-3 kinase (PI3K). However, the mechanisms by which ERK and PI3K exert neuroprotection are not completely understood. Because ERK1/2 and PI3K both stimulate serum response element (SRE)-mediated gene expression, and serum response factor (SRF) is indispensable for SRE-mediated transcription, we investigated whether SRF contributes to ERK1/2 and PI3K neuroprotection. To accomplish this goal, we used an established experimental paradigm in which BDNF protects postnatal cortical neurons against both trophic deprivation and camptothecin-induced DNA damage. BDNF protection against camptothecin is mediated primarily by ERK1/2 activation, whereas its protection against trophic deprivation is mainly through stimulation of the PI3K pathway (Hetman et al., 1999). Here we demonstrate that expression of a wild-type SRF is sufficient to protect postnatal cortical neurons against camptothecin or trophic deprivation. Expression of a dominant-negative SRF partially reversed BDNF neuroprotection against both apoptotic insults. Moreover, the dominant-negative SRF inhibited neuroprotection against trophic withdrawal afforded by expression of a constitutive active PI3K. In addition, protection against camptothecin by expression of constitutive active mitogen-activated protein kinase kinase 1, an upstream kinase that activates ERK1/2, was also blocked by expression of the dominant-negative SRF. These data suggest that SRF is both necessary and sufficient for BDNF neuroprotection of cortical neurons against trophic deprivation and DNA damage. Our data provide a direct demonstration of a biological function of SRF in neurons and a novel downstream neuroprotective mechanism common to both ERK1/2 and PI3K pathways.

c-Src and hydrogen peroxide mediate transforming growth factor-beta1-induced smooth muscle cell-gene expression in 10T1/2 cells

OBJECTIVE

Transforming growth factor-beta1 (TGF-beta1) controls the expression of numerous genes, including smooth muscle cell (SMC)-specific genes and extracellular matrix protein genes. Here we investigated whether c-Src plays a role in TGF-beta1 signaling in mouse embryonic fibroblast C3H10T1/2 cells.

METHODS AND RESULTS

TGF-beta1 induction of the SMC contractile protein SM22alpha gene expression was inhibited by PP1 (an inhibitor of Src family kinases) or by C-terminal Src kinase (a negative regulator of c-Src). Induction of SM22alpha by TGF-beta1 was markedly attenuated in SYF cells (c-Src(-), Yes(-), and Fyn(-)) compared with Src(++) cells (c-Src(++), Yes(-), and Fyn(-)). PP1 also inhibited the TGF-beta1-induced expression of serum response factor (SRF), a transcription factor regulating the SMC marker gene expression. Confocal immunofluorescence analysis showed that TGF-beta1 stimulates production of hydrogen peroxide. Antioxidants such as catalase or NAD(P)H oxidase inhibitors such as apocynin inhibited the TGF-beta1-induced expression of SM22alpha. Furthermore, we demonstrate that TGF-beta1 induction of the plasminogen activator inhibitor-1 (PAI-1) gene, which is known to be dependent on Smad but not on SRF, is inhibited by PP1 and apocynin.

CONCLUSIONS

Our results suggest that TGF-beta1 activates c-Src and generates hydrogen peroxide through NAD(P)H oxidase, and these signaling pathways lead to the activation of specific sets of genes, including SM22alpha and PAI-1. TGF-beta1 controls the expression of numerous genes, including SM22alpha and PAI-1. We investigated whether c-Src plays a role in TGF-beta1 signaling. TGF-beta1 induction of such genes was significantly reduced in Src family tyrosine kinase-deficient cells, and Csk and pharmacological inhibitors for Src family kinases or antioxidants inhibit the effects of TGF-beta1. These results indicate that c-Src and hydrogen peroxide are required for TGF-beta1 signaling.

Mechanical Strain Inhibits Airway Smooth Muscle Gene Transcription via Protein Kinase C Signaling

Mechanical strain affects airway myocyte phenotype, cytoskeletal architecture, proliferation, and contractile function. We hypothesized that (i) short-term mechanical strain modulates transcription of smooth muscle-specific gene promoters for SM22 and smooth muscle myosin heavy chain (smMHC); and (ii) strain-induced change is mediated by altered actin polymerization in association with activation of protein kinase C (PKC). Primary cultured canine tracheal myocytes were transiently transfected with luciferase reporter plasmids harboring a murine SM22, human smMHC, or artificial serum response factor (SRF)-specific gene promoter and then subjected to cyclic strain for 48 h. This strain protocol significantly reduced transcriptional activity of SM22 and smMHC promoters and an artificial SRF-dependent promoter by 55 +/- 5.9%, 57 +/- 6.4%, and 75 +/- 7.9%, respectively, with concomitant reduction in F/G actin ratio by 31 +/- 8%. PKC inhibitors, GF109203X or Gö6976, significantly attenuated these affects. Similar to strain, strain-independent activation of PKC inhibited SM22, smMHC, and SRF-dependent promoter activity by 61 +/- 4%, 66 +/- 5%, and 28 +/- 15%, respectively, and reduced the F/G actin ratio by 30 +/- 5%. Gel shift assay revealed that PKC activation led to decreased binding of the required transcription factor, SRF, to CArG elements in the SM22 promoter. These data suggest a previously unknown role for PKC isoforms in mechanosensitive signaling in airway myocytes that is associated with coordinated regulation of actin cytoskeletal dynamics and smooth muscle-specific gene transcription.

Traditional Chinese drug ShuXueTong facilitates angiogenesis during wound healing following traumatic brain injury

ETHNOPHARMACOLOGICAL RELEVANCE

ShuXueTong injection is a traditional Chinese drug designed to treat the patients of "blood stasis and stagnation (yu xue yu zhi)", including subacute brain trauma. However, the mechanism of the therapeutic effect of ShuXueTong on traumatic brain injury is unknown yet.

AIM OF THE STUDY

We hypothesized that ShuXueTong may promote the brain wound healing by facilitating angiogenesis. Thus this study was designed to explore this hypothesis.

MATERIALS AND METHODS

By means of microvessel count, Western blotting, immunocytochemistry, methyl thiazolyl tetrazolium assay and etc., the effect of ShuXueTong on the angiogenesis of brain wound was studied and then its influence on the VEGF/VEGFR-2 pathway were explored.

RESULTS

ShuXueTong facilitates angiogenesis in the brain wound and improves the neurological function of the traumatized rats. VEGF expression in the lesion was elevated due to ShuXueTong induction. The in vitro experiment revealed VEGFR-2 and SRF expression in the endothelial cells were enhanced when exposed to ShuXueTong for merely 1d. Moreover, ShuXueTong promoted the endothelial cell proliferation via the VEGF/VEGFR-2 pathway.

CONCLUSIONS

The mechanism of the therapeutic effect of ShuXueTong on traumatic brain injury lies at least partly in the enhanced angiogenesis in the lesion.

Cellular Toxicity Induced by SRF-Mediated Transcriptional Squelching

The transcriptional activator serum response factor (SRF) is a member of the immediate early gene family known to promote embryonic development, cell growth, and myogenesis through interaction with multiple nuclear protein factors. Previous studies have shown that SRF possesses potent transcriptional activation domains that can interfere with gene expression at artificially high expression levels through "transcriptional squelching." The current work sought to characterize toxicological aspects of SRF-mediated transcriptional squelching. An adenoviral expression system driven by the potent cytomegalovirus promoter was used to achieve up to a 50-fold increase in SRF protein levels. The overexpressed SRF is nuclear localized and interferes with gene expression independent of specific promoter interaction as expected for transcriptional squelching. SRF-mediated squelching elicits robust cell killing affecting multiple cell types including normal and abnormal proliferating cells as well as postmitotic cells such as cardiomyocytes in culture, and the cell killing is more pronounced than that mediated by the tumor suppressor protein p53. Although both the DNA-binding and transcriptional activation domains of SRF are normally required for the physiological roles of SRF, only the transcriptional activation domain is required for cell killing. Unlike c-myc-induced cell killing, squelching-induced cell death does not require serum withdrawal and cannot be effectively attenuated by blocking the caspase and calpain proteolytic pathways or by overexpression of the antiapoptotic gene bcl-xL. These findings suggest transcriptional squelching may be engineered for killing cancer cells, and the SRF gene may represent a novel molecular target for cancer therapeutics.

Laser microdissection-based analysis of mRNA expression in human coronary arteries with intimal thickening

Intimal thickening is an early phase of atherosclerosis characterized by differentiation of plaque smooth muscle cells (SMCs) from a contractile to a synthetic phenotype. We used laser microdissection (LMD) plus real-time RT-PCR to quantify mRNAs for calponin-1 and smoothelin, markers of the contractile phenotype, and for serum response factor (SRF), a regulator of SMC differentiation, in intimal and medial SMCs of human coronary arteries with intimal thickening. RNA expression was also analyzed by ISH and protein expression was detected by IHC. LMD plus RT-PCR found similar levels of SRF mRNA in intimal and medial SMCs, while medial mRNA levels for calponin-1 and smoothelin were higher. ISH confirmed that smoothelin mRNA levels in media exceeded those in intima, whereas SRF mRNA levels were similar at both sites. For calponin-1 and smoothelin, protein levels mirrored respective mRNA levels. By contrast, more medial than intimal SRF protein was present. Our results indicate that intimal SMCs exhibit a largely synthetic phenotype, perhaps reflecting lower intimal levels of SRF protein; ISH and LMD plus real-time RT-PCR provide comparable results; as a valuable alternative to ISH, LMD plus RT-PCR allows parallel measurement of several transcripts; and tissue gene expression studies must measure both protein and mRNA levels.

Rac3-Mediated Transformation Requires Multiple Effector Pathways

Our initial characterization of Rac3, a close relative of the small GTPase Rac1, established its ability to promote membrane ruffling, transformation, and activation of c-jun transcriptional activity. The finding that Rac3 is transforming, and its similarity to Rac1, a protein that has a well-established connection to many processes important for cancer progression, prompted further investigation into Rac3 transformation. We used effector domain mutants (EDMs) to explore the relationship among Rac signaling, transformation, and effector usage. All Rac3 EDMs tested (N26D, F37L, Y40C, and N43D) retained the ability to promote membrane ruffling and focus formation. In contrast, only the N43D mutant promoted anchorage independence. This differs from Rac1, where both N26D and N43D mutants were impaired in both types of transformation. To learn more about the signaling pathways involved, we did luciferase reporter assays and glutathione S-transferase pull-down assays for effector binding. We found evidence for a functional link between activation of phospholipase Cbeta2 by Rac3 and signaling to the serum response factor (SRF). Surprisingly, we also found that Rac3 binds poorly to the known Rac1 effectors mixed lineage kinases 2 and 3 (MLK2 and MLK3). Transcription of cyclin D1 was the only pathway that correlated with growth in soft agar. Our experiments show that activation of membrane ruffling and transcriptional activation of c-jun, SRF, or E2F are not sufficient to promote anchorage-independent growth mediated by Rac3. Instead, multiple effector pathways are required for Rac3 transformation, and these overlap partially but not completely with those used by Rac1.

Maintaining serum response factor activity in the older heart equal to that of the young adult is associated with better cardiac response to isoproterenol stress

To understand the effect of transcription regulation in modulating cardiac aging, we sought to study the role of serum response factor (SRF), a key transcription factor in the heart that is normally increased with senescence and also in congestive heart failure. A Tet-Off gene expression system was used for cardiac-specific over-expression of a mutant SRF protein. In these binary transgenic mice, there is no age-related increase in SRF protein expression; in fact, there appeared to be a mild reduction of SRF protein (Mild-R SRF Tg). The older, middle-aged (15 mo) Mild-R SRF Tg mice appeared healthier and were better able to maintain their left ventricular systolic pressure (LVSP) in response to moderate â-adrenergic stimulation compared with age-matched Non-Tg mice, which demonstrated a negative ionotropic response. The Mild-R SRF Tg hearts had lower mRNA expression of BNP (p < 0.05), and the sodium calcium exchanger (p < 0.05), compared to Non-Tg. Mild-R SRF Tg had higher mRNA levels of SERCA2 (p < 0.05) and ryanodine receptor 2 (p < 0.05) compared to Non-Tg hearts. These findings suggest that preventing the age-associated increase in SRF is associated with better preserved intracellular calcium handling and functional response to stress; it might be advantageous for the older adult heart. This mouse model could be helpful in elucidating the molecular mechanisms underlying certain age-related changes in cardiac reserve capacity and response to stress.

Smooth muscle gamma-actin promoter regulation by RhoA and serum response factor signaling

Smooth muscle gamma-actin (SMGA) is both an early marker of smooth muscle cell differentiation, which demonstrates an expression pattern restricted to smooth muscle and the post meiotic spermatocyte. Serum response factor (SRF) DNA-binding is an important regulator of muscle differentiation, including SMGA expression during smooth muscle cell differentiation. RhoA, a low molecular weight GTPase protein, can regulate cardiac, skeletal, and smooth muscle differentiation through SRF-dependent mechanisms. This study's purpose was to examine RhoA expression during smooth muscle cell development, and determine if the SMGA promoter activity is sensitive to RhoA-mediated signaling through SRF. Additionally, the study identified the promoter regulation modifying SMGA expression by RhoA signaling. Western blot analysis of embryonic chick gizzard whole protein extracts during 5 to 14 days of development demonstrated a large induction of RhoA (10-fold) and beta1 integrin expression at day 8, which corresponds to the time SMGA expression and differentiation are occurring. Transient transfections in CV-1 fibroblast cells demonstrated that co-overexpression of SRF and RhoA could induce a 40-fold induction of -176 bp SMGA promoter activity. Mutational analysis demonstrated that serum response element (SRE)-1, but not SRE2, was necessary for RhoA/SRF activation of the SMGA promoter. Deletion analysis revealed that although SRE1 was necessary for SMGA promoter activation by RhoA and SRF, it was not sufficient, implicating a possible obligatory role of additional promoter sequences in the response. Overexpression of a mutated SRF protein that was unable to bind DNA demonstrated that the 40-fold RhoA/SRF activation was largely dependent on SRF binding to the SMGA promoter. Thus, as the SMGA promoter appears to be a target of RhoA-mediated transcriptional regulation, the uncovering of these signaling mechanisms effecting SMGA promoter activity should provide a regulatory paradigm that can then be examined during the regulation of other smooth muscle genes.

Role of serum response factor expression in prostate cancer biochemical recurrence

BACKGROUND

Up to a third of prostate cancer patients fail curative treatment strategies such as surgery and radiation therapy in the form of biochemical recurrence (BCR) which can be predictive of poor outcome. Recent clinical trials have shown that men experiencing BCR might benefit from earlier intervention post-radical prostatectomy (RP). Therefore, there is an urgent need to identify earlier prognostic biomarkers which will guide clinicians in making accurate diagnosis and timely decisions on the next appropriate treatment. The objective of this study was to evaluate Serum Response Factor (SRF) protein expression following RP and to investigate its association with BCR.

MATERIALS AND METHODS

SRF nuclear expression was evaluated by immunohistochemistry (IHC) in TMAs across three international radical prostatectomy cohorts for a total of 615 patients. Log-rank test and Kaplan-Meier analyses were used for BCR comparisons. Stepwise backwards elimination proportional hazard regression analysis was used to explore the significance of SRF in predicting BCR in the context of other clinical pathological variables. Area under the curve (AUC) values were generated by simulating repeated random sub-samples.

RESULTS

Analysis of the immunohistochemical staining of benign versus cancer cores showed higher expression of nuclear SRF protein expression in cancer cores compared with benign for all the three TMAs analysed (P < 0.001, n = 615). Kaplan-Meier curves of the three TMAs combined showed that patients with higher SRF nuclear expression had a shorter time to BCR compared with patients with lower SRF expression (P < 0.001, n = 215). Together with pathological T stage T3, SRF was identified as a predictor of BCR using stepwise backwards elimination proportional hazard regression analysis (P = 0.0521). Moreover ROC curves and AUC values showed that SRF was better than T stage in predicting BCR at year 3 and 5 following radical prostatectomy, the combination of SRF and T stage had a higher AUC value than the two taken separately.

CONCLUSIONS

SRF assessment by IHC following RP could be useful in guiding clinicians to better identify patients for appropriate follow-up and timely treatment.

[Serum response factor participates in RhoA-induced endothelial cell F-actin rearrangements]

RhoA is one of the main members of RhoGTPase family involved in cell morphology, smooth muscle contraction, cytoskeletal microfilaments and stress fiber formation. It has been demonstrated that RhoA modulates endothelial cell permeability by its effect on F-actin rearrangement, but the molecular mechanism of rearrangement of actin cytoskeleton remains unclear. Recent studies prove that RhoA/Rho kinase regulates smooth muscle specific actin dynamics by activating serum response factor (SRF)-dependent transcription. To further investigate the molecular mechanism of the rearrangement of vascular endothelial cell actin cytoskeleton, we explored the relationship between the activation of SRF and F-actin rearrangement induced by RhoA in human umbilical vein endothelial cells (HUVECs). HUVECs were infected with the constitutively active forms of RhoA (Q63LRhoA) or the dominant negative forms of RhoA(T19NRhoA) using retrovirus vector pLNCX-Q63LRhoA or pLNCX-T19NRhoA, the positive clone was obtained by G418 selection. The expression and distribution of SRF in normal and infected cells were evaluated by immunohistochemistry and Western blot in complete medium and in serum-free medium. The effect of F-actin polymerization was detected by Rhodamine-Phalloidine staining. Infection of PLNCX-Q63LRhoA induced F-actin rearrangement and stress fiber formation in HUVECs, as well as enhanced the expression of SRF in the nuclei. In contrast, the cells infected with T19NRhoA showed no distinct changes. With serum deprivation, the expression of SRF increased obviously in both normal and infected HUVECs, but the subcellular localization of SRF was evidently different. In HUVECs, the localization of SRF was in the nuclei after 3 d with serum deprivation, but it was redistributed outside the nuclei after 5 d with serum deprivation. In cells infected with Q63LRhoA, the immunolocalization of SRF was always in the nuclei compared with HUVECs infected with T19NRhoA, which was almost always localized in the cytoplasm. In HUVECs, the rearrangement of F-actin and formation of stress fiber increased after 3 d with serum deprivation, but appeared decreased and unpolymerized after 5 d with serum deprivation. The polymerization of F-actin and the formation of stress fiber in HUVECs infected with Q63LRhoA kept during the period of serum-free culture, whereas the rearrangement of F-actin in cells infected with T19NRhoA was not found. These results suggest that RhoA influences endothelial F-actin rearrangement in part by regulating the expression and subcellular localization of SRF.