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Drosophila pico and its mammalian ortholog lamellipodin activate serum response factor and promote cell proliferation.

Lyulcheva E, Taylor E, Michael M, Vehlow A, Tan S, Fletcher A, Krause M, Bennett D - Dev. Cell (2008)

Bottom Line: Conversely, pico overexpression reduced G:F actin ratios and promoted tissue overgrowth in an epidermal growth factor (EGF) receptor (EGFR)-dependent manner.We show that pico and lamellipodin share the ability to activate serum response factor (SRF), a transcription factor that responds to reduced G:F-actin ratios via its co-factor Mal.We propose that MRL proteins link EGFR activation to mitogenic SRF signaling via changes in actin dynamics.

View Article: PubMed Central - PubMed

Affiliation: Department of Zoology, Oxford University, South Parks Road, Oxford OX1 3PS, UK.

ABSTRACT
MIG-10/RIAM/lamellipodin (MRL) proteins link activated Ras-GTPases with actin regulatory Ena/VASP proteins to induce local changes in cytoskeletal dynamics and cell motility. MRL proteins alter monomeric (G):filamentous (F) actin ratios, but the impact of these changes had not been fully appreciated. We report here that the Drosophila MRL ortholog, pico, is required for tissue and organismal growth. Reduction in pico levels resulted in reduced cell division rates, growth retardation, increased G:F actin ratios and lethality. Conversely, pico overexpression reduced G:F actin ratios and promoted tissue overgrowth in an epidermal growth factor (EGF) receptor (EGFR)-dependent manner. Consistently, in HeLa cells, lamellipodin was required for EGF-induced proliferation. We show that pico and lamellipodin share the ability to activate serum response factor (SRF), a transcription factor that responds to reduced G:F-actin ratios via its co-factor Mal. Genetics data indicate that mal/SRF levels are important for pico-mediated tissue growth. We propose that MRL proteins link EGFR activation to mitogenic SRF signaling via changes in actin dynamics.

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Reduction or Elevation of pico Levels Does Not Induce ApoptosisThe basal view of wing disc epithelia is shown. Discs expressing picoIR with ptc-GAL4 have a reduced zone of GFP-labeled cells compared with discs expressing GFP alone (control). Conversely, discs expressing ectopic pico displayed an expanded zone of GFP-labeled cells. Some cells were located basally, but did not show elevated caspase staining and did not have pyknotic nuclei (arrow). Expression of E2F and Dp resulted in high levels of caspase staining and nuclei were pyknotic (arrowhead). Coexpression of pico was unable to suppress E2F/Dp-induced cell death. Merged images show activated caspase staining in blue to visualize apoptotic cells, ptc-GAL4-expressing cells visualized by GFP in green, and propidium-labeled nuclei in red. All images were taken with identical settings to permit comparison of the intensity of activated caspase.
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fig4: Reduction or Elevation of pico Levels Does Not Induce ApoptosisThe basal view of wing disc epithelia is shown. Discs expressing picoIR with ptc-GAL4 have a reduced zone of GFP-labeled cells compared with discs expressing GFP alone (control). Conversely, discs expressing ectopic pico displayed an expanded zone of GFP-labeled cells. Some cells were located basally, but did not show elevated caspase staining and did not have pyknotic nuclei (arrow). Expression of E2F and Dp resulted in high levels of caspase staining and nuclei were pyknotic (arrowhead). Coexpression of pico was unable to suppress E2F/Dp-induced cell death. Merged images show activated caspase staining in blue to visualize apoptotic cells, ptc-GAL4-expressing cells visualized by GFP in green, and propidium-labeled nuclei in red. All images were taken with identical settings to permit comparison of the intensity of activated caspase.

Mentions: Mutations that slow the rate of cell proliferation can show intrinsic survival defects. Conversely, strong proliferative stimuli, such as the oncogenes E2F and Dp, can induce apoptosis, and net proliferation only occurs when apoptosis is simultaneously prevented. To determine whether cells expressing different levels of pico undergo cell death, we examined the effect of expressing pico or picoIR in positively marked (GFP-positive) cells with ptc-GAL4. Cells undergoing apoptosis were identified by activated caspase 3 staining. Compared to controls expressing GFP alone, the zone of GFP-positive cells in discs ectopically expressing picoIR was narrower and contained fewer cells (Figure 4). We did not observe elevated activated caspase 3 staining, and all nuclei appeared normal, suggesting that cells with reduced pico levels did not have an intrinsic survival defect. Ectopic expression of pico under control of ptc-GAL4 resulted in an expansion of the number of ptc > GFP cells. Pico expressing cells often exhibited an abnormal distribution toward the basal side of the wing disc epithelium, despite appearing morphologically normal (Figure 4). The effect of pico overexpression on adult wing size was not enhanced when apoptosis was suppressed by coexpression of the caspase inhibitor p35 (data not shown). Therefore, stimulation of growth by pico was not associated with an increase in apoptosis. Studies of the miRNA bantam have shown that genes stimulating cell proliferation can simultaneously suppress apoptosis (Brennecke et al., 2003). Therefore, we wondered whether pico could suppress proliferation-induced apoptosis caused by ectopic E2F and Dp. As previously reported, cells overexpressing E2F and Dp under the control of ptc-GAL4 showed pyknotic nuclei and elevated levels of activated caspase 3 in basal optical sections of wing discs, indicative of apoptosis (Figure 4) (Brennecke et al., 2003). Coexpression of pico with E2F/Dp had no effect on the levels of activated caspase (Figure 4). Therefore, stimulation of growth and proliferation by pico is not associated with an increase in apoptosis, and pico is not capable of suppressing apoptosis induced by proliferative stimuli from E2F and DP oncogenes.


Drosophila pico and its mammalian ortholog lamellipodin activate serum response factor and promote cell proliferation.

Lyulcheva E, Taylor E, Michael M, Vehlow A, Tan S, Fletcher A, Krause M, Bennett D - Dev. Cell (2008)

Reduction or Elevation of pico Levels Does Not Induce ApoptosisThe basal view of wing disc epithelia is shown. Discs expressing picoIR with ptc-GAL4 have a reduced zone of GFP-labeled cells compared with discs expressing GFP alone (control). Conversely, discs expressing ectopic pico displayed an expanded zone of GFP-labeled cells. Some cells were located basally, but did not show elevated caspase staining and did not have pyknotic nuclei (arrow). Expression of E2F and Dp resulted in high levels of caspase staining and nuclei were pyknotic (arrowhead). Coexpression of pico was unable to suppress E2F/Dp-induced cell death. Merged images show activated caspase staining in blue to visualize apoptotic cells, ptc-GAL4-expressing cells visualized by GFP in green, and propidium-labeled nuclei in red. All images were taken with identical settings to permit comparison of the intensity of activated caspase.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2691947&req=5

fig4: Reduction or Elevation of pico Levels Does Not Induce ApoptosisThe basal view of wing disc epithelia is shown. Discs expressing picoIR with ptc-GAL4 have a reduced zone of GFP-labeled cells compared with discs expressing GFP alone (control). Conversely, discs expressing ectopic pico displayed an expanded zone of GFP-labeled cells. Some cells were located basally, but did not show elevated caspase staining and did not have pyknotic nuclei (arrow). Expression of E2F and Dp resulted in high levels of caspase staining and nuclei were pyknotic (arrowhead). Coexpression of pico was unable to suppress E2F/Dp-induced cell death. Merged images show activated caspase staining in blue to visualize apoptotic cells, ptc-GAL4-expressing cells visualized by GFP in green, and propidium-labeled nuclei in red. All images were taken with identical settings to permit comparison of the intensity of activated caspase.
Mentions: Mutations that slow the rate of cell proliferation can show intrinsic survival defects. Conversely, strong proliferative stimuli, such as the oncogenes E2F and Dp, can induce apoptosis, and net proliferation only occurs when apoptosis is simultaneously prevented. To determine whether cells expressing different levels of pico undergo cell death, we examined the effect of expressing pico or picoIR in positively marked (GFP-positive) cells with ptc-GAL4. Cells undergoing apoptosis were identified by activated caspase 3 staining. Compared to controls expressing GFP alone, the zone of GFP-positive cells in discs ectopically expressing picoIR was narrower and contained fewer cells (Figure 4). We did not observe elevated activated caspase 3 staining, and all nuclei appeared normal, suggesting that cells with reduced pico levels did not have an intrinsic survival defect. Ectopic expression of pico under control of ptc-GAL4 resulted in an expansion of the number of ptc > GFP cells. Pico expressing cells often exhibited an abnormal distribution toward the basal side of the wing disc epithelium, despite appearing morphologically normal (Figure 4). The effect of pico overexpression on adult wing size was not enhanced when apoptosis was suppressed by coexpression of the caspase inhibitor p35 (data not shown). Therefore, stimulation of growth by pico was not associated with an increase in apoptosis. Studies of the miRNA bantam have shown that genes stimulating cell proliferation can simultaneously suppress apoptosis (Brennecke et al., 2003). Therefore, we wondered whether pico could suppress proliferation-induced apoptosis caused by ectopic E2F and Dp. As previously reported, cells overexpressing E2F and Dp under the control of ptc-GAL4 showed pyknotic nuclei and elevated levels of activated caspase 3 in basal optical sections of wing discs, indicative of apoptosis (Figure 4) (Brennecke et al., 2003). Coexpression of pico with E2F/Dp had no effect on the levels of activated caspase (Figure 4). Therefore, stimulation of growth and proliferation by pico is not associated with an increase in apoptosis, and pico is not capable of suppressing apoptosis induced by proliferative stimuli from E2F and DP oncogenes.

Bottom Line: Conversely, pico overexpression reduced G:F actin ratios and promoted tissue overgrowth in an epidermal growth factor (EGF) receptor (EGFR)-dependent manner.We show that pico and lamellipodin share the ability to activate serum response factor (SRF), a transcription factor that responds to reduced G:F-actin ratios via its co-factor Mal.We propose that MRL proteins link EGFR activation to mitogenic SRF signaling via changes in actin dynamics.

View Article: PubMed Central - PubMed

Affiliation: Department of Zoology, Oxford University, South Parks Road, Oxford OX1 3PS, UK.

ABSTRACT
MIG-10/RIAM/lamellipodin (MRL) proteins link activated Ras-GTPases with actin regulatory Ena/VASP proteins to induce local changes in cytoskeletal dynamics and cell motility. MRL proteins alter monomeric (G):filamentous (F) actin ratios, but the impact of these changes had not been fully appreciated. We report here that the Drosophila MRL ortholog, pico, is required for tissue and organismal growth. Reduction in pico levels resulted in reduced cell division rates, growth retardation, increased G:F actin ratios and lethality. Conversely, pico overexpression reduced G:F actin ratios and promoted tissue overgrowth in an epidermal growth factor (EGF) receptor (EGFR)-dependent manner. Consistently, in HeLa cells, lamellipodin was required for EGF-induced proliferation. We show that pico and lamellipodin share the ability to activate serum response factor (SRF), a transcription factor that responds to reduced G:F-actin ratios via its co-factor Mal. Genetics data indicate that mal/SRF levels are important for pico-mediated tissue growth. We propose that MRL proteins link EGFR activation to mitogenic SRF signaling via changes in actin dynamics.

Show MeSH
Related in: MedlinePlus