Controlling long-term signaling: receptor dynamics determine attenuation and refractory behavior of the TGF-β pathway

Clinical development of galunisertib (LY2157299 monohydrate), a small molecule inhibitor of transforming growth factor-beta signaling pathway

Transforming growth factor-beta (TGF-β) signaling regulates a wide range of biological processes. TGF-β plays an important role in tumorigenesis and contributes to the hallmarks of cancer, including tumor proliferation, invasion and metastasis, inflammation, angiogenesis, and escape of immune surveillance. There are several pharmacological approaches to block TGF-β signaling, such as monoclonal antibodies, vaccines, antisense oligonucleotides, and small molecule inhibitors. Galunisertib (LY2157299 monohydrate) is an oral small molecule inhibitor of the TGF-β receptor I kinase that specifically downregulates the phosphorylation of SMAD2, abrogating activation of the canonical pathway. Furthermore, galunisertib has antitumor activity in tumor-bearing animal models such as breast, colon, lung cancers, and hepatocellular carcinoma. Continuous long-term exposure to galunisertib caused cardiac toxicities in animals requiring adoption of a pharmacokinetic/pharmacodynamic-based dosing strategy to allow further development. The use of such a pharmacokinetic/pharmacodynamic model defined a therapeutic window with an appropriate safety profile that enabled the clinical investigation of galunisertib. These efforts resulted in an intermittent dosing regimen (14 days on/14 days off, on a 28-day cycle) of galunisertib for all ongoing trials. Galunisertib is being investigated either as monotherapy or in combination with standard antitumor regimens (including nivolumab) in patients with cancer with high unmet medical needs such as glioblastoma, pancreatic cancer, and hepatocellular carcinoma. The present review summarizes the past and current experiences with different pharmacological treatments that enabled galunisertib to be investigated in patients.

Response to Nodal morphogen gradient is determined by the kinetics of target gene induction

Morphogen gradients expose cells to different signal concentrations and induce target genes with different ranges of expression. To determine how the Nodal morphogen gradient induces distinct gene expression patterns during zebrafish embryogenesis, we measured the activation dynamics of the signal transducer Smad2 and the expression kinetics of long- and short-range target genes. We found that threshold models based on ligand concentration are insufficient to predict the response of target genes. Instead, morphogen interpretation is shaped by the kinetics of target gene induction: the higher the rate of transcription and the earlier the onset of induction, the greater the spatial range of expression. Thus, the timing and magnitude of target gene expression can be used to modulate the range of expression and diversify the response to morphogen gradients.

DOI:http://dx.doi.org/10.7554/eLife.05042.001

How a cell can tell where it is in a developing embryo has fascinated scientists for decades. The pioneering computer scientist and mathematical biologist Alan Turing was the first person to coin the term ‘morphogen’ to describe a protein that provides information about locations in the body. A morphogen is released from a group of cells (called the ‘source’) and as it moves away its activity (called the ‘signal’) declines gradually. Cells sense this signal gradient and use it to detect their position with respect to the source. Nodal is an important morphogen and is required to establish the correct identity of cells in the embryo; for example, it helps determine which cells should become a brain or heart or gut cell and so on.

The zebrafish is a widely used model to study animal development, in part because its embryos are transparent; this allows cells and proteins to be easily observed under a microscope. When Nodal acts on cells, another protein called Smad2 becomes activated, moves into the cell's nucleus, and then binds to specific genes. This triggers the expression of these genes, which are first copied into mRNA molecules via a process known as transcription and are then translated into proteins. The protein products of these targeted genes control cell identity and movement.

Several models have been proposed to explain how different concentrations of Nodal switch on the expression of different target genes; that is to say, to explain how a cell interprets the Nodal gradient. Dubrulle et al. have now measured factors that underlie how this gradient is interpreted. Individual cells in zebrafish embryos were tracked under a microscope, and Smad2 activation and gene expression were assessed. Dubrulle et al. found that, in contradiction to previous models, the amount of Nodal present on its own was insufficient to predict the target gene response. Instead, their analysis suggests that the size of each target gene's response depends on its rate of transcription and how quickly it is first expressed in response to Nodal.

These findings of Dubrulle et al. suggest that timing and transcription rate are important in determining the appropriate response to Nodal. Further work will be now needed to find out whether similar mechanisms regulate other processes that rely on the activity of morphogens.

DOI:http://dx.doi.org/10.7554/eLife.05042.002

Response to comment on "Controlling long-term signaling: receptor dynamics determine attenuation and refractory behavior of the TGF-β pathway"-Smad2/3 activity does not predict the dynamics of transcription

Using an integrative experimental and computational modeling approach to dissect the signaling dynamics of the transforming growth factor-β to Smad (TGF-β/Smad) pathway, we discovered that previous exposure to ligand desensitizes cells, rendering them refractory to further acute TGF-β stimulation. We demonstrated that this refractory behavior, which also explains signal attenuation, is caused by the fast depletion from the cell surface of signaling-competent receptors upon TGF-β binding and their slow replenishment, which is the rate-limiting step for regaining full competence for acute ligand induction. In their Comment, Warmflash and colleagues suggest that receptor dynamics do not necessarily reflect the dynamics of TGF-β target gene transcription. We argue that to understand receptor dynamics, phosphorylated Smad2 abundance is the optimal readout, because this directly reflects receptor activity. Target gene transcription, in contrast, is influenced by many other factors in addition to nuclear abundance of activated Smad complexes and is thus a poor readout for receptor dynamics. Warmflash et al. also claim that our results are inconsistent with parts of the literature, in particular with data published by Zi et al. (Mol. Syst. Biol. 7, 492, 2011) and by Sorre et al. (Dev. Cell 20, 334, 2014). However, we show with our mathematical model that our results are consistent with the data in question.

Comment on "Controlling long-term signaling: receptor dynamics determine attenuation and refractory behavior of the TGF-β pathway"-Smad2/3 activity does not predict the dynamics of transcription

The transforming growth factor-β (TGF-β) pathway plays a fundamental role in development and disease. Despite its importance, the dynamics of signaling activity downstream of ligand stimulation have remained largely unexplored. The recent study by Vizán et al. demonstrates that loss of signaling-capable receptors from the cell surface leads to a refractory period during which cells are incapable of responding to additional signals. In this letter and in our previous work, we show that although receptor dynamics determine Smad2/3 activity, signaling activity at the level of transcription terminates far earlier in a receptor- and Smad2/3-independent manner. Thus, Smad2/3 activity does not reveal the dynamics of transcription, and downstream measures must be examined directly.

Caveolin-1 is required for TGF-β-induced transactivation of the EGF receptor pathway in hepatocytes through the activation of the metalloprotease TACE/ADAM17

Transforming growth factor-beta (TGF-β) plays a dual role in hepatocytes, inducing both pro- and anti-apoptotic responses, whose balance decides cell fate. Survival signals are mediated by the epidermal growth factor receptor (EGFR) pathway, which is activated by TGF-β in these cells. Caveolin-1 (Cav1) is a structural protein of caveolae linked to TGF-β receptors trafficking and signaling. Previous results have indicated that in hepatocytes, Cav1 is required for TGF-β-induced anti-apoptotic signals, but the molecular mechanism is not fully understood yet. In this work, we show that immortalized Cav1(-/-) hepatocytes were more sensitive to the pro-apoptotic effects induced by TGF-β, showing a higher activation of caspase-3, higher decrease in cell viability and prolonged increase through time of intracellular reactive oxygen species (ROS). These results were coincident with attenuation of TGF-β-induced survival signals in Cav1(-/-) hepatocytes, such as AKT and ERK1/2 phosphorylation and NFκ-B activation. Transactivation of the EGFR pathway by TGF-β was impaired in Cav1(-/-) hepatocytes, which correlated with lack of activation of TACE/ADAM17, the metalloprotease responsible for the shedding of EGFR ligands. Reconstitution of Cav1 in Cav1(-/-) hepatocytes rescued wild-type phenotype features, both in terms of EGFR transactivation and TACE/ADAM17 activation. TACE/ADAM17 was localized in detergent-resistant membrane (DRM) fractions in Cav1(+/+) cells, which was not the case in Cav1(-/-) cells. Disorganization of lipid rafts after treatment with cholesterol-binding agents caused loss of TACE/ADAM17 activation after TGF-β treatment. In conclusion, in hepatocytes, Cav1 is required for TGF-β-mediated activation of the metalloprotease TACE/ADAM17 that is responsible for shedding of EGFR ligands and activation of the EGFR pathway, which counteracts the TGF-β pro-apoptotic effects. Therefore, Cav1 contributes to the pro-tumorigenic effects of TGF-β in liver cancer cells.

2013: Signaling breakthroughs of the year

The editorial staff and distinguished scientists in the field of cell signaling nominated diverse research as advances for 2013. Breakthroughs in understanding how spatial and temporal signals control cellular behavior ranged from filtering high-frequency stimuli to interpreting circadian inputs. This year's nominations also highlight the importance of understanding cell signaling in the context of physiology and disease, such as links between the nervous system and cancer. Furthermore, the application of new techniques to study cell signaling--such as optogenetics, DNA editing with CRISPR-Cas9, and sequencing untranslated regions of transcripts--continues to expand the realm and impact of signaling research.