Regulation of ASK1 signaling by scaffold and adaptor proteins

Abstract

The mitogen-activated protein kinase (MAPK) signaling pathway is a three-tiered kinase cascade where mitogen-activated protein kinase kinase kinases (MAP3Ks) lead to the activation of mitogen-activated protein kinase kinases (MAP2K), and ultimately MAPK proteins. MAPK signaling can promote a diverse set of biological outcomes, ranging from cell death to proliferation. There are multiple mechanisms which govern MAPK output, such as the duration and strength of the signal, cellular localization to upstream and downstream binding partners, pathway crosstalk and the binding to scaffold and adaptor molecules. This review will focus on scaffold and adaptor proteins that bind to and regulate apoptosis signal-regulating kinase 1 (ASK1), a MAP3K protein with a critical role in mediating stress response pathways.

Introduction

The MAPK signaling cascade is composed of many MAP3K, MAP2K and MAPK proteins which can assemble into various pathways to drive a wide array of cellular outcomes. Proper organization and localization of these kinases is critical to ensuring an incoming signal elicits the proper biological response. One mechanism that provides this coordination is the binding of scaffolding and adaptor proteins.

Scaffold and adaptor proteins often contain multiple binding sites to aid in the assembly of multi-protein complexes. While both scaffold and adaptor proteins sometimes serve the same function, the term “scaffold” is generally reserved for a protein which links two or more target proteins, while “adaptor” is usually used to describe those which connect two proteins (Bhattacharyya et al., 2006, Flynn, 2001, Langeberg and Scott, 2015, Pan et al., 2012, Pawson, 2007). By interacting with multiple proteins within a cascade, scaffolds and adaptor can insulate downstream signaling, impact cross-talk with other pathways, target proteins to a particular subcellular location, coordinate feedback loops, and alter enzymatic activity or conformation of their binding partners (Bhattacharyya et al., 2006, Good et al., 2011, Harris et al., 2001, Keshet and Seger, 2010, Langeberg and Scott, 2015, Pan et al., 2012, Shaul and Seger, 2007, Shaw and Filbert, 2009, Witzel et al., 2012, Zeke et al., 2009). They are widely used by many signaling pathways, including the MAPK cascade.

MAPK scaffolds were first identified in yeast where the MAP3K protein, Ste11, interacts with either the Ste5 or Pbs2 scaffolding proteins in response to pheromones or osmotic stress, respectively (Choi et al., 1994, Kranz et al., 1994, Marcus et al., 1994, Posas and Saito, 1997, Printen and Sprague, 1994). Ste5 insulates the Ste11-Ste7-Fus3 pathway to promote the mating response, while Pbs2 drives glycerol synthesis via Ste11-Pbs2-Hog1 signaling (Gustin et al., 1998, Whitmarsh et al., 1998). These scaffolds are critical in coordinating the proper response to a given stimuli, as expression of chimeric Ste5 and Pbs2 proteins can swap the response to pheromones and activate the osmoresponse instead of mating (Bashor et al., 2008, Harris et al., 2001, Park et al., 2003, Tatebayashi et al., 2003).

While the MAPK cascade is conserved in eukaryotic cells, the protein diversity and complexity of this pathway has expanded throughout evolution (Widmann et al., 1999). This can especially be seen in the MAP3K tier of the cascade. Yeast cells have 4 well-described MAP3Ks, while mammalian cells have at least 20, making it the most abundant and diverse level of the MAPK cascade (Cuevas et al., 2007, Johnson et al., 2005, Uhlik et al., 2004). As the initiators of MAPK signaling, the large expansion of MAP3K proteins likely mediates downstream MAPK activation in response to the many complex stimuli in mammalian cells (Cuevas et al., 2007, Widmann et al., 1999). Importantly, activation of different MAP3K proteins can selectively promote one biological outcome over another. Scaffolding and adaptor proteins help organize and regulate the activity of the MAPK pathway to link together the proper biological outcome to an input signaling (Bashor et al., 2008, Bhattacharyya et al., 2006, Good et al., 2011, Harris et al., 2001, Pan et al., 2012, Park et al., 2003, Shaw and Filbert, 2009, Witzel et al., 2012, Zeke et al., 2009).

ASK1 is a MAP3K protein which directly phosphorylates MKK3/6 and MKK4/7 to lead to activation of downstream kinases, p38 and JNK, respectively (Ichijo et al., 1997). The most well-established role of ASK1 is as a pro-apoptotic signaling protein activated by a variety of cellular stressors, such as oxidative and endoplasmic reticulum (ER) stress (Hattori et al., 2009, Shiizaki et al., 2013). Over-expression of ASK1 or of a constitutively active form of the protein is sufficient to induce cell death under a variety of experimental conditions (Hatai et al., 2000, Ichijo et al., 1997, Saitoh et al., 1998).

ASK1 activity has been linked to many human diseases where ASK1-dependent cell death has both positive or negative effects on pathogenesis. In the brain, ASK1 promotes neuronal cell death that is characteristic of many neurodegenerative diseases and cerebral ischemia/reperfusion (I/R) injury (Guo et al., 2017, Lee et al., 2012, Nishitoh et al., 2002, Sturchler et al., 2011, Zhang and Zhang, 2002). In some cancers and viral infections, ASK1 activity is important in mediating chemotherapy-induced cell death and restricting viral replication and survival (Brozovic and Osmak, 2007, Chen et al., 1999, Cheng et al., 2014, Geleziunas et al., 2001, Maruoka et al., 2003, Miyakawa et al., 2015, Okazaki et al., 2015, Ryuno et al., 2017, Yuan et al., 2003). Additionally, ASK1 signaling can promote cell proliferation and inflammation, two biological processes that can drive tumor development and inflammatory diseases (Guo et al., 2010, Hayakawa et al., 2010, Hayakawa et al., 2011, Hayakawa et al., 2012, Iriyama et al., 2009, Mnich et al., 2010). Because of its vital role in cell fate determination, ASK1 activity and signaling is tightly regulated in the cells, in part through the binding of scaffold adaptor proteins. Furthermore, the loss of these interactions, either through genetic or pharmacological approaches, can impact ASK1-mediating signaling in these disorders.