Signal-transducing protein phosphorylation cascades mediated by Ras/Rho proteins in the mammalian cell: the potential for multiplex signalling

Abstract

The features of three distinct protein phosphorylation cascades in mammalian cells are becoming clear. These signalling pathways link receptor-mediated events at the cell surface or intracellular perturbations such as DNA damage to changes in cytoskeletal structure, vesicle transport and altered transcription factor activity. The best known pathway, the Ras-->Raf-->MEK-->ERK cascade [where ERK is extracellular-signal-regulated kinase and MEK is mitogen-activated protein (MAP) kinase/ERK kinase], is typically stimulated strongly by mitogens and growth factors. The other two pathways, stimulated primarily by assorted cytokines, hormones and various forms of stress, predominantly utilize p21 proteins of the Rho family (Rho, Rac and CDC42), although Ras can also participate. Diagnostic of each pathway is the MAP kinase component, which is phosphorylated by a unique dual-specificity kinase on both tyrosine and threonine in one of three motifs (Thr-Glu-Tyr, Thr-Phe-Tyr or Thr-Gly-Tyr), depending upon the pathway. In addition to activating one or more protein phosphorylation cascades, the initiating stimulus may also mobilize a variety of other signalling molecules (e.g. protein kinase C isoforms, phospholipid kinases, G-protein alpha and beta gamma subunits, phospholipases, intracellular Ca2+). These various signals impact to a greater or lesser extent on multiple downstream effectors. Important concepts are that signal transmission often entails the targeted relocation of specific proteins in the cell, and the reversible formation of protein complexes by means of regulated protein phosphorylation. The signalling circuits may be completed by the phosphorylation of upstream effectors by downstream kinases, resulting in a modulation of the signal. Signalling is terminated and the components returned to the ground state largely by dephosphorylation. There is an indeterminant amount of cross-talk among the pathways, and many of the proteins in the pathways belong to families of closely related proteins. The potential for more than one signal to be conveyed down a pathway simultaneously (multiplex signalling) is discussed. The net effect of a given stimulus on the cell is the result of a complex intracellular integration of the intensity and duration of activation of the individual pathways. The specific outcome depends on the particular signalling molecules expressed by the target cells and on the dynamic balance among the pathways.

The ras-related GTPase rac1 regulates a proliferative pathway selectively utilized by G-protein coupled receptors

“…G-protein coupled receptors (GPCR's), tyrosine kinase-linked receptors and JAK/STAT-linked receptors each activate intracellular kinase cascades, which in turn mediate complex cellular responses such as proliferation (Denhardt, 1996). The monomeric GTPase ras plays a pivotal role in linking cell surface receptors to intracellular signaling pathways and recently it has become clear that the ras-related GTPase rac performs similar functions.…”

“…The m1 muscarinic receptor, which transforms NIH3T3 cells in response to agonist stimulation (Gutkind et al, 1991), has been shown to activate both the MAP kinase and Jun kinase cascades (Coso et al, 1995b), which are each preferentially regulated by ras and rac, respectively (Denhardt, 1996;Coso et al, 1995;Minden et al, 1995). The closely related m5 muscarinic receptor also transforms via a ras-dependent pathway (Mattingly et al, 1994).…”

“…The dbl proteins are themselves oncogenic, stimulate JNK activation and many show exchange factor activity towards the racrelated GTPases (Hart et al, 1994;Horii et al, 1994;Cerione and Zheng, 1996;Denhardt, 1996). Candidate molecules which could act upstream of these exchange factors include the src-like non-receptor tyrosine kinases, Pyk2 and the adaptor molecule shc (Dikic et al, 1996;Chen et al, 1996).…”

“…Three distinct protein phosphorylation cascades have been described which mediate intracellular responses to extracellular signals (Denhardt, 1996), including the MAP/ERK kinase cascade (Marshall, 1995;Davis, 1993) which is preferentially activated by mitogens and growth factors and the SAPK/JNK and/ or p38 kinase cascades which are preferentially activated by cytokines and cellular stress Derijard et al, 1994). The MAP/ERK kinase cascade is linked to inputs from cell surface receptors by ras while rac and rac-related GTPases activate the SAPK/JNK pathway (Coso et al, 1995;Minden et al, 1995).…”

Phenotype-assisted transcriptome analysis identifies FOXM1 downstream from Ras–MKK3–p38 to regulate in vitro cellular invasion

“…Oncogenic Ras activates multiple signal transduction pathways, which either consist of stressor mitogen-activated protein kinases (SAPKs and MAPKs), including the p38, JNK/SAPK and p42/p44 MAPK/ERK pathways (Marshall, 1996). In general, growth factors and tumor promoters stimulate extracellular signal-regulated kinases (ERKs), while JNKs and p38 isoforms are activated by environmental stress such as UV light, osmotic stress but also inflammatory cytokines (Denhardt, 1996). Both, MAPKs and SAPKs are regulated by upstream kinases (Raman et al, 2007) and consist of a three module system transducing the signal through subsequent phosphorylation into the nucleus.…”