# Ras Isoforms from Lab Benches to Lives--What Are We Missing and How Far Are We?
## 5 Missing Pieces in the Ras Puzzle  
### Multiple Receptors Activating the Same Relay Protein-Ras  
Since its discovery as an oncogene, the prime focus of Ras research has been on its activation by RTKs. Most studies were unidimensional and undermined the isoform-specific signaling and functions. However, Ras is also activated by certain receptors primarily present on immune cells. [PERSON] et al. in 1996 demonstrated that cluster of differentiation 40 (CD40), a tumor necrotic factor receptor (TNFR) family member, activated Ras. Treatment of Daudi B cells with anti-CD40 led to the activation of Ras which further correlated with the stimulation of Ras-related C3 botulinum toxin substrate 1 (Racl), MEK-1, and PI3K [104]. This finding was further fortified by [PERSON] et al., who showed that in murine macrophages, there exist activator and effector specificities for Ras isoforms wherein, a weak CD40 signal activated N-Ras via SOS, whereas a strong CD40 signal activated H-/K-Ras via Ras guanyl nucleotide-releasing protein (Ras-GRP). This signaling specificity of isoforms was further reflected in their specificity for effectors. H-/K-Ras showed specificity for PI3K at high strength of CD40 signal, meanwhile, N-Ras associated with Raf1 at a low strength of CD40 signaling [21]. Ras activation through stimulation of T cell receptors (TCRs) was first shown in human peripheral blood (Figure 5). Upon TCR stimulation p21 Ras was rapidly activated, as hypothesized then, due to a decrease in the activity of GAP on PKC stimulation [105]. Ras signaling plays a pivotal role in the early growth and maturation of T cells in the thymus [106] and the two main GEFs involved in the activation of Ras in T cells are SOS1 and RasGRP1. While SOS1 is required for the normal function of TCRs, a study in Jurkat T cells showed that following TCR stimulation, the activity of Ras GRP and GTP binding to Ras increased [107]. In addition, RTKs independently activate Ras through the Raf-MEK-ERK pathway to control dimeric transcription factor activator protein 1 (AP1) and NFAT. This leads to an increase in the expression of the IL-2 gene that drives T cell proliferation [108]. Not only TCRs but B cell receptors (BCR) activate Ras as well. On B cell receptor ligation, phosphorylation of its immunoreceptor tyrosine-based activation motifs (ITAMs) takes place which initiates signals to activate Ras via SOS. BCR stimulation causes the adaptor proteins Grb2 and Shc to form a ternary complex with SOS. Activated Ras recruits and activates Raf which further phosphorylates p21-activated kinases (PAK) and MEK1/2, which in turn activated ERK1/2 [109]. Ras GEFs control N-Ras activation by both individual actions and in concert. In both T and B cells, Ras-GRP is the dominant GEF for Ras activation and it also primes the Ras binding to the allosteric pocket of SOS. This creates a positive feedback loop that regulates the activity of Ras and provides a robust mechanism of activation of Ras even if a few lymphocyte receptors are engaged [110]. Not only the receptors of the adaptive immune system, but those of the innate immune system also activate Ras. Pattern recognition receptors (PRRs) like TLRs are found to activate Ras isoforms. [PERSON] et al. found that upon stimulation of TLR 2, 3, and 4, by their agonists Pam\({}_{3}\)CSK\({}_{4}\), poly (I:C), and lipopolysaccharide-acrylamide (LPS) respectively, the expression of the PTM enzyme ICMT and Ras increased in macrophages. Similar effects were seen in the colon, stomach, and liver of mice with colitis, gastritis, and hepatitis. These results suggest that Ras plays an important role in TLR signaling that activates AP-1 mediated inflammatory response [111]. Another study reported that in macrophages, TLR9 ligand CpG ODN activates Ras in a dose-dependent manner. Following activation, Ras associates with TLR9 and promotes IRAK1/TRAF6 complex formation, leading to the activation of MAPK and NF-\(\kappa\)B [112].