Inverted signaling hierarchy between RAS and RAC in T-lymphocytes

Abstract

In order to generate coherent biological responses to extracellular stimuli, cells have established synergistic and antagonistic crosstalk between pathways with similar or opposing functions, respectively. Two routes cooperating in the generation of mitogenic and cytoskeletal functions are those induced by Ras and Rho/Rac GTPases. In these signaling interactions, Rho/Rac proteins have been always placed in a downstream position respect to Ras in all cell systems analysed so far. In this report, we describe that such signaling hierarchy does not apply to T-lymphocytes. Thus, we show that both Rac1 GDP/GTP exchange factors such as Vav and constitutively active versions of Rac1 can promote the effective stimulation of the Ras pathway in T-lymphocytes. The molecular link for this new type of pathway interconnectivity is RasGRP1, a diacylglyceroldependent GDP/GTP exchange factor for Ras that translocates to the plasma membrane in a Vav-and Rac1-dependent manner. The effect of the Vav/Rac1 pathway on the Ras pathway is highly dependent on the activity of phospholipase C-c, the key cellular supplier of intracellular diacylglycerol. Signaling experiments suggest that this crosstalk represents a signaling strategy used by the T-cell receptor to promote robust biological responses of both the Rac/Rho and Ras pathways upon antigen engagement.

Role of chimaerins, a group of Rac-specific GTPase activating proteins, in T-cell receptor signaling

“…To study whether a similar regulation occurred in T-cells, we monitored the changes in the subcellular localization of EGFP-tagged α2-and β□-chimaerin upon the treatment of Jurkat cells with anti-CD3 antibodies for 5 min. As positive control, we used an EGFP fused to RasGRP1, a DAGdependent GEF for Ras that has been shown before to translocate to the plasma membrane of lymphocytes upon similar stimulation conditions [52,53]. Unexpectedly, we found that the stimulation of the TCR did not induce any translocation of chimaerin family members to membrane structures (Fig 5A, see first and second rows of panels from top).…”

Rac1 Protein Regulates Glycogen Phosphorylase Activation and Controls Interleukin (IL)-2-dependent T Cell Proliferation

“…It has been reported that T cell receptor engagement by antigen promotes the rapid activation of these GTPases together with an array of other signaling molecules (12,13). Subsequently, Rac proteins transduce signals through intermediate signaling molecules, most notably PKC (14), phosphatidylinositol 3-kinase (PI3K) (5), calcium (15), and PLC␥1 (13).…”

“…Subsequently, Rac proteins transduce signals through intermediate signaling molecules, most notably PKC (14), phosphatidylinositol 3-kinase (PI3K) (5), calcium (15), and PLC␥1 (13). Importantly, Rac1 controls the translocation of RasGRP1 (a guanine nucleotide exchange factor for Ras) to the actin juxtamembrane structures to facilitate Ras/ERK pathway activation (12). The integration of these signaling events reaches the nucleus via AP-1, nuclear factor of activated T cells, NF-B, and JNK.…”

Rac1/p21‐activated kinase pathway controls retinoblastoma protein phosphorylation and E2F transcription factor activation in B lymphocytes

“…This molecular structure and its potential to establish intermolecular interactions result in a complex and flexible platform for both recognition and processing information as well as the transduction of signals to the cell cytosol and the nucleus. B lymphocytes may be maintained in a quiescent state and stimulation of BCR leads to potent activation of signalling molecules, such as the small GTPases of the Ras superfamily [2][3][4][5] mediating the activation of intracellular signalling cascades [2][3][4][5][6][7][8][9]. Briefly, BCR stimulates the activity of both the Syk [10,11] and the Src family of protein tyrosine kinases [12] leading to the activation of the B cell signalosome, which includes the adapter protein BLNK (B cell linker-protein), Bruton's tyrosine kinase (BTK) and phospholipase Cc2 (PLCc2) [13,14].…”

“…Briefly, BCR stimulates the activity of both the Syk [10,11] and the Src family of protein tyrosine kinases [12] leading to the activation of the B cell signalosome, which includes the adapter protein BLNK (B cell linker-protein), Bruton's tyrosine kinase (BTK) and phospholipase Cc2 (PLCc2) [13,14]. PLCc2 in its active state, hydrolyses the membrane component phosphatidylinositol-4,5-bisphosphate (PIP 2 ), resulting in the second messengers inositol-1,4,5-trisphosphate (IP 3 ) and diacylglycerol (DAG). IP 3 binds to IP 3 receptors in specialized areas of the endoplasmic reticulum triggering Ca 2+ release [15], while DAG remains attached to the inner region of the plasma membrane and serves to recruit and contribute to activating the cytosolic protein kinase C (PKC) [16].…”