Coordinate interactions of protein tyrosine kinases and protein tyrosine phosphatases in T-cell receptor-mediated signalling

PTPRO-related CD8+ T-cell signatures predict prognosis and immunotherapy response in patients with breast cancer

BACKGROUND

Poor immunogenicity and extensive immunosuppressive T-cell infiltration in the tumor immune microenvironment (TIME) have been identified as potential barriers to immunotherapy success in "immune-cold" breast cancers. Thus, it is crucial to identify biomarkers that can predict immunotherapy efficacy. Protein tyrosine phosphatase receptor type O (PTPRO) regulates multiple kinases and pathways and has been implied to play a regulatory role in immune cell infiltration in various cancers.

METHODS

ESTIMATE and single-sample gene set enrichment analysis (ssGSEA) were performed to uncover the TIME landscape. The correlation analysis of PTPRO and immune infiltration was performed to characterize the immune features of PTPRO. Univariate and multivariate Cox analyses were applied to determine the prognostic value of various variables and construct the PTPRO-related CD8⁺ T-cell signatures (PTSs). The Kaplan-Meier curve and the receiver operating characteristic (ROC) curve were used to estimate the performance of PTS in assessing prognosis and immunotherapy response in multiple validation datasets.

RESULTS

High PTPRO expression was related to high infiltration levels of CD8⁺ T cells, as well as macrophages, activated dendritic cells (aDCs), tumor-infiltrating lymphocytes (TILs), and Th1 cells. Given the critical role of CD8⁺ T cells in the TIME, we focused on the impact of PTPRO expression on CD8⁺ T-cell infiltration. The prognostic PTS was then constructed using the TCGA training dataset. Further analysis showed that the PTS exhibited favorable prognostic performance in multiple validation datasets. Of note, the PTS could accurately predict the response to immune checkpoint inhibitors (ICIs).

CONCLUSION

PTPRO significantly impacts CD8⁺ T-cell infiltration in breast cancer, suggesting a potential role of immunomodulation. PTPRO-based PTS provides a new immune cell paradigm for prognosis, which is valuable for immunotherapy decisions in cancer patients.

Grb2, a simple adapter with complex roles in lymphocyte development, function, and signaling

Lymphocyte development, activation, and tolerance depend on antigen receptor signaling transduced via multiple intracellular signalosomes. These signalosomes are assembled by different adapters. Given that signaling molecules can be either positive or negative regulators for a biochemical target, the complex of a target with different regulator may dictate the final signaling outcome. Grb2 is a simple adapter known to be involved in a variety of growth factor receptor signaling. However, its role in antigen receptor signaling as well as lymphocyte development and function has emerged only recently. Despite its simple molecular structure, recent experiments show that Grb2 may play a complex role in T and B-cell antigen receptor signaling. In this article, we review recent findings about the physiological role of Grb2 in T and B-cell development and activation and summarize the current mechanistic understanding of how Grb2 exerts its function following T and B-cell antigen receptor stimulation.

Regulation of lymphocyte development and activation by the LAT family of adapter proteins

Transmembrane adapter proteins (TRAPs) are critical components of signaling pathways in lymphocytes, linking antigen receptor engagement to downstream cellular processes. While these proteins lack intrinsic enzymatic activity, their phosphorylation following receptor ligation allows them to function as scaffolds for the assembly of multi-molecular signaling complexes. Many TRAPs have recently been discovered, and numerous studies demonstrate their roles in the positive and negative regulation of lymphocyte maturation, activation, and differentiation. One such example is the linker for activation of T cells (LAT) family of adapter proteins. While LAT has been shown to play an indispensable role in T-cell and mast cell function, the other family members, linker for activation of B cells (LAB) and linker for activation of X cells (LAX), are necessary to fine-tune immune responses. In addition to its well-established role in the positive regulation of lymphocyte activation, LAT exerts an inhibitory effect on T-cell receptor-mediated signaling. Furthermore, LAT, along with LAB and LAX, plays a crucial role in establishing and maintaining tolerance. Here, we review recent data concerning the regulation of lymphocyte development and activation by the LAT family of proteins.

Upregulation of cAMP-specific PDE-4 activity following ligation of the TCR complex on thymocytes is blocked by selective inhibitors of protein kinase C and tyrosyl kinases

We have previously shown that the major cAMP phosphodiesterase (PDE) isoforms present in murine thymocytes are the cGMP-stimulated PDE activity (PDE-2) and the cAMP-specific PDE activity (PDE-4), and that these isoforms are differentially regulated following ligation of the TCR (Michie, A.M., Lobban, M. D., Mueller, T., Harnett, M. M., and Houslay, M.D. [1996] Cell. Signalling 8, 97-110). We show here that the anti-CD3-stimulated elevation in PDE-4 activity in murine thymocytes is dependent on protein tyrosine kinase and protein kinase C (PKC)-mediated signals as the TCR-coupled increase in PDE-4 activity can be abrogated by both the tyrosine kinase inhibitor, genistein, and the PKC selective inhibitors chelerythrine and staurosporine. Moreover, the PKC-activating phorbol ester, phorbol-12-myristate, 13-acetate (PMA) caused an increase in PDE-4 activity, similar to that observed in cells challenged with anti-CD3 monoclonal antibodies and which was not additive with cochallenge using anti-CD3 antibodies. Both the PMA- and the anti-CD3 antibody-mediated increases in PDE-4 activity were blocked by treatment with either cycloheximide or actinomycin D. Despite the upregulation of PDE-4 activity consequent to TCR ligation, intracellular cAMP levels increased on challenge of thymocytes with anti-CD3 antibody, indicating that adenylate cyclase activity was also increased by TCR ligation. It is suggested that the anti-CD3-mediated increase in PDE-4 activity was owing to a rapid PKC-dependent induction of PDE-4 activity following crosslinking of the TCR complex. This identifies "crosstalk" occurring between the PKA and PKC signaling pathways initiated by ligation of the antigen receptor in murine thymocytes. That both adenylate cyclase and PDE-4 activities were increased may indicate the presence of compartmentalized cAMP responses present in these cells.

Role of protein phosphatases in the regulation of nonspecific cytotoxic cell activity

In vitro and in vivo experiments were conducted to determine the effects of the protein phosphatase (PPase) inhibitors sodium fluoride (fluoride), sodium orthovanadate (vanadate), and lithium chloride on nonspecific cytotoxic cell (NCC) lysis of target cells. Both vanadate and fluoride stimulated NCC activity. Lithium chloride had no effects. Optimum enhancement for "normal" NCC was at low effector:target cell ratios and at least 30 min treatment was required to achieve maximum activation effects. Fluoride, but not vanadate activation effects were largely reversible. Vanadate, 2.5-10 mM, produced a 5-10-fold increase in cytotoxicity at 25:1 E:T, whereas less than twofold increases were produced by these concentrations at 100:1. NCC activity from "stressed" fish that had essentially no cytotoxic activity were also activated by vanadate. In vitro preincubation of NCC with 10-20 mM fluoride or 2.5-10 mM vanadate produced up to a 20-fold increase in stressed cytotoxicity. Combined treatments with 2.5 mM vanadate and 20 mM fluoride produced even greater responses. In vivo responses to vanadate were also determined. Treatment of catfish by immersion in 50 microM vanadate produced significant increases in cytotoxicity by 24-48 h posttreatment. Activation of cytotoxicity was not accompanied by increases in percentage of NCC (small lymphocyte content) or in total cell numbers in anterior kidney tissue. These studies indicated that levels of NCC activity are partially regulated by control of dephosphorylation of membrane proteins. Inability of NCC from stressed fish to lyse IM-9 target cells was reversed (probably) by disruption of an equilibrium between kinase and phosphatase activities. Normal NCC were "superactivated" only under conditions were they were in limiting numbers. These data show that phosphatases must be considered as active participants in regulation of signal transduction processes.

Tyrosine phosphorylation of alpha tubulin in human T lymphocytes

N-terminal sequencing of the 55- and 50-kDa polypeptides affinity purified on a phosphotyrosine monoclonal antibody column from activated Jurkat T cells identified alpha and beta tubulin. Two-dimensional gel analysis indicated that alpha tubulin was directly phosphorylated on tyrosine. beta Tubulin was not detectably tyrosine phosphorylated but was precipitated by anti-phosphotyrosine (PTyr) antibody by virtue of its association with the alpha subunit as a heterodimer. Phosphotyrosyl alpha tubulin was not incorporated into intact microtubules and was all in the unpolymerized soluble fraction. These results suggest that tyrosine phosphorylation of alpha tubulin may inhibit the ability of this subunit to polymerize into microtubules. Stimulation of Jurkat T cells via T cell receptor increased the amount of tubulin precipitated by the anti-PTyr antibody. These data raise the possibility that the polymerization of tubulin heterodimers may be regulated by phosphorylation on tyrosine during T cell activation.

Redistribution of activated pp60c-src to integrin-dependent cytoskeletal complexes in thrombin-stimulated platelets

Thrombin stimulation of platelets induces a transient increase in the specific activity of pp60c-src followed by a redistribution of pp60c-src to the Triton X-100-insoluble, cytoskeleton-rich fraction. Concomitant with the observed increase in pp60c-src activity was a rapid dephosphorylation of tyrosine 527 in 10 to 15% of pp60c-src molecules. In addition, we found that pp60c-src from the Triton-insoluble fraction was phosphorylated on tyrosine 416, the autophosphorylation site which is phosphorylated in activated oncogenic variants of pp60src. Furthermore, in platelets from patients with Glanzmann's thrombasthenia (which are deficient in the integrin receptor GPIIb-IIIa), pp60c-src was not translocated to the Triton-insoluble fraction, and there was a sustained increase in pp60c-src activity following thrombin treatment. These results suggest that pp60c-src is rapidly activated in thrombin-stimulated platelets, potentially by a protein tyrosine phosphatase, before it translocates to a cytoskeletal fraction, where many of its potential substrates are found. The evidence that the cytoskeletal association of pp60c-src is dependent upon engagement of the integrin receptor GPIIb-IIIa suggests that integrin-cytoskeletal complexes may serve to compartmentalize and anchor activated enzymes involved in signal transduction.

Redistribution of activated pp60c-src to integrin-dependent cytoskeletal complexes in thrombin-stimulated platelets

Thrombin stimulation of platelets induces a transient increase in the specific activity of pp60c-src followed by a redistribution of pp60c-src to the Triton X-100-insoluble, cytoskeleton-rich fraction. Concomitant with the observed increase in pp60c-src activity was a rapid dephosphorylation of tyrosine 527 in 10 to 15% of pp60c-src molecules. In addition, we found that pp60c-src from the Triton-insoluble fraction was phosphorylated on tyrosine 416, the autophosphorylation site which is phosphorylated in activated oncogenic variants of pp60src. Furthermore, in platelets from patients with Glanzmann's thrombasthenia (which are deficient in the integrin receptor GPIIb-IIIa), pp60c-src was not translocated to the Triton-insoluble fraction, and there was a sustained increase in pp60c-src activity following thrombin treatment. These results suggest that pp60c-src is rapidly activated in thrombin-stimulated platelets, potentially by a protein tyrosine phosphatase, before it translocates to a cytoskeletal fraction, where many of its potential substrates are found. The evidence that the cytoskeletal association of pp60c-src is dependent upon engagement of the integrin receptor GPIIb-IIIa suggests that integrin-cytoskeletal complexes may serve to compartmentalize and anchor activated enzymes involved in signal transduction.

Signal transduction by integrin receptors for extracellular matrix: cooperative processing of extracellular information

Adhesion receptors allow cells to interact with a dynamic and information-rich environment of extracellular matrix molecules. The integrin family of adhesion receptors transduces signals from the extracellular matrix that regulate growth, gene expression and differentiation, as well as cell shape, motility and cytoskeletal architecture. Recent data support the hypothesis that integrins transduce signals cooperatively with other classes of adhesion receptors or with growth factor receptors. Furthermore, the ability of integrins to interact with the cytoskeleton appears to be fundamental to their mechanism for signal transduction.

Specificity of cell adhesion in development: the cadherin superfamily

Cadherins are major cell-surface receptors involved in specific cell adhesion during development. Recent results reveal the existence of a growing array of related molecules involved in various forms of cell-cell adhesion, including that mediated by desmosomes. Comparisons with other families of adhesion receptors suggest testable models for functions of the emerging cadherin superfamily in development and disease.