SOCS1 expression in cancer cells: potential roles in promoting antitumor immunity.

Suppressor of cytokine signaling 1 (SOCS1) is a potent regulator immune cell responses and a proven tumor suppressor. Inhibition of SOCS1 in T cells can boost antitumor immunity, whereas its loss in tumor cells increases tumor aggressivity. Investigations into the tumor suppression mechanisms so far focused on tumor cell-intrinsic functions of SOCS1. However, it is possible that SOCS1 expression in tumor cells also regulate antitumor immune responses in a cell-extrinsic manner via direct and indirect mechanisms. Here, we discuss the evidence supporting the latter, and its implications for antitumor immunity.

Multi-functional adaptor SKAP1: regulator of integrin activation, the stop-signal, and the proliferation of T cells.

T-cell activation is a complex process involving a network of kinases and downstream molecular scaffolds or adaptors that integrate surface signals with effector functions. One key immune-specific adaptor is Src kinase-associated phosphoprotein 1 (SKAP1), which is also known as src kinase-associated protein of 55 kDa (SKAP55). This mini-review explains how SKAP1 plays multiple roles in regulating integrin activation, the "stop-signal", and the optimization of the cell cycling of proliferating T cells through interactions with various mediators, including the Polo-like kinase 1 (PLK1). Ongoing research on SKAP1 and its binding partners will likely provide important insights into the regulation of immune function and have implications for the development of new treatments for disease states such as cancer and autoimmunity.

SOCS1 controls liver regeneration by regulating HGF signaling in hepatocytes.

BACKGROUND & AIMS: Frequent repression of the Socs1 (suppressor of cytokine signaling 1) gene in hepatocellular carcinoma (HCC) and increased susceptibility of SOCS1-deficient mice to hepatocarcinogens suggest a tumor suppressor role for SOCS1 in the liver, but the underlying mechanisms remain unclear. Here we investigated the role of SOCS1 in regulating hepatocyte proliferation following partial hepatectomy and HGF stimulation. METHODS: Because Socs1(-/-) mice die prematurely due to deregulated IFNγ signaling, we used Socs1(-/-)Ifng(-/-) mice to study the role of SOCS1 in liver regeneration following partial hepatectomy. We examined the activation of signaling molecules downstream of IL-6 and hepatocyte growth factor (HGF) receptors in the regenerating liver, primary hepatocytes, and in human hepatoma cells. We examined the interaction between SOCS1 and the HGF receptor c-Met by reciprocal immunoprecipitation. RESULTS: Socs1(-/-)Ifng(-/-) mice displayed accelerated liver regeneration with increased DNA synthesis compared to Ifng(-/-) and wild type mice. The regenerating liver of Socs1(-/-)Ifng(-/-) mice did not show increased IL-6 signaling, but displayed earlier phosphorylation of Gab1, a signaling adaptor downstream of c-Met. Following HGF stimulation, hepatocytes from Socs1(-/-)Ifng(-/-) mice displayed increased phosphorylation of c-Met and Gab1, cell migration and proliferation. Accordingly, SOCS1 overexpression attenuated HGF-induced phosphorylation of c-Met, Gab1, and ERK1/2 in hepatoma cells, and decreased their proliferation and migration. SOCS1 interacted with the Tpr-Met, an oncogenic form of the Met receptor. CONCLUSIONS: SOCS1 attenuates c-Met signaling and thus negative regulation of HGF signaling could be an important mechanism underlying the anti-tumor role of SOCS1 in the liver.

The MNK1/2-eIF4E Axis Supports Immune Suppression and Metastasis in Postpartum Breast Cancer.

Breast cancer diagnosed within 10 years following childbirth is defined as postpartum breast cancer (PPBC) and is highly metastatic. Interactions between immune cells and other stromal cells within the involuting mammary gland are fundamental in facilitating an aggressive tumor phenotype. The MNK1/2-eIF4E axis promotes translation of prometastatic mRNAs in tumor cells, but its role in modulating the function of nontumor cells in the PPBC microenvironment has not been explored. Here, we used a combination of in vivo PPBC models and in vitro assays to study the effects of inactivation of the MNK1/2-eIF4E axis on the protumor function of select cells of the tumor microenvironment. PPBC mice deficient for phospho-eIF4E (eIF4ES209A) were protected against lung metastasis and exhibited differences in the tumor and lung immune microenvironment compared with wild-type mice. Moreover, the expression of fibroblast-derived IL33, an alarmin known to induce invasion, was repressed upon MNK1/2-eIF4E axis inhibition. Imaging mass cytometry on PPBC and non-PPBC patient samples indicated that human PPBC contains phospho-eIF4E high-expressing tumor cells and CD8+ T cells displaying markers of an activated dysfunctional phenotype. Finally, inhibition of MNK1/2 combined with anti-PD-1 therapy blocked lung metastasis of PPBC. These findings implicate the involvement of the MNK1/2-eIF4E axis during PPBC metastasis and suggest a promising immunomodulatory route to enhance the efficacy of immunotherapy by blocking phospho-eIF4E. SIGNIFICANCE: This study investigates the MNK1/2-eIF4E signaling axis in tumor and stromal cells in metastatic breast cancer and reveals that MNK1/2 inhibition suppresses metastasis and sensitizes tumors to anti-PD-1 immunotherapy.

Metastatic Breast Carcinoma-Associated Fibroblasts Have Enhanced Protumorigenic Properties Related to Increased IGF2 Expression.

PURPOSE: The microenvironment of metastatic breast cancer is incompletely characterized, despite prior evidence that it plays a key role in the biology of metastasis. A major component of the tumor stroma is the carcinoma-associated fibroblast (CAF), which has been shown to communicate with other stromal and cancer cells to create a protumorigenic milieu. Our study was designed to characterize human CAFs from different metastatic sites. EXPERIMENTAL DESIGN: We collected eight carcinoma-associated fibroblasts (mCAFs) from different metastatic sites and compared them with CAFs from primary tumors (pCAFs) and with normal breast fibroblasts (NFs). Molecular profiles and effects on breast cancer cell growth, on response to doxorubicin and on T-cell proliferation were compared. RESULTS: We observed marked differences in mCAFs compared with pCAFs and NFs with respect to in vitro proliferation and effects on breast cancer cell migration, spheroid growth, invasion, response to doxorubicin, and in vivo tumor growth. We found marked transcriptomic differences between mCAFs and pCAFs, including increased expression of IFN-related genes and IGF2 in the former. Cluster analysis revealed two groups of mCAFs, with the liver mCAFs clustering together, with increased PDGFA expression. Treatment with an antibody against insulin-like growth factors (BI836845) inhibited growth of mixed mCAF-tumor cell xenografts in vivo. Also, mCAFs had a suppressive effect on T-cell proliferation. CONCLUSIONS: This is the first comparative analysis of a set of CAFs from metastatic sites in breast cancer. It revealed a marked protumorigenic effect in these mCAFs, which occurs in part through increased expression of IGF2.

Attenuation of MET-mediated migration and invasion in hepatocellular carcinoma cells by SOCS1.

AIM: To investigate the role of suppressor of cytokine signaling 1 (SOCS1) in regulating MET-mediated invasive potential of hepatocellular carcinoma (HCC) cells. METHODS: Stable derivatives of mouse (Hepa1-6) and human (hep3B, HepG2) HCC cell lines expressing SOCS1 or control vector were evaluated for their ability to migrate towards hepatocyte growth factor (HGF) in the transwell migration assay, invade extracellular matrix in response to HGF stimulation in a 3-D invasion assay by confocal microscopy, and to undergo anchorage-independent proliferation in semisolid agar. Following intravenous and intrasplenic inoculation into NOD.scid.gamma mice, the ability of Hepa cells to form othotopic tumors was evaluated. Following HGF stimulation of Hepa and Hep3B cells, expression of proteins implicated in epithelial-to-mesenchymal transition was evaluated by western blot and qRT-PCR. RESULTS: SOCS1 expression in mouse and human HCC cells inhibited HGF-induced migration through matrigel. In the 3-D invasion assay, HGF stimulation induced invasion of HCC cells across type-I collagen matrix, and SOCS1 expression significantly reduced the depth of invasion. SOCS1 expression also reduced the number and size of colonies formed by anchorage-independent growth in semisolid agar. Following intravenous inoculation, control Hepa cell formed large tumor nodules that obliterated the liver whereas the SOCS1-expressing Hepa cells formed significantly smaller nodules. Tumors formed by SOCS1-expressing cells showed reduced phosphorylation of STAT3 and ERK that was accompanied by reduced levels of MET protein expression. HGF stimulated Hepa cells expressing SOCS1 showed increased expression of E-cadherin and decreased expression of EGR1, SNAI1 and ZEB1. Comparable results were obtained with Hep3B cells. SOCS1 expressing HCC cells also showed reduced levels of EGR1 and SNAI1 transcripts. CONCLUSION: Our findings indicate that loss of SOCS1-dependent control over epithelial-to-mesenchymal transition may contribute to MET-mediated migration, invasion and metastatic growth of HCC.

Regulation of Hedgehog signaling by ubiquitination.

The Hedgehog (Hh) signaling pathway plays crucial roles both in embryonic development and in adult stem cell function. The timing, duration and location of Hh signaling activity need to be tightly controlled. Abnormalities of Hh signal transduction lead to birth defects or malignant tumors. Recent data point to ubiquitination-related posttranslational modifications of several key Hh pathway components as an important mechanism of regulation of the Hh pathway. Here we review how ubiquitination regulates the localization, stability and activity of the key Hh signaling components.

Regulation of MET receptor signaling by SOCS1 and its implications for hepatocellular carcinoma.

The SOCS1 gene is a frequent target of epigenetic repression in hepatocellular carcinoma. Many other types of cancer also harbor methylated SOCS1 gene. Besides, recent studies implicate microRNAs targeting SOCS1 in cancer progression. These findings suggest a broad tumor suppressor role of SOCS1 and have stimulated the quest to elucidate the underlying molecular mechanisms. The essential physiological function of SOCS1 is to attenuate interferon gamma signaling in immune cells. SOCS1 binds activated JAK kinases and the receptor chains of several cytokines, some of which are implicated in cancer progression. SOCS1 also facilitates ubiquitination and proteasomal degradation of many signaling molecules downstream of cytokine and growth factor receptors. We have shown that SOCS1 inhibits signaling via the hepatocyte growth factor receptor c-MET in hepatocytes. Aberrant MET signaling, implicated in the progression of many types of cancers, also contributes to the development of chemoresistance to tyrosine kinase inhibitors and drugs targeting other oncogenic signaling pathways. Here, we discuss the SOCS1-dependent regulation of MET signaling as an important mechanism underlying the tumor suppressor role of SOCS1 that is relevant not only to hepatocellular carcinoma but also to other types of cancers.