Presence and functions of immune components in the tumor microenvironment

Organ selectivity in metastasis: regulation by chemokines and their receptors

Cancer metastasis results from a non-random process, in which organ selectivity by the tumor cells is largely determined by factors that are expressed at the remote organs that eventually turn into preferred sites of metastasis formation. These factors support the consecutive steps required for metastasis formation, including tumor cell adhesion to microvessel walls, extravasation into target tissue and migration. Of the different components that regulate organ selectivity, instrumental roles were recently attributed to chemokines and their receptors. The present review presents the rationale standing behind the first studies looking at the potential involvement of chemokine-related components in organ selectivity. Based on these studies and many others that followed, the current paradigm is that chemokines that are expressed at specific organs determine to large extent organ specificity by promoting tumor cell adhesion to microvessel walls, by facilitating processes of extravasation into the target tissue and by inducing tumor cell migration. Moreover, chemokines can possibly support additional steps that are required for "successful" establishment of metastases, such as tumor cell proliferation and survival. The review focuses on the CXCL12-CXCR4 pair as the role model in our current understanding of chemokine involvement in organ selectivity. This review also describes the prominent roles played by CCR7 and its corresponding chemokine ligands (CCL21, CCL19) in lymph node metastasis, and of the CCR10-CCL27 axis in melanoma skin survival and metastasis. Overall, the present discussion describes chemokines as important constituents of the tumor microenvironment at metastatic sites, dictating directionality of chemokine receptor-expressing tumor cells, facilitating their adhesion and extravasation, and eventually contributing to organ selectivity.

CXCL10 promotes invasion-related properties in human colorectal carcinoma cells

CXCL10 was recently shown to exert antimalignancy functions by influencing the tumor microenvironment. Here, we have taken a different approach, investigating the effects of CXCL10 directly on tumor-promoting functions in colorectal carcinoma (CRC) cells. CXCL10 expression was detected in preferred metastatic sites of CRC (liver, lungs, and lymph nodes), and its CXCR3 receptor was expressed by eight CRC cell lines (detected: reverse transcription-PCR and/or flow cytometry). Detailed analysis was done on two cell lines derived from primary CRC tumors (SW480, KM12C) and their metastatic descendents (SW620 and KM12SM). The three known variants of CXCR3 (CXCR3-A, CXCR3-B, and CXCR3-alt) were detected in all four cell lines. CXCR3 expression was also observed on colorectal tumor cells in biopsies of CRC patients (immunohistochemistry). CXCL10 and CXCR3 expression were potently induced in CRC cells by Interferon gamma and all four CRC cell lines responded to CXCL10 by extracellular signal-regulated kinase 1/2 dephosphorylation. The chemokine did not affect tumor cell growth or angiogenesis-related functions in the tumor cells, such as CXCL8 and vascular endothelial growth factor secretion. Importantly, CXCL10 significantly up-regulated invasion-related properties in CRC cells: It promoted matrix metalloproteinase 9 expression and induced CRC cell migration. Of note, CXCL10-induced migration was detected only in the two metastatic cells and not in their primary counterparts. Also, CXCL10 promoted the adhesion of metastatic cells to laminin. These results suggest that CXCL10 can be exploited by CRC cells toward their progression, thus possibly antagonizing the antimalignancy effects of the chemokine on the tumor microenvironment. Therefore, care should be taken when considering CXCL10 as a therapeutic antitumor modality for CRC treatment.

The tumor microenvironment in the post-PAGET era

The research area of tumor microenvironment is considered, at present, to be an important factor in tumorigenesis and especially in tumor progression. The present mini review is focused on three principles characterizing the nature of the tumor microenvironment. We first discuss the regulatory functions of the tumor microenvironment and the complexity of the combinatorial signaling pathways operating in it. We then address the aspect that the tumor microenvironment incorporates both pro and anti malignancy factors and that a balance between these factors regulates tumor progression. Thirdly we provide evidence that the non-tumor cells in the tumor microenvironment and their products may be different from those of their counterparts residing in non-tumor microenvironments. The conclusion of this mini review is that the tumor microenvironment, by exerting regulatory functions and selective pressures drives cancer cells into one of several molecular evolution pathways thereby determining and shaping their malignancy phenotype.

cAMP-mediated inhibition of DNA replication and S phase progression: involvement of Rb, p21Cip1, and PCNA

cAMP exerts an antiproliferative effect on a number of cell types including lymphocytes. This effect of cAMP is proposed to be mediated by its ability to inhibit G1/S transition. In this report, we provide evidence for a new mechanism whereby cAMP might inhibit cellular proliferation. We show that elevation of intracellular levels of cAMP inhibits DNA replication and arrests the cells in S phase. The cAMP-induced inhibition of DNA synthesis was associated with the increased binding of p21Cip1 to Cdk2-cyclin complexes, inhibition of Cdk2 kinase activity, dephosphorylation of Rb, and dissociation of PCNA from chromatin in S phase cells. The ability of cAMP to inhibit DNA replication and trigger release of PCNA from chromatin required Rb and p21Cip1 proteins, since both processes were only marginally affected by increased levels of cAMP in Rb-/- and p21Cip1-/- 3T3 fibroblasts. Importantly, the implications of cAMP-induced inhibition of DNA synthesis in cancer treatment was demonstrated by the ability of cAMP to reduce apoptosis induced by S phase-specific cytotoxic drugs. Taken together, these results demonstrate a novel role for cAMP in regulation of DNA synthesis and support a model in which activation of cAMP-dependent signaling protects cells from the effect of S phase-specific antitumor agents.