Inflammatory conditions induce IRES-dependent translation of cyp24a1

Rapid alterations in protein expression are commonly regulated by adjusting translation. In addition to cap-dependent translation, which is e.g. induced by pro-proliferative signaling via the mammalian target of rapamycin (mTOR)-kinase, alternative modes of translation, such as internal ribosome entry site (IRES)-dependent translation, are often enhanced under stress conditions, even if cap-dependent translation is attenuated. Common stress stimuli comprise nutrient deprivation, hypoxia, but also inflammatory signals supplied by infiltrating immune cells. Yet, the impact of inflammatory microenvironments on translation in tumor cells still remains largely elusive. In the present study, we aimed at identifying translationally deregulated targets in tumor cells under inflammatory conditions. Using polysome profiling and microarray analysis, we identified cyp24a1 (1,25-dihydroxyvitamin D3 24-hydroxylase) to be translationally upregulated in breast tumor cells co-cultured with conditioned medium of activated monocyte-derived macrophages (CM). Using bicistronic reporter assays, we identified and validated an IRES within the 5′ untranslated region (5′UTR) of cyp24a1, which enhances translation of cyp24a1 upon CM treatment. Furthermore, IRES-dependent translation of cyp24a1 by CM was sensitive to phosphatidyl-inositol-3-kinase (PI3K) inhibition, while constitutive activation of Akt sufficed to induce its IRES activity. Our data provide evidence that cyp24a1 expression is translationally regulated via an IRES element, which is responsive to an inflammatory environment. Considering the negative feedback impact of cyp24a1 on the vitamin D responses, the identification of a novel, translational mechanism of cyp24a1 regulation might open new possibilities to overcome the current limitations of vitamin D as tumor therapeutic option

Macrophages attenuate the transcription of CYP1A1 in breast tumor cells and enhance their proliferation.

While aberrant cells are routinely recognized and removed by immune cells, tumors eventually escape innate immune responses. Infiltrating immune cells are even corrupted by the tumor to acquire a tumor-supporting phenotype. In line, tumor-associated macrophages are well-characterized to promote tumor progression and high levels of tumor-infiltrating macrophages are a poor prognostic marker in breast cancer. Here, we aimed to further decipher the influence of macrophages on breast tumor cells and determined global gene expression changes in three-dimensional tumor spheroids upon infiltration of macrophages. While various tumor-associated mRNAs were upregulated, expression of the cytochrome P450 family member CYP1A1 was markedly attenuated. Repression of CYP1A1 in tumor cells was elicited by a macrophage-shaped tumor microenvironment rather than by direct tumor cell-macrophage contacts. In line with changes in RNA expression profiles, macrophages enhanced proliferation of the tumor cells. Enhanced proliferation and macrophage presence further correlated with reduced CYP1A1 expression in patient tumors when compared with normal tissue. These findings are of interest in the context of combinatory therapeutic approaches involving cytotoxic and immune-modulatory compounds

Macrophages attenuate the transcription of CYP1A1 in breast tumor cells and enhance their proliferation

While aberrant cells are routinely recognized and removed by immune cells, tumors eventually escape innate immune responses. Infiltrating immune cells are even corrupted by the tumor to acquire a tumor-supporting phenotype. In line, tumor-associated macrophages are well-characterized to promote tumor progression and high levels of tumor-infiltrating macrophages are a poor prognostic marker in breast cancer. Here, we aimed to further decipher the influence of macrophages on breast tumor cells and determined global gene expression changes in three-dimensional tumor spheroids upon infiltration of macrophages. While various tumor-associated mRNAs were upregulated, expression of the cytochrome P450 family member CYP1A1 was markedly attenuated. Repression of CYP1A1 in tumor cells was elicited by a macrophage-shaped tumor microenvironment rather than by direct tumor cell-macrophage contacts. In line with changes in RNA expression profiles, macrophages enhanced proliferation of the tumor cells. Enhanced proliferation and macrophage presence further correlated with reduced CYP1A1 expression in patient tumors when compared with normal tissue. These findings are of interest in the context of combinatory therapeutic approaches involving cytotoxic and immune-modulatory compounds

sST2 translation is regulated by FGF2 via an hnRNP A1-mediated IRES-dependent mechanism

Translation is an energy-intensive process and tightly regulated. Generally, translation is initiated in a cap-dependent manner. Under stress conditions, typically found within the tumor microenvironment in association with e.g. nutrient deprivation or hypoxia, cap-dependent translation decreases, and alternative modes of translation initiation become more important. Specifically, internal ribosome entry sites (IRES) facilitate translation of specific mRNAs under otherwise translation-inhibitory conditions. This mechanism is controlled by IRES trans-acting factors (ITAF), i.e. by RNA-binding proteins, which interact with and determine the activity of selected IRESs. We aimed at characterizing the translational regulation of the IL-33 decoy receptor sST2, which was enhanced by fibroblast growth factor 2 (FGF2). We identified and verified an IRES within the 5'UTR of sST2. Furthermore, we found that MEK/ERK signaling contributes to FGF2-induced, sST2-IRES activation and translation. Determination of the sST2-5'UTR structure by in-line probing followed by deletion analyses identified 23 nucleotides within the sST2-5'UTR to be required for optimal IRES activity. Finally, we show that the RNA-binding protein heterogeneous ribonucleoprotein A1 (hnRNP A1) binds to the sST2-5'UTR, acts as an ITAF, and thus controls the activity of the sST2-IRES and consequently sST2 translation. Specifically, FGF2 enhances nuclear-cytoplasmic translocation of hnRNP A1, which requires intact MEK/ERK activity. In summary, we provide evidence that the sST2-5'UTR contains an IRES element, which is activated by a MEK/ERK-dependent increase in cytoplasmic localization of hnRNP A1 in response to FGF2, enhancing the translation of sST2

Cyp24a1 contains an IRES element.

<p>(A) Sequence of the human cyp24a1-5′UTR. (B) Schematic representation of the bicistronic control (phpRF) and cyp24a1-5′UTR-containing (phpR-cyp-F) luciferase constructs used for reporter assays. (C) Bicistronic reporter plasmids phpRF (white bars) and phpR-cyp-F (black bars) were transfected into MCF7 cells. 24 h after transfection <i>renilla</i> and <i>firefly</i> luciferase activities were measured and data are presented as means ± SEM relative to phpRF (n≥3, ** p<0.01). (D) RNA isolated from cells transfected with phpRF or phpR-cyp-F was DNAse treated and reverse transcribed. <i>Upper panel</i>: PCR was performed with specific primers to amplify full length RL or R-cyp-L mRNAs. PCR products were visualized <i>via</i> agarose gel electrophoresis and ethidium bromide staining. Data are representative for at least 3 independent experiments. <i>Lower panel</i>: RT-qPCR analysis of the amount of <i>firefly</i> mRNA normalized to <i>renilla</i> mRNA. Data are presented as means ± SEM (n≥3). (E) <i>In vitro-</i>transcribed mRNAs of the control (hpRF, white bars) or the cyp24a1-5′UTR-containing vector (hpR-cyp-F, black bars) were transfected into MCF7 cells. 24 h after transfection <i>renilla</i> and <i>firefly</i> luciferase activities were measured. Luciferase activities are given relative to hpRF and data are presented as means ± SEM (n≥3, ** p<0.01).</p

Polysome profile of MCF7 cells.

<p>Representative profile of MCF7 lysates at 254 nm as determined during polysomal fractionation (<i>upper panel</i>). Equal aliquots of RNA isolated from single fractions were analyzed using denaturing agarose gel electrophoresis to verify 28S and 18S rRNA content as indicators for ribosome distribution (<i>lower panel</i>).</p

CM induces cyp24a1 translation.

<p>MCF7 cells were treated with Ctr or CM for 4(A) and cyp24a1 (B) was analyzed in single fractions using RT-qPCR. The distribution of the respective mRNAs across the individual gradients was determined relative to the total RNA extracted from the gradients. Results from a representative experiment are given in A and B. (C+D) Changes of gapdh (C) and cyp24a1 (D) mRNA distribution induced by CM were normalized to Ctr. (E) cyp24a1 distribution (from D) was normalized to gapdh distribution (from C). Distribution changes are presented as means ± SEM (n≥3, * p<0.05, ** p<0.01, *** p<0.001).</p

Cyp24a1 translation is initiated in part cap-independently.

<p>MCF7 cells were treated with rapamycin [100 nM] for 4 h and subjected to polysomal fractionation. RNA from single fractions was isolated and gapdh (A) and cyp24a1 (B) mRNA distribution changes were analyzed separately as described before. Data are presented as means ± SEM (n≥3).</p

CM induces cyp24a1 IRES activity in an Akt-dependent manner.

<p>(A) MCF7 cells were transfected with phpR-cyp-F. 48 h after transfection cells were treated for 4 h with Ctr, CM, or CM supplemented with LY294002 [10 µM] or SB203580 [10 µM]. IRES activity was calculated as ratio of <i>firefly</i> to <i>renilla</i> luciferase activities and is given relative to Ctr. Data are presented as means ± SEM (n≥3, * p<0.05). (B) <i>(upper panel)</i> HEK293 cells overexpressing HA-tagged myr Akt were transfected with phpR-cyp-F. 48 h after transfection IRES activity was calculated as ratio of <i>firefly</i> to <i>renilla</i> luciferase activities and is given relative to control vector transfected cells. Data are presented as means ± SEM (n≥3, * p<0.05). <i>(lower panel)</i> HEK293 cells stably overexpressing HA-tagged myr Akt were serum starved for 48 h. Protein expression and S6-phosphorylation was determined by Western analysis. (C) MCF7 cells were treated for 4 h with CM or CM in combination with LY294002 [10 µM] followed by polysomal fractionation. Changes in cyp24a1 mRNA distribution were analyzed as described before. Data of pooled polysomal fractions (7–10) are presented as means ± SEM (n≥3, * p<0.05).</p