A role for cAMP in angiotensin II mediated inhibition of cell growth in AT1A receptor-transfected CHO-K1 cells

Expression of the BAD pathway is a marker of triple-negative status and poor outcome

Triple-negative breast cancer (TNBC) has few therapeutic targets, making nonspecific chemotherapy the main treatment. Therapies enhancing cancer cell sensitivity to cytotoxic agents could significantly improve patient outcomes. A BCL2-associated agonist of cell death (BAD) pathway gene expression signature (BPGES) was derived using principal component analysis (PCA) and evaluated for associations with the TNBC phenotype and clinical outcomes. Immunohistochemistry was used to determine the relative expression levels of phospho-BAD isoforms in tumour samples. Cell survival assays evaluated the effects of BAD pathway inhibition on chemo-sensitivity. BPGES score was associated with TNBC status and overall survival (OS) in breast cancer samples of the Moffitt Total Cancer Care dataset and The Cancer Genome Atlas (TCGA). TNBC tumours were enriched for the expression of phospho-BAD isoforms. Further, the BPGES was associated with TNBC status in breast cancer cell lines of the Cancer Cell Line Encyclopedia (CCLE). Targeted inhibition of kinases known to phosphorylate BAD protein resulted in increased sensitivity to platinum agents in TNBC cell lines compared to non-TNBC cell lines. The BAD pathway is associated with triple-negative status and OS. TNBC tumours were enriched for the expression of phosphorylated BAD protein compared to non-TNBC tumours. These findings suggest that the BAD pathway it is an important determinant of TNBC clinical outcomes.

Overexpression of Na+/Mg2+ exchanger SLC41A1 attenuates pro-survival signaling

The Na⁺/Mg²⁺ exchanger SLC41A1 (A1), a key component of intracellular Mg homeostasis (IMH), is the major cellular Mg²⁺ efflux system, and its overexpression decreases [Mg²⁺]_{intracellular}. IMH plays an important role in the regulation of many cellular processes, including cellular signaling. However, whether the overexpression of A1 and the consequent drop of [Mg²⁺]_i impact on intracellular signaling is unknown. To examine the latter, we utilized dynamic mass redistribution (DMR) assay, PathScan^® RTK signaling antibody (PRSA) array, confirmatory Western blot (WB) analyses of phosphorylation of kinases selected by PRSA, and mag-fura 2-assisted fast filter spectrometry (FFS). We demonstrate here that the overexpression of A1 quantitatively and qualitatively changes the DMR signal evoked by the application of PAR-1-selective activating peptide and/or by changing [Mg²⁺]_{extracellular} in HEK293 cells. PRSA profiling of the phosphorylation of important signaling nodes followed by confirmatory WB has revealed that, in HEK293 cells, A1 overexpression significantly attenuates the phosphorylation of Akt/PKB on Thr³⁰⁸ and/or Ser⁴⁷³ and of Erk1/2 on Thr²⁰²/Tyr²⁰⁴ in the presence of 0 or 1 mM (physiological) Mg²⁺ in the bath solution. The latter is also true for SH-SY5Y and HeLa cells. Overexpression of A1 in HEK293 cells significantly lowers [Mg²⁺]_i in the presence of [Mg²⁺]_e = 0 or 1 mM. This correlates with the observed attenuation of prosurvival Akt/PKB - Erk1/2 signaling in these cells. Thus, A1 expression status and [Mg²⁺]_e (and consequently also [Mg²⁺]_i) modulate the complex physiological fingerprint of the cell and influence the activity of kinases involved in anti-apoptotic and, hence, pro-survival events in cells.

Intermedin suppresses pressure overload cardiac hypertrophy through activation of autophagy

Left ventricular hypertrophy is a maladaptive response to pressure overload and an important risk factor for heart failure. Intermedin (IMD), a multi-functional peptide, plays important roles in cardiovascular protection. In this study, we revealed an autophagy-dependent mechanism involved in IMD’s protection against cardiac remodeling and cardiomyocyte death in heart hypertrophy. We observed that transverse aortic contraction (TAC) induction, Ang II or ISO exposure induced remarkable increase in the expression of endogenous IMD and its receptor components, CRLR, RAMP1 and RAMP3, in mouse hearts and H9c2 cell cultures, respectively. Furthermore, the heart size, heart weight/body weight ratios, cardiomyocyte size and apoptosis, interstitial collagen, hypertrophic markers including ANP and BNP expression were also significantly increased, which were effectively suppressed by IMD supplementation. In addition, IMD induced capillary angiogenesis and improved functions in hypertrophic hearts. We further observed that IMD induced strong autophagy in hypertrophic hearts and cultured cells, which was paralleling with the decrease in cardiomyocyte size and apoptosis. Furthermore, an autophagy inhibitor, 3-MA, was used to block the IMD-augmented autophagy level, and then the protection of IMD on cardiomyocyte hypertrophy and apoptosis was almost abrogated. We also observed that IMD supplementation stirred intracellular cAMP production, and augmented the ERK1/2 phosphorylation induced by Ang II/ISO exposure in H9c2 cells. In addition, we inhibited PI3K, PKA and MAPK/ERK1/2 signaling pathways by using wortamannin, H89 and PD98059, respectively, in H9c2 cells co-incubating with both IMD and Ang II or ISO, and observed that these inhibitors effectively reduced IMD-augmented autophagy level, but only H89 and PD98059 pre-incubation abrogated the anti-apoptotic action of IMD. These results indicate that the endogenous IMD and its receptor complexes are induced in hypertrophic cardiomyocytes and proposed to play an important role in the pathogenesis of cardiac hypertrophy, and the autophagy stirred by IMD supplementation is involved in its protection against cardiomyocyte hypertrophy and apoptosis through the activation of both cAMP/PKA and MAPK/ERK1/2 pathways.

Evidence for involvement of 3'-untranslated region in determining angiotensin II receptor coupling specificity to G-protein

The mRNA 3'-untranslated region (3'-UTR) of many genes has been identified as an important regulator of the mRNA transcript itself as well as the translated product. Previously, we demonstrated that Chinese-hamster ovary-K1 cells stably expressing angiotensin receptor subtypes (AT(1A)) with and without 3'-UTR differed in AT(1A) mRNA content and its coupling with intracellular signalling pathways. Moreover, RNA mobility-shift assay and UV cross-linking studies using the AT(1A) 3'-UTR probe identified a major mRNA-binding protein complex of 55 kDa in Chinese-hamster ovary-K1 cells. In the present study, we have determined the functional significance of the native AT(1A) receptor 3'-UTR in rat liver epithelial (WB) cell lines by co-expressing the AT(1A) 3'-UTR sequence 'decoy' to compete with the native receptor 3'-UTR for its mRNA-binding proteins. PCR analysis using specific primers for the AT(1A) receptor and [(125)I]angiotensin II (AngII)-binding studies demonstrated the expression of the native AT(1A) receptors in WB (B(max)=2.7 pmol/mg of protein, K(d)=0.56 nM). Northern-blot analysis showed a significant increase in native receptor mRNA expression in 3'-UTR decoy-expressing cells, confirming the role of 3'-UTR in mRNA destabilization. Compared with vehicle control, AngII induced DNA and protein synthesis in wild-type WB as measured by [(3)H]thymidine and [(3)H]leucine incorporation respectively. Activation of [(3)H]thymidine and [(3)H]leucine correlated with a significant increase in cell number (cellular hyperplasia). In these cells, AngII stimulated GTPase activity by AT(1) receptor coupling with G-protein alpha i. We also delineated that functional coupling of AT(1A) receptor with G-protein alpha i is an essential mechanism for AngII-mediated cellular hyperplasia in WB by specifically blocking G-protein alpha i activation. In contrast with wild-type cells, stable expression of the 3'-UTR 'decoy' produced AngII-stimulated protein synthesis and cellular hypertrophy as demonstrated by a significant increase in [(3)H]leucine incorporation and no increase in [(3)H]thymidine incorporation and cell number. Furthermore, [(125)I]AngII cross-linking and immunoprecipitation studies using specific G-protein alpha antibodies showed that in wild-type cells, the AT(1A) receptor coupled with G-protein alpha i, whereas in cells expressing the 3'-UTR 'decoy', the AT(1A) receptor coupled with G-protein alpha q. These findings indicate that the 3'-UTR-mediated changes in receptor function may be mediated in part by a switch from G-protein alpha i to G-protein alpha q coupling of the receptor. Our results suggest that the 3'-UTR-mediated post-transcriptional modification of the AT(1A) receptor is critical for regulating tissue-specific receptor functions.

Functional role for the angiotensin II receptor (AT1A) 3'-untranslated region in determining cellular responses to agonist: evidence for recognition by RNA binding proteins

We demonstrate a functional role for the 3'-untranslated region (3'-UTR) of the angiotensin II (Ang II) receptor subtype AT1A mRNA in Chinese hamster ovary (CHO-K1) cells by stably transfecting the coding region of the receptor gene with or without the 845 bp 3'-UTR. Two cell lines expressing similar levels of cell-surface receptors (with 3'-UTR, Bmax=571 fmol/mg protein; without 3'-UTR, Bmax=663 fmol/mg protein) were used in the present study. Both cell lines expressed high-affinity receptors (with 3'-UTR, Kd=0.83 nM; without 3'-UTR, Kd=0.82 nM), and binding studies with 125I-labelled Ang II in the presence of GTP[S] demonstrated that both coupled to heterotrimeric G-proteins. Despite these similarities, significant differences were observed for receptor-mediated cell signalling pathways. In cells without the 3'-UTR, Ang II stimulated an increase in cAMP accumulation (11-fold above control) and in cells with the 3'-UTR no stimulation was observed, which was consistent with previous observations in most endogenous Ang II receptor (AT1)-expressing cells. Activation of cAMP by Ang II in cells without the 3'-UTR correlated with an inhibition of DNA synthesis, determined by [3H]thymidine incorporation. Ang II-mediated responses were blocked by EXP3174, a selective non-peptide receptor antagonist. We also observed differences in the transient profiles of intracellular calcium between cells with and without the 3'-UTR in response to Ang II. In cells with the 3'-UTR, a sustained level of intracellular calcium was observed after Ang II stimulation, whereas cells without the 3'-UTR displayed a full return to basal level within 50 s of Ang II treatment. Even though the expressed exogenous gene is under the control of a constitutively expressing promoter (cytomegalovirus promoter), Northern-blot analysis revealed a considerably greater accumulation of AT1A mRNA in cells without the 3'-UTR compared with cells with the 3'-UTR. Analysis of the decay rate of the AT1A mRNA in cells with and without the 3'-UTR revealed that the normally unstable AT1A receptor mRNA became highly stable by removing its 3'-UTR, identifying a role for the 3'-UTR in mRNA destabilization. Interestingly, both cells express similar levels of receptors at the cell surface, suggesting that the 3'-UTR is also involved in the efficient translation and/or translocation of the receptor protein to the plasma membrane. We hypothesized that these 3'-UTR-mediated functions of the receptor are regulated by RNA-binding proteins. To identify possible RNA-binding proteins for the AT1A 3'-UTR, cellular extracts were prepared from parental CHO-K1 cells and 3'-UTR-binding assays, electrophoretic mobility-shift assays and UV crosslinking studies were performed. A major cellular protein of 55 kDa was identified, which specifically interacted with the 3'-UTR. Our data suggest that the 3'-UTR of the AT1A can control specific receptor functions, perhaps via selective recognition of the 3'-UTR by RNA-binding proteins.