cAMP stimulation of rodent steroidogenic cells produces two StAR transcripts, a major 3.5 kb and a minor 1.6 kb mRNA, differing only in their 3′ untranslated regions (3′ UTR). They exhibit very different responses to stimulation and removal of 8-Br-cAMP, with the 3.5 kb form increasing and declining much more rapidly than the 1.6 kb form. The 3′ end of the 3.5 kb StAR mRNA contains three conserved AU-rich element (AURE) motifs that mediate fast mRNA turnover in over 900 genes in the human genome. In this paper, we explore post-transcriptional regulation in steroidogenic and non-steroidogenic cells using expression vectors containing StAR or luciferase with different StAR 3′ UTRs. We show that the basal steady-state levels of StAR or luciferase protein and mRNA are five to eight times lower with the 3′ UTR of 3.5 kb StAR compared with that of the 1.6 kb 3′ UTR. Examination of transcript stability by direct mRNA transfection showed only a 1.5-fold increase in the rate of cytoplasmic decay of the 3.5 kb mRNA relative to the 1.6 kb mRNA. However, the long 3′ UTR caused a fivefold decrease in the rate of appearance of mature cytoplasmic mRNA despite transcription from the same promoter. This is attributed to less efficient nuclear processing of immature transcripts prior to export to cytoplasm. Selective 3′ UTR sequence substitutions, deletions, and mutations showed that this loss of expression is produced additively by specific sequences in a 700-base basal instability region and by non-specific length effects. These mechanisms are selectively enhanced in steroidogenic cells. The AURE contribute a smaller basal destabilization effect selective for steroidogenic cells that is removed by their mutations. Inclusion of introns in the 3.5 kb StAR vector enhances StAR expression, suggesting the effects of introns complexes on nuclear processing. Br-cAMP provides an additional means to rapidly modulate StAR expression independent of transcription by attenuating the nuclear and cytoplasmic instability mechanisms within the extended 3′ UTR.
Colin R Jefcoate and Jinwoo Lee
Cholesterol is an important regulator of cell signaling, both through direct impacts on cell membranes and through oxy-metabolites that activate specific receptors (steroids, hydroxy-cholesterols, bile acids). Cholesterol moves slowly through and between cell membranes with the assistance of specific binding proteins and transfer processes. The prototype cholesterol regulator is the Steroidogenesis Acute Regulatory (STAR), which moves cholesterol into mitochondria, where steroid synthesis is initiated by cytochrome P450 11A1 in multiple endocrine cell types. CYP27A1 generates hydroxyl cholesterol metabolites that activate LXR nuclear receptors to control cholesterol homeostatic and transport mechanisms. LXR regulation of cholesterol transport and storage as cholesterol ester droplets is shared by both steroid-producing cells and macrophage. This cholesterol signaling which is crucial to brain neuron regulation by astrocytes and microglial macrophage, is mediated by ApoE and is sensitive to disruption by β-amyloid plaques. sm-FISH delivers appreciable insights into signaling in single cells, by resolving single RNA molecules as mRNA and by quantifying pre-mRNA at gene loci. sm-FISH has been applied to problems in physiology, embryo development and cancer biology, where single cell features have critical impacts. sm-FISH identifies novel features of STAR transcription in adrenal and testis cells, including asymmetric expression at individual gene loci, delayed splicing and 1:1 association of mRNA with mitochondria. This may represent a functional unit for the translation-dependent cholesterol transfer directed by STAR, which integrates into mitochondrial fusion dynamics. Similar cholesterol dynamics repeat with different players in the cycling of cholesterol between astrocytes and neurons in the brain, which may be abnormal in neurodegenerative diseases.
Jun Yang, Peter J Fuller, James Morgan, Hirotaka Shibata, Colin D Clyne and Morag J Young
The mineralocorticoid receptor (MR) is a member of the nuclear receptor superfamily. Pathological activation of the MR causes cardiac fibrosis and heart failure, but clinical use of MR antagonists is limited by the renal side effect of hyperkalemia. Coregulator proteins are known to be critical for nuclear receptor-mediated gene expression. Identification of coregulators, which mediate MR activity in a tissue-specific manner, may allow for the development of novel tissue-selective MR modulators that confer cardiac protection without adverse renal effects. Our earlier studies identified a consensus motif among MR-interacting peptides, MPxLxxLL. Gem (nuclear organelle)-associated protein 4 (GEMIN4) is one of the proteins that contain this motif. Transient transfection experiments in HEK293 and H9c2 cells demonstrated that GEMIN4 repressed agonist-induced MR transactivation in a cell-specific manner. Furthermore, overexpression of GEMIN4 significantly decreased, while knockdown of GEMIN4 increased, the mRNA expression of specific endogenous MR target genes. A physical interaction between GEMIN4 and MR is suggested by their nuclear co-localization upon agonist treatment. These findings indicate that GEMIN4 functions as a novel coregulator of the MR.
Dongxing Zhu, Neil C W Mackenzie, Jose Luis Millan, Colin Farquharson and Vicky E MacRae
The process of vascular calcification shares many similarities with that of skeletal mineralisation and involves the deposition of hydroxyapatite crystals in arteries and cardiac valves. However, the cellular mechanisms responsible have yet to be fully elucidated. In this study, we employed microarray analysis to demonstrate the upregulation of more than >9000 genes during the calcification of murine vascular smooth muscle cells (VSMCs), of which the most significantly, differentially expressed gene was Igf2. Following the validation of increased IGF2 expression by RT-qPCR and immunoblotting in calcifying murine VSMCs, IGF2 expression was further demonstrated in the calcified aorta of the Enpp1−/− mouse model of medial aortic calcification. Having confirmed that IGF1R and IGF2R were expressed in cultured murine VSMCs, cell-signalling studies in these cells revealed that IGF2 (50 ng/ml) significantly stimulated the phosphorylation of Akt and Erk1/2 (P<0.05). These results potentially indicate that IGF2 may mediate VSMC calcification via the stimulation of Erk1/2 and Akt signalling. This study suggests that the increased IGF2 expression in calcifying VSMCs may reflect the well-established prenatal role of IGF2, particularly as the osteogenic phenotypic transition of VSMCs in a calcified environment recapitulates many of the events occurring during embryonic development. A full understanding of the importance of IGF2 in this pathological process will lead to a better understanding of the aetiology of vascular calcification.
Soojin Kim, Daksh Thaper, Samir Bidnur, Paul Toren, Shusuke Akamatsu, Jennifer L Bishop, Colin Colins, Sepideh Vahid and Amina Zoubeidi
Neuroendocrine (NE) differentiation of advanced prostate adenocarcinoma following androgen receptor (AR) axis-directed therapy is becoming increasingly recognized. Several models of this transdifferentiation provide insight into its molecular pathogenesis and have highlighted the placental gene PEG10 for further study. Using our unique model of enzalutamide resistance (ENZR) and NE differentiation, we studied PEG10/AR interplay in enzalutamide treatment-resistant cell lines 42DENZR and 42FENZR compared to LNCaP and castration-resistant 16DCRPC cells. ENZR cell lines with positive terminal NE marker status also displayed higher baseline expression of PEG10 compared to LNCaP and 16DCRPC. Antagonism of AR activity increased PEG10 expression followed by an increase in terminal NE markers. Conversely, stimulating AR activity via androgen supplementation reversed PEG10 and NE marker expression in a time and dose-dependent manner. These results were supported by human data showing that PEG10 expression is highest in NEPC and that AR-dependent gene, PSA, is negatively correlated with PEG10 in adenocarcinoma. Further, ChIP assay confirmed binding of activated AR to the PEG10 enhancer, decreasing PEG10 expression. While PEG10 did not drive NEPC, its knockdown reduced NE markers in our cell lines. Moreover, PEG10 knockdown in vitro- and in vivo-attenuated tumor growth. Overall, these observations indicate that PEG10 is an AR-repressed gene which modulates NE markers in ENZR cells and targeting PEG10 in advanced prostate cancer with NE features is a rational and viable option.