Transcriptional regulation of the murine surfactant protein-A gene by B-Myb

Differential Regulation of Human Surfactant Protein A Genes, SFTPA1 and SFTPA2, and Their Corresponding Variants

The human SFTPA1 and SFTPA2 genes encode the surfactant protein A1 (SP-A1) and SP-A2, respectively, and they have been identified with significant genetic and epigenetic variability including sequence, deletion/insertions, and splice variants. The surfactant proteins, SP-A1 and SP-A2, and their corresponding variants play important roles in several processes of innate immunity as well in surfactant-related functions as reviewed elsewhere [1]. The levels of SP-A have been shown to differ among individuals both under baseline conditions and in response to various agents or disease states. Moreover, a number of agents have been shown to differentially regulate SFTPA1 and SFTPA2 transcripts. The focus in this review is on the differential regulation of SFTPA1 and SFTPA2 with primary focus on the role of 5′ and 3′ untranslated regions (UTRs) and flanking sequences on this differential regulation as well molecules that may mediate the differential regulation.

MYBL2 disrupts the Hippo-YAP pathway and confers castration resistance and metastatic potential in prostate cancer

Rationale: Resistance to androgen-deprivation therapy (ADT) associated with metastatic progression remains a challenging clinical task in prostate cancer (PCa) treatment. Current targeted therapies for castration-resistant prostate cancer (CRPC) are not durable. The exact molecular mechanisms mediating resistance to castration therapy that lead to CRPC progression remain obscure. Methods: The expression of MYB proto-oncogene like 2 (MYBL2) was evaluated in PCa samples. The effect of MYBL2 on the response to ADT was determined by in vitro and in vivo experiments. The survival of patients with PCa was analyzed using clinical specimens (n = 132) and data from The Cancer Genome Atlas (n = 450). The mechanistic model of MYBL2 in regulating gene expression was further detected by subcellular fractionation, western blotting, quantitative real-time PCR, chromatin immunoprecipitation, and luciferase reporter assays. Results: MYBL2 expression was significantly upregulated in CRPC tissues and cell lines. Overexpression of MYBL2 could facilitate castration-resistant growth and metastatic capacity in androgen-dependent PCa cells by promoting YAP1 transcriptional activity via modulating the activity of the Rho GTPases RhoA and LATS1 kinase. Importantly, targeting MYBL2, or treatment with either the YAP/TAZ inhibitor Verteporfin or the RhoA inhibitor Simvastatin, reversed the resistance to ADT and blocked bone metastasis in CRPC cells. Finally, high MYBL2 levels were positively associated with TNM stage, total PSA level, and Gleason score and predicted a higher risk of metastatic relapse and poor prognosis in patients with PCa. Conclusions: Our results reveal a novel molecular mechanism conferring resistance to ADT and provide a strong rationale for potential therapeutic strategies against CRPC.

Transcriptional control of lung morphogenesis

The vertebrate lung consists of multiple cell types that are derived primarily from endodermal and mesodermal compartments of the early embryo. The process of pulmonary organogenesis requires the generation of precise signaling centers that are linked to transcriptional programs that, in turn, regulate cell numbers, differentiation, and behavior, as branching morphogenesis and alveolarization proceed. This review summarizes knowledge regarding the expression and proposed roles of transcription factors influencing lung formation and function with particular focus on knowledge derived from the study of the mouse. A group of transcription factors active in the endodermally derived cells of the developing lung tubules, including thyroid transcription factor-1 (TTF-1), beta-catenin, Forkhead orthologs (FOX), GATA, SOX, and ETS family members are required for normal lung morphogenesis and function. In contrast, a group of distinct proteins, including FOXF1, POD1, GLI, and HOX family members, play important roles in the developing lung mesenchyme, from which pulmonary vessels and bronchial smooth muscle develop. Lung formation is dependent on reciprocal signaling among cells of both endodermal and mesenchymal compartments that instruct transcriptional processes mediating lung formation and adaptation to breathing after birth.

Surfactant protein A promoter directs the expression of Cre recombinase in brain microvascular endothelial cells of transgenic mice

Brain microvascular endothelial cells (ECs) have unique characteristics distinguished from peripheral ECs and play important roles in blood-brain barrier (BBB). To investigate the physiological control of the brain ECs, we generated a transgenic mouse line in which the expression of Cre recombinase was driven by the promoter of the mouse surfactant protein A (SP-A) gene. The Cre activity was detected in blood vessels of brain, alveolar type II cells of lung and epithelium of gland stomach. In brain ECs, the Cre activity started at embryonic day 11.5, indicating that the subpopulation of ECs in brain could be molecularly defined at early embryonic stages. The use of SP-A-Cre mice should facilitate analysis of gene function in the brain ECs.

B-Myb acts as a repressor of human COL1A1 collagen gene expression by interacting with Sp1 and CBF factors in scleroderma fibroblasts

We investigated the role of B-Myb, a cell-cycle-regulated transcription factor, in the expression of the alpha1 (I) pro-collagen gene (COL1A1) in scleroderma fibroblasts. Scleroderma or SSc (systemic sclerosis) is a fibrotic disease characterized by excessive production of extracellular matrix components, especially type I collagen. Northern-blot analysis showed an inverse relationship between COL1A1 mRNA expression and that of B-Myb during exponential cell growth and during quiescence in human SSc fibroblasts. Overexpression of B-Myb in SSc fibroblasts was correlated with decreased COL1A1 mRNA expression. Transient transfections localized the down-regulatory effect of B-Myb to a region containing the proximal 174 bp of the COL1A1 promoter that does not contain B-Myb consensus binding sites. Gel-shift analysis, using nuclear extracts from normal and SSc fibroblasts transfected with B-Myb, showed no differences in DNA-protein complex formation when compared with the nuclear extracts from mock-transfected cells. However, we found that B-Myb decreases Sp1 (specificity protein 1) and CBF (CCAAT-binding factor) binding for their specific sites localized in the 174 bp COL1A1 proximal promoter. These results were also confirmed using B-Myb-immunodepleted nuclear extracts. Furthermore, immunoprecipitation assays using SSc nuclear extracts demonstrated a physical interaction of B-Myb with Sp1 and CBF transcription factors, and also an interaction between Sp1 and CBF. In addition, by employing full-length or deleted B-Myb cDNA construct, we found that B-Myb down-regulates the COL1A1 proximal promoter through its C-terminal domain. Thus these results suggest that B-Myb may be an important factor in the pathway(s) regulating collagen production in SSc fibroblasts.

Stimulation of the murine Uchl1 gene promoter by the B-Myb transcription factor

It has been reported that human lung cancers frequently overexpress both the ubiquitous cell cycle transcription factor B-myb and the ubiquitin carboxyterminal hydrolase UCHL1, an enzyme whose expression is normally limited to neurons and neuroendocrine cells in the lung. A possible explanation for the co-expression of these markers is that Uchl1 is subject to transcriptional regulation by B-Myb, and in tumors the ectopic expression of UCHL1 is a direct consequence of B-Myb overexpression. We have tested this hypothesis in the mouse model system by cloning the murine Uchl1 promoter and analyzing its regulation by murine B-Myb. Expression of a luciferase reporter gene driven by the Uchl1 promoter was induced by cotransfected B-Myb, but induction was not dependent on the presence of a myb consensus binding site identified in the promoter region. B-Myb induction was dependent on the context of the Uchl1 TATA box, as has been reported for other genes. Transgenic mice expressing a truncated, constitutively active form of B-Myb in the lung epithelium showed elevated expression of UCHL1 protein. We conclude that B-Myb can stimulate expression of the Uchl1 both in cultured cells and in vivo.

TTF-1 phosphorylation is required for peripheral lung morphogenesis, perinatal survival, and tissue-specific gene expression

Thyroid transcription factor-1 (TTF-1) is a 43-kDa, phosphorylated member of the Nkx2 family of homeodomain-containing proteins expressed selectively in lung, thyroid, and the central nervous system. To assess the role of TTF-1 and its phosphorylation during lung morphogenesis, mice bearing a mutant allele, in which seven serine phosphorylation sites were mutated, Titf1PM/PM, were generated by homologous recombination. Although heterozygous Titf1PM/+ mice were unaffected, homozygous Titf1PM/PM mice died immediately following birth. In contrast to Titf1 null mutant mice, which lack peripheral lung tissues, bronchiolar and peripheral acinar components of the lung were present in the Titf1PM/PM mice. Although lobulation and early branching morphogenesis were maintained in the mutant mice, abnormalities in acinar tubules and pulmonary hypoplasia indicated defects in lung morphogenesis later in development. Although TTF-1PM protein was readily detected within the nuclei of pulmonary epithelial cells at sites and abundance consistent with that of endogenous TTF-1, expression of a number of known TTF-1 target genes, including surfactant proteins and secretoglobulin 1A, was variably decreased in the mutant mice. Vascular endothelial growth factor mRNA was decreased in association with decreased formation of peripheral pulmonary blood vessels. Genes mediating surfactant homeostasis, vasculogenesis, host defense, fluid homeostasis, and inflammation were highly represented among those regulated by TTF-1. Thus, in contrast to the null Titf1 mutation, the Titf1PM/PM mutant substantially restored lung morphogenesis. Direct and indirect transcriptional targets of TTF-1 were identified that are likely to play important roles in lung formation and function.

B-Myb repressor function is regulated by cyclin A phosphorylation and sequences within the C-terminal domain

B-Myb is a widely expressed member of the myb oncogene family that has been shown to act as either an activator or repressor of gene transcription in a cell-type-specific fashion. For example, in aortic smooth muscle cells B-Myb represses transcription of the alpha2(V) collagen gene. Recently, phosphorylation of B-Myb by cyclin A was shown to enhance greatly its ability to transactivate. Here, we have tested the effects of cyclin A on the ability of B-Myb to repress. We report that coexpression of cyclin A abolished repression of the alpha2(V) collagen promoter, whereas a dominant-negative cyclin-dependent kinase 2 (cdk2) enhanced repression by ectopic and endogenous B-Myb protein. Mutation of 10 of 22 putative cyclin A sites, which greatly reduces the effects of cyclin A on transactivation by B-Myb, had no effect on the ability of cyclin A to alleviate B-Myb-mediated repression of alpha2(V) collagen promoter activity. Furthermore, the stability of the mutant B-Myb protein was largely unaffected by cyclin A, although ectopic expression of cyclin A enhanced the rate of decay of wild-type B-Myb protein. Thus, the mechanisms of repression and activation appear distinct, for example, mediated by different critical phosphorylation sites or protein-protein interactions. B-Myb mutants with either deletion of aa 374-581 (B-Myb-Mut3) or C-terminal truncation beyond aa 491 (B-Myb-491) positively regulated alpha2(V) collagen promoter activity, and were not affected by cyclin A. Thus, our findings indicate that the ability of B-Myb to function as a repressor of matrix promoter activity is abolished by cyclin A, and maps the sites mediating negative regulation by B-Myb to the region between aa 491 and 582.

Effects of B-Myb on gene transcription: phosphorylation-dependent activity ans acetylation by p300

The transcription factor B-Myb is a cell-cycle regulated phosphoprotein involved in cell cycle progression through the transcriptional regulation of many genes. In this study, we show that the promoter of the fibroblast growth factor-4 (FGF-4) gene is strongly activated by B-Myb in HeLa cells and it can serve as a novel diagnostic tool for assessing B-Myb activity. Specifically, B-Myb deletion mutants were examined and domains of B-Myb required for activation of the FGF-4 promoter were identified. Using phosphorylation-deficient mutant forms of B-Myb, we also show that phosphorylation is essential for B-Myb activity. Moreover, a mutant form of B-Myb, which lacks all identified phosphorylation sites and which has little activity, can function as a dominant-negative and suppress wild-type B-Myb activity. Acetylation is another post-translational modification known to affect the activity of other Myb family members. We show that B-Myb is acetylated by the co-activator p300. We also show that the bromo and histone acetyltransferase domains of p300 are sufficient to interact with and acetylate B-Myb. These data indicate that phosphorylation of B-Myb is an essential modification for activity and that acetylation of B-Myb may play a role in B-Myb activity.

GATA-6 activates transcription of surfactant protein A

Surfactant protein A (SP-A) is a member of the collectin family of innate host defense molecules expressed primarily in respiratory epithelial cells of the lung. SP-A concentrations are influenced by both cell-specific and ubiquitous nuclear proteins that regulate SP-A gene transcription in a cell-selective and temporally regulated manner. In this work, a consensus GATA-binding site (GBS) was identified at positions -69 to -64 of the mouse SP-A gene. The transcriptional activity of wild-type SP-A reporter constructs in HeLa cells was increased 5-10-fold when cotransfected with a GATA-6 expression plasmid. Deletion of the GBS completely blocked transactivation by GATA-6. Transfection of a construct expressing GATA-6-engrailed fusion protein inhibited basal expression of the SP-A/chloramphenicol acetyltransferase construct in MLE-15 cells. Nuclear extract proteins from MLE-15 cells bound to the GBS in the mouse SP-A gene, and a supershifted band was detected with a GATA-6-specific antibody. Transactivation of the wild-type SP-A constructs by GATA-6 increased transcriptional activity 7-10-fold, whereas thyroid transcription factor-1 (TTF-1) increased the activity of these constructs 12-18-fold. The effects of cotransactivating with both GATA-6 and TTF-1 expression constructs were additive. However, mutation of the TTF-1-binding sites alone or in combination decreased GATA-6 transactivation. Likewise, mutation of the GBS blocked TTF-1 activation of the SP-A promoter. In situ hybridization demonstrated GATA-6 mRNA in the peripheral epithelial cells of fetal mouse lung, consistent with the sites of SP-A expression. GATA-6 is expressed in respiratory epithelial cells and binds to a cis-acting element in the SP-A gene promoter, activating the transcriptional activity of the gene.