Polyadenylation-mediated translational regulation of maternal P450(11beta) mRNA in frog oocytes

RNA polyadenylation sites on the genomes of microorganisms, animals, and plants

Pre–messenger RNA (mRNA) 3′-end cleavage and subsequent polyadenylation strongly regulate gene expression. In comparison with the upstream or downstream motifs, relatively little is known about the feature differences of polyadenylation [poly(A)] sites among major kingdoms. We suspect that the precise poly(A) sites are very selective, and we therefore mapped mRNA poly(A) sites on complete and nearly complete genomes using mRNA sequences available in the National Center for Biotechnology Information (NCBI) Nucleotide database. In this paper, we describe the mRNA nucleotide [i.e., the poly(A) tail attachment position] that is directly in attachment with the poly(A) tail and the pre-mRNA nucleotide [i.e., the poly(A) tail starting position] that corresponds to the first adenosine of the poly(A) tail in the 29 most-mapped species (2 fungi, 2 protists, 18 animals, and 7 plants). The most representative pre-mRNA dinucleotides covering these two positions were UA, CA, and GA in 17, 10, and 2 of the species, respectively. The pre-mRNA nucleotide at the poly(A) tail starting position was typically an adenosine [i.e., A-type poly(A) sites], sometimes a uridine, and occasionally a cytidine or guanosine. The order was U>C>G at the attachment position but A>>U>C≥G at the starting position. However, in comparison with the mRNA nucleotide composition (base composition), the poly(A) tail attachment position selected C over U in plants and both C and G over U in animals, in both A-type and non-A-type poly(A) sites. Animals, dicot plants, and monocot plants had clear differences in C/G ratios at the poly(A) tail attachment position of the non-A-type poly(A) sites. This study of poly(A) site evolution indicated that the two positions within poly(A) sites had distinct nucleotide compositions and were different among kingdoms.

Salt-inducible kinase is involved in the ACTH/cAMP-dependent protein kinase signaling in Y1 mouse adrenocortical tumor cells

The involvement of salt-inducible kinase, a recently cloned protein serine/threonine kinase, in adrenal steroidogenesis was investigated. When Y1 mouse adrenocortical tumor cells were stimulated by ACTH, the cellular content of salt-inducible kinase mRNA, protein, and enzyme activity changed rapidly. Its level reached the highest point in 1-2 h and returned to the initial level after 8 h. The mRNA levels of cholesterol side-chain cleavage cytochrome P450 and steroidogenic acute regulatory protein, on the other hand, began to rise after a few hours, reaching the highest levels after 8 h. The salt-inducible kinase mRNA level in ACTH-, forskolin-, or 8-bromo-cAMP-treated Kin-7 cells, mutant Y1 with less cAMP-dependent PKA activity, remained low. However, Kin-7 cells, when transfected with a PKA expression vector, expressed salt-inducible kinase mRNA. Y1 cells that overexpressed salt-inducible kinase were isolated, and the mRNA levels of steroidogenic genes in these cells were compared with those in the parent Y1. The level of cholesterol side-chain cleavage cytochrome P450 mRNA in the salt-inducible kinase-overexpressing cells was markedly low compared with that in the parent, while the levels of Ad4BP/steroidogenic factor-1-, ACTH receptor-, and steroidogenic acute regulatory protein-mRNAs in the former were similar to those in the latter. The ACTH-dependent expression of cholesterol side-chain cleavage cytochrome P450- and steroidogenic acute regulatory protein-mRNAs in the salt-inducible kinase-overexpressing cells was significantly repressed. The promoter activity of the cholesterol side-chain cleavage cytochrome P450 gene was assayed by using Y1 cells transfected with a human cholesterol side-chain cleavage cytochrome P450 promoter-linked reporter gene. Addition of forskolin to the culture medium enhanced the cholesterol side-chain cleavage cytochrome P450 promoter activity, but the forskolin-dependently activated promoter activity was inhibited when the cells were transfected with a salt-inducible kinase expression vector. This inhibition did not occur when the cells were transfected with a salt-inducible kinase (K56M) vector that encoded an inactive kinase. The salt-inducible kinase's inhibitory effect was also observed when nonsteroidogenic, nonAd4BP/steroidogenic factor-1 -expressing, NIH3T3 cells were used for the promoter assays. These results suggested that salt-inducible kinase might play an important role(s) in the cAMP-dependent, but Ad4BP/steroidogenic factor-1-independent, gene expression of cholesterol side-chain cleavage cytochrome P450 in adrenocortical cells.

Cloned blood-brain barrier adenosine transporter is identical to the rat concentrative Na+ nucleoside cotransporter CNT2

Adenosine transport into brain is regulated by the activity of the adenosine transporter located at the brain capillary endothelial wall, which forms the blood-brain barrier (BBB) in vivo. To facilitate cloning of BBB adenosine transporters, poly A+ RNA was purified from isolated rat brain capillaries for production of a rat BBB cDNA library in the pSPORT vector. The cloned RNA (cRNA) generated from in vitro transcription of this library was injected into frog oocytes followed by measurement of [3H]-adenosine uptake. After dilutional cloning, a full-length, 2905-nucleotide adenosine transporter cDNA, designated clone A-11, was isolated. The A-11 clone yielded [3H]-adenosine flux ratios of 400 to 500 after injection of cRNA in oocytes. The adenosine uptake was sodium-dependent and insensitive to inhibition by S-(4-nitrobenzyl)-6-thioinosine (NBTI). The Km and Vmax of adenosine transport in the cRNA-injected oocytes were 23.1 +/- 3.7 micromol/L and 10.8 +/- 0.9 pmol/oocyte. min. The K0.5 for sodium was 2.4 +/- 0.1 mmol/L, with a Hill coefficient (n) of 1.06 +/- 0.07. DNA sequence analysis indicated the rat BBB A-11 adenosine cDNA was identical to rat concentrative nucleoside transporter type 2 (CNT2). The adenosine transporter activity of the rat BBB A-11 CNT2 clone is 50-fold more active than previously reported rat CNT2 clones. In summary, these studies describe the expression cloning of CNT2 from a rat BBB library and show that the pattern of sodium dependency and NBTI insensitivity of the cloned CNT2 are identical to patterns of adenosine transport across the BBB in vivo. These results suggest that BBB adenosine transport in vivo is mediated by CNT2, which would make CNT2 one of the few known sodium-dependent cotransporters that mediate substrate transport across the BBB in the blood to brain direction.

Characterization of a proximal element in the rat preadipocyte factor-1 (Pref-1) gene promoter

Preadipocyte factor-1 (Pref-1) was shown to negatively regulate adipocyte differentiation. We recently reported that ZOG, a rat homolog of Pref-1, was specifically expressed in the adrenal zona glomerulosa. Results of the investigation of Pref-1 expression in preadipocyte and in undifferentiated adrenal cortex suggested that down-regulation of Pref-1 gene was closely correlated with the differentiation process. In this study we demonstrate that an upstream region (from -76 to -47) of the rat Pref-1 gene was essential for its expression in adrenocortical carcinoma-derived H295R cells. A nucleotide sequence found in this region, GCGTGGGCGTGGGCGGGGG (Egr/GC-box), seemed to contain three elements, two early growth response (Egr) elements and one GC-box, overlapping each other. Mutations of four or five nucleotides in a 7-nucleotides-stretch in the midst of the Egr/GC-box eliminated the binding of Sp1/3, abolished the activation by Egr-factor(s) and diminished the Pref-1 promoter activity. When mutations were introduced into the outside of the middle portion, the binding of Sp1/3 to the Egr/GC-box was abolished similarly. However, the decrease in the promoter activity was less than that found with the construct mutated at the middle. These results indicated that an element present at the 7-nucleotides-stretch in the midst of the Egr/GC-box might be important for the Pref-1 promoter activity, and this proximal element was possibly activated by a still-unidentified nuclear factor(s). This element would function as the promoter of the Pref-1 gene in H295R cells, but not in HeLa cells.