Characterization of a minimal promoter element required for transcription of the mouse type II beta regulatory subunit (RII beta) of cAMP-dependent protein kinase

Protein kinase A regulatory subunits in human adipose tissue: decreased R2B expression and activity in adipocytes from obese subjects

OBJECTIVE

In human adipocytes, the cAMP-dependent pathway mediates signals originating from beta-adrenergic activation, thus playing a key role in the regulation of important metabolic processes, i.e., lipolysis and thermogenesis. Cyclic AMP effects are mainly mediated by protein kinase A (PKA), whose R2B regulatory isoform is the most expressed in mouse adipose tissue, where it protects against diet-induced obesity and fatty liver development. The aim of the study was to investigate possible differences in R2B expression, PKA activity, and lipolysis in adipose tissues from obese and nonobese subjects.

RESEARCH DESIGN AND METHODS

The expression of the different PKA regulatory subunits was evaluated by immunohistochemistry, Western blot, and real-time PCR in subcutaneous and visceral adipose tissue samples from 20 nonobese and 67 obese patients. PKA activity and glycerol release were evaluated in total protein extract and adipocytes isolated from fresh tissue samples, respectively.

RESULTS

Expression techniques showed that R2B was the most abundant regulatory protein, both at mRNA and protein level. Interestingly, R2B mRNA levels were significantly lower in both subcutaneous and visceral adipose tissues from obese than nonobese patients and negatively correlated with BMI, waist circumference, insulin levels, and homeostasis model assessment of insulin resistance. Moreover, both basal and stimulated PKA activity and glycerol release were significantly lower in visceral adipose tissue from obese patients then nonobese subjects.

CONCLUSIONS

Our results first indicate that, in human adipose tissue, there are important BMI-related differences in R2B expression and PKA activation, which might be included among the multiple determinants involved in the different lipolytic response to beta-adrenergic activation in obesity.

Cyclic AMP signaling and gene regulation

Cyclic adenosine monophosphate (cAMP) is a ubiquitous second messenger produced in cells in response to hormones and nutrients. The production of cAMP is dependent upon the actions of many different proteins that affect its synthesis and degradation. An important function of cAMP is to activate the phosphorylating enzyme, protein kinase A. The key roles of cAMP and protein kinase A in the phosphorylation and regulation of enzyme substrates involved in intermediary metabolism are well known. A newly discovered role for protein kinase A is in the phosphorylation and activation of transcription factors that are critical for the control of the transcription of genes in response to elevated levels of cAMP.

Characterization of the 5'-flanking region of the gene for the cAMP-inducible protein kinase A subunit, RIIbeta, in Sertoli cells

Activation of cyclic AMP-dependent protein kinases (protein kinase A, PKA) by gonadotropins and cyclic AMP (cAMP) plays an important role in the regulation of testicular functions. A regulatory subunit, RIIbeta, of PKA is transcriptionally induced in rat Sertoli cells in response to treatment with cAMP. The present study addresses regulatory mechanisms leading to increased transcription of the rat RIIbeta gene. We have localized a footprint which overlaps one of the major transcription initiation sites in the basal promoter (-293 to -123). One of the proteins binding this sequence belongs to the NF-1 family of transcription factors. We also observed binding to a basic helix-loop-helix (bHLH) response element. Furthermore, transfection studies of various 5'-deletions of the rat RIIbeta gene in primary cultures of rat Sertoli cells and in peritubular cells revealed the presence of an upstream region (-723 to -395, cAMP-responsive region) inhibiting basal expression from the rat RIIbeta gene only in Sertoli cells. This region was found to enhance cAMP responsiveness in Sertoli cells but not in peritubular cells. Interactions with downstream elements seemed to be important for the function of the cAMP-responsive region. Although some short stretches reveal homology to the cAMP-responsive regions of other slowly cAMP-responding genes, and an AP-1-like element is present, no strong resemblance to any known regulatory element responsive to cAMP is found.

Promoter sequences in the RI beta subunit gene of cAMP-dependent protein kinase required for transgene expression in mouse brain

Neural-specific expression of the mouse regulatory type-I beta (RI beta) subunit gene of cAMP-dependent protein kinase is controlled by a fragment of genomic DNA comprised of a TATA-less promoter flanked by 1.5 kilobases of 5'-upstream sequence and a 1.8-kilobase intron. This DNA contains a complex arrangement of transcription factor binding motifs, and previous experiments have shown that many of these are recognized by proteins found in brain nuclear extract. To identify sequences critical for RI beta expression in functional neurons, we performed a deletion analysis in transgenic mice. Evidence is presented that the GC-rich proximal promoter is responsible for cell type-specific expression in vivo because RI beta DNA containing as little as 17 base pairs (bp) of 5'-upstream sequence was functional in mouse brain. One likely regulatory element coincides with the start of transcription and includes an EGR-1 motif and 3 consecutive SP1 sites within a 21-bp interval. Maximal RI beta promoter activity required the adjacent 663 bp of 5'-upstream DNA where most, but not all, of the regulatory activity was localized between position -663 and -333. A 37-bp direct repeat lies within this region that contains 2 basic helix-loop-helix binding sites, each of which are overlapped by two steroid hormone receptor half-sites, and a shared AP1 consensus sequence. Intron I sequences were also tested, and deletion of a 388-bp region containing numerous Sp1-like sequences lowered transgene activity significantly. These results have identified specific regions of the RI beta promoter that are required for the expression of this signal transduction protein in mouse neurons.

Ovarian cell differentiation: a cascade of multiple hormones, cellular signals, and regulated genes

During the development of preovulatory follicles, tonic levels of FSH (and steroid) induce expression of aromatase, the LH receptor, and RII beta in a coordinate manner. Despite the similar temporal increase in steady-state levels of mRNA encoding these proteins, the cis-acting DNA elements and trans-acting factors regulating each gene are distinct (Richards, 1993). Whereas the aromatase gene has a TATA motif and a single transcriptional initiation site (Fitzpatrick and Richards, 1993), both the LH receptor (Wang et al., 1992; Tsai-Morris et al., 1993) and RII beta (Kurten et al., 1992; Luo et al., 1992) genes have promoters that are GC rich, lack TATA motifs, and initiate transcription at multiple sites. The aromatase promoter appears to be regulated, in part, by SF-1, a CRE-like region, and possibly another or overlapping region binding an Ad3BP-like factor. The RII beta promoter has a region that binds several nuclear proteins, whose identity is not yet known. Likewise, the LH receptor promoter elements have yet to be clearly defined (Figures 2, 4, and 25; Kurten et al., 1992). FSH can also induce the expression of at least three immediate-early genes that encode novel kinases or kinase-like proteins (Figure 25). One of these is called serum-inducible kinase (snk) (Simmons et al., 1992), another is serum and glucocorticoid regulated kinase (sgk) (Webster et al., 1993), and a third is called pole kinase (Clay et al., 1993). Steady-state levels of snk and sgk mRNA are induced rapidly (within a few hours) by FSH in granulosa cells prior to the appearance of transcripts for aromatase, LH receptor, and RII beta (T. Alliston and J. S. Richards, in preparation). The functional role of these kinases in the initial response of granulosa cells to tonic (not surge) levels of FSH remains to be elucidated. The cellular signaling pathways mediating the effects of the LH surge appear equally or more complex (Fig. 25). Based on data presented herein, as well as on analyses of the cloned and expressed LH receptor (Guderman et al., 1992), it is clear that low concentrations of LH stimulate adenylyl cyclase, cAMP production, and activation of protein kinase A. Higher (surge) concentrations of LH also increase IP3 and activation of protein kinase C. GnRH has been used in several studies to examine the ability of the protein kinase C pathway to mimic effects of high LH.(ABSTRACT TRUNCATED AT 250 WORDS)

Cloning and characterization of the gene for the catalytic subunit of cAMP-dependent protein kinase in the aquatic fungus Blastocladiella emersonii

We have isolated and characterized cDNA and genomic DNA clones encoding the catalytic subunit (C) of cAMP-dependent protein kinase in the aquatic fungus Blastocladiella emersonii. The C-subunit amino acid sequence derived from the nucleotide sequence predicts a basic polypeptide of 424 residues, excluding the initiator methionine, which by amino-terminal sequence analysis has been shown to be absent from the mature protein. The Blastocladiella C presents a 70-amino-acid extension at the amino terminus, when aligned to the mouse C alpha subunit, being one of the largest C subunits already characterized. The B. emersonii C-gene-coding region is interrupted by three introns, ranging in size over 57-69 bp. The positions of the introns are quite different from those found in other species, suggesting a considerable amount of evolutionary drift in the gene structure. The 5'-flanking region lacks recognizable TATA or CCAAT sequences, is remarkably high in GC content (70%), and primer extension experiments indicate that transcription initiates from multiple sites. Several sequence motifs were identified in the promoter region which could be involved in the developmental control of this gene.