Synthesis and conservation of ribosomal proteins during compensatory renal hypertrophy

Dysregulation of RNA polymerase I transcription during disease

Transcription of the ribosomal RNA genes by the dedicated RNA polymerase I enzyme and subsequent processing of the ribosomal RNA are fundamental control steps in the synthesis of functional ribosomes. Dysregulation of Pol I transcription and ribosome biogenesis is linked to the etiology of a broad range of human diseases. Diseases caused by loss of function mutations in the molecular constituents of the ribosome, or factors intimately associated with RNA polymerase I transcription and processing are collectively termed ribosomopathies. Ribosomopathies are generally rare and treatment options are extremely limited tending to be more palliative than curative. Other more common diseases are associated with profound changes in cellular growth such as cardiac hypertrophy, atrophy or cancer. In contrast to ribosomopathies, altered RNA polymerase I transcriptional activity in these diseases largely results from dysregulated upstream oncogenic pathways or by direct modulation by oncogenes or tumor suppressors at the level of the RNA polymerase I transcription apparatus itself. Ribosomopathies associated with mutations in ribosomal proteins and ribosomal RNA processing or assembly factors have been covered by recent excellent reviews. In contrast, here we review our current knowledge of human diseases specifically associated with dysregulation of RNA polymerase I transcription and its associated regulatory apparatus, including some cases where this dysregulation is directly causative in disease. We will also provide insight into and discussion of possible therapeutic approaches to treat patients with dysregulated RNA polymerase I transcription. This article is part of a Special Issue entitled: Transcription by Odd Pols.

Inhibition of cellular autophagy in kidney tubular cells stimulated to grow by unilateral nephrectomy

Cytoplasmic growth (hypertrophy) presupposes a positive metabolic balance brought about by increased anabolic and/or decreased catabolic processes. Degradation of cytoplasmic components takes place in autophagic vacuoles (AVs) whose volume fraction may be taken as a measure of the relative rate of degradation of cytoplasmic components. Male adult Sprague-Dawley rats (n = 80) were unilaterally nephrectomized (n = 40) or sham-operated (n = 40) and were killed 3.5-57.5 h p.o. The volume density of AVs in parenchymal cells of renal cortical convoluted tubules was determined morphometrically by systematic evaluation of large test fields in the electron microscope. During compensatory renal growth, the volume densities of autophagic vacuoles were reduced at day 0 (3.5-8 h p.o.), day 1 (20.5-33.5 h p.o.) and day 2 (44.5-57.5 h p.o.) by 49% (p less than 0.01), 43% (p less than 0.05), and 19% (n.s.), respectively, when compared with sham-operated controls. No decrease, and even an increase, in the AV-volume fraction was found in liver parenchymal cells of the unilaterally nephrectomized animals. This indicates that inhibition of autophagy is not a general response after unilateral nephrectomy, but is confined to the growing kidney, where it may represent a significant factor in the increase of cytoplasmic mass.

Protein kinase C activity in compensatory kidney growth

Very little information exists about the intracellular mechanisms mediating remnant kidney hypertrophy after reduction of renal mass. The present experiments demonstrate that the activity of a key enzyme in cellular proliferation, protein kinase C, is enhanced in the kidney during post-uninephrectomy hypertrophy. As additional evidence of "in vivo" enzyme activation, the enzyme activity migrates from the cytosolic to the particulate fraction of the cells and the number of sites of phorbol dibutyrate binding is enhanced in kidney membranes 24 hr after contralateral nephrectomy. The stimulation of the enzyme is higher than the increase in kidney weight, although both phenomena are linearly related.

Specific gene expression during compensatory renal hypertrophy in the rat

The compensatory growth of the kidney which is induced by unilateral nephrectomy is a highly regulated process resulting principally in hypertrophy of the remaining kidney. The events which regulate this process are unknown. We have examined the levels of transcripts for the proto-oncogenes, myc, H-ras, K-ras, and fos, and the cellular genes, H4 histone, ornithine aminotransferase, and gamma-glutamyl transpeptidase, following unilateral nephrectomy in the rat. The pattern of expression of c-myc, c-H-ras, and c-K-ras during compensatory growth of the kidney differs from the pattern of expression of these proto-oncogenes during liver regeneration, in which, unlike the kidney, hyperplasia rather than hypertrophy predominates. The lack of change in the abundance of these proto-oncogene transcripts following unilateral nephrectomy suggests a primary relationship between the expression of these proto-oncogenes and DNA synthesis and indicates there may be separate signals for cell growth, one to double cell size and one to replicate DNA. Increased mRNA transcripts for the enzymes ornithine aminotransferase and gamma-glutamyl transpeptidase were induced in the contralateral kidney after nephrectomy. The time course of expression for these two enzymes differs. The early expression of the gamma-glutamyl transpeptidase gene may indicate an involvement of this glutathione-metabolizing enzyme during renal compensatory growth, while the function of the delayed increase in ornithine aminotransferase transcripts in the remaining kidney is not apparent.

Inhibited autophagic degradation during ACTH-stimulated growth of rat adrenal zona fasciculata

Autophagic vacuoles (AVs) in adrenal parenchymal cells of the outer part of the zona fasciculata were investigated by means of electron microscopic morphometry in 40 adult male Sprague Dawley rats. Four test groups and four control groups received twice daily subcutaneous injections of ACTH (12.5 U/kg body weight) or physiological saline, respectively, for 3, 6, 9 and 12 days. At 12 days the total dry weight of the adrenals was increased in ACTH-treated animals from 17.2 to 71.0 mg (about fourfold) and the average volume of zona fasciculata cells from 1,500 to 5,100 microns3 (about threefold). The relatively larger increase in total mass reflects mitotic activity in the growing adrenal tissue. The calculated growth rate of total tissue expressed as the percentage increase in dry weight per day was 12% at day 3 and decreased to 4.6% at day 12 of ACTH stimulation. During the time interval investigated the cytoplasmic volume fraction of autophagic vacuoles was significantly reduced from 10 X 10(-5) in controls to 1 X 10(-5) in ACTH-treated animals (p less than 0.001). Assuming a half-life for AVs of 7.9 min (Papadopoulos and Pfeifer 1986), autophagic degradation of cytoplasm can be calculated as 1.3%/day in controls, but only 0.15%/day in ACTH-treated animals. Thus growth of 1.15%/day can be attributed to inhibition of autophagic degradation. Although this is only part of total growth, it is evident from the present results that anticatabolic growth induction is utilized nearly to its maximum in adrenocortical fasciculata cells under the influence of ACTH.

Kidney function after unilateral nephrectomy

Immediately after unilateral nephrectomy ( uNX ) some different mechanisms of compensatory adaptation begin to act followed by a restoration of sufficient kidney function in a short time period. Beside biochemical changes early compensatory hypertrophy of the remaining kidney occurs. Simultaneously, functional adaptations of renal blood flow, glomerular filtration and exertion of electrolyte and xenobiotics take place. With a suitable pretreatment it is principally possible to accelerate the regeneration phase. Thus the phase of reduced excretion capacity of tubularly secreted xenobiotics after removal of one kidney can be shortened or prevented.

Humoral factors in regulation of compensatory renal hypertrophy

Compensatory renal hypertrophy in transplanted kidneys shows that the major regulators are humoral. Crosscirculation experiments are confirmatory indicating further that the regulators are short-lived and must be consistently present during the early phases of hypertrophy. Controls are difficult to achieve in other systems; variables relate to nutrition, means of assay, pharmacokinetics, and abnormalities produced by the experimental model itself. A simple hypothesis to account for the events precipitating renal hypertrophy might integrate the onsets of renal hyperemia soon after contralateral nephrectomy with the activation of pre-existing stimulators specific for the kidney.

Influence of inhibitors of protein synthesis on restitution of tubular transport capacity after unilateral nephrectomy

One day after unilateral nephrectomy (uNX), excretion of p-aminohippurate (PAH) reaches 80% of control values in rats. A stimulation of tubular transport by repeated administration of xenobiotics shortened the phase of diminished PAH excretion capacity following. The relative extent of compensation (%) after uNX is not affected significantly by inhibitors of protein synthesis. These inhibitors influence the time course of compensation after uNX. Administration of azauracil, fluorouracil and neomycin, respectively, causes a dose dependent reduction of PAH excretion in nephrectomized and sham operated animals. This effect is also provable following pretreatment with cyclopenthiazide, which can stimulate the PAH elimination. A stimulated renal function after uNX can also be suppressed by high doses of inhibitors of protein synthesis. In contrast to this, small doses of these substances produce a stimulation of renal PAH excretion. The extent of this stimulation reached the same degree as after cyclopenthiazide pretreatment found in preliminary experiments. An additional treatment with cyclopenthiazide does not additionally increase PAH excretion. These results indicate that processes of compensatory growth as well as induction of renal tubular transport are caused by increased protein synthesis.

Decreased rate of protein breakdown during nutritional recovery of mouse kidney

After a 5-day protein-depletion diet, there is a 23% decrease in kidney protein content in mice. When mice are shifted to a normal diet, protein mass is restored to its normal value 15 h after the nutritional change. The mechanism for this protein-depleted mice have a protein synthesis efficiency in vivo equal to 71% of that in normal kidneys, and normal values were reached within 6 h after the nutritional shift; 2) protein degradation rates were estimated either from the difference between the synthesis of kidney proteins and the net protein increase or by the disappearance of radioactivity from kidney proteins previously labeled by the administration of NaH14CO3 to mice. By both procedures, a large decrease in protein degradation rates during the recovery through a complete diet was observed.