Conservation of ribosomal RNA during compensatory renal hypertrophy. A major mechanism in RNA accretion.

Abstract

After removal of one mouse kidney, compensatory hypertrophy in the remaining kidney is marked in 2 days by a 20% average increase in ribosomal RNA (rRNA) per cell. Both 28S and 18S RNA are conserved during the initial stages of compensatory renal hypertrophy to an extent sufficient to account for the rest of the observed accumulation of rRNA. Like some cultured cells, the kidney conserves rRNA during physiological growth.2 days after unilateral nephrectomy in rats and mice, the amount of RNA per cell in the remaining kidney is increased 20-40% (6,8,9,19,22,32). Cell division and DNA synthesis are not pronounced, nor are they necessary for compensatory renal hypertrophy.Increases in total renal RNA during compensatory hypertrophy are likely to be due to an increase in the quantity of ribosomal RNA (rRNA), since rRNA accounts for at least 85% of cellular RNA (I 3). However, an increased rate of rRNA synthesis could not actually be demonstrated (11). An alternative explanation for the accumulation of RNA is that rRNA is degraded slower than normal while its rate of synthesis continues unchanged. Although the turnover rate of rRNA in mouse kidneys several days after the onset of renal hypertrophy is the same as that in kidneys of sham nephrectomized mice (3, 22), few measurements of the degradation rate of rRNA have been made during the first stages of growth, when the rate of accretion of RNA is most pronounced (10).We show that the rate of degradation of both 28S and 18S RNA during the first 4 days of compensatory renal growth is distinctly slower than in nongrowing kidney. MATERIALS AND METHODS Animals 40-day old male Charles River mice (Charles RiverBreeding Laboratories, Wilmington, Mass.) were used for unilateral nephrectomy and controls as described (23). Injections of radiochemicals were intraperitoneal. Isolation of Ribosmes and RNATwo kidneys were homogenized in 10 ml of ice-cold RSB buffer with 10 strokes of the tight pestle of a Dounce homogenizer (Kontes Glass Co., Vineland, N. J.). After removal of 1 ml of the homogenate for estimation of RNA and DNA, the remainder was centrifuged at 10,000 g for 10 rain. MgCI~ was added to the supernate to a final concentration of 70 mM, and the preparation was left for 45 min in ice. After centrifugation at 10,000 g for 10 rain. the ribosomal pellet was resuspended.For preparation of polyribosomes the cell homogenate was centrifuged at 10,000 g for 12 min. The sapernate was layered on a discontinuous sucrose gradient (1.5 ml of 1.5 M sucrose over 2.5 ml of 2.0 M sucrose in 0.25 M NaCI, 0.05 M MgCI~, and 0.01 M Tris, pH 7.4 at 20~ and centrifuged at 50,000 rpm in a Spinco type 65 rotor (Beckman Instruments, Inc., Spinco Div., Palo Alto, Calif.) for 4 h to deposit the polysomes.Polysome and ribosome preparations were dissolved in 548

Synthesis and conservation of ribosomal proteins during compensatory renal hypertrophy

“…These experiments confirm previous data (Melvin et al, 1976) that an increase of the rate at which some ribosomes are produced cannot explain the accumulation of ribosomes in the remaining kidney for the first few days after unilateral nephrectomy in the mouse. At that time, the number of ribosomes per cell in the remaining kidney is increased about 10%.…”

“…Since both we (Melvin et al, 1976) and Cortes et al (1976) derived opposite conclusions about the rates of rRNA synthesis from measurements of the rates at which radiolabelled UTP enters RNA, correction for possible changes in the nucleotide precursor pool must be more complex * Present address: Department of Biochemistry, University of Aberdeen, Aberdeen AB9 lAS, Scotland, U.K. than would appear. We have therefore investigated the rate of synthesis of ribosomal proteins as an index of the rate of production of ribosomes during compensatory renal hypertrophy.…”

“…At that time, the number of ribosomes per cell in the remaining kidney is increased about 10%. Since the fractional synthetic rate is about 15% per day (Melvin et al, 1976), an average increase of 60% in the rate of ribosome synthesis would be required to account for the observed accumulation of ribosomes in the first 4 days. Nonetheless, there is no change in the rate of incorporation of leucine into total protein or in the rate of incorporation of methionine into ribosomal protein or rRNA.…”

“…Our view is that a decreased rate of degradation (conservation) of mature rRNA is sufficient to account for most of the increment during the first 2 days and that an increased rate of synthesis becomes progressively more important thereafter Melvin et al, 1976). These processes lead to the new steady state with both increased rates of rRNA synthesis and degradation and an expanded pool of ribosomes, restoring the rate of turnover to that of normal kidneys (Malt & Lemaitre, 1968).…”

“…Three proposals (Hill, 1975;Melvin et al, 1976;Cortes et al, 1976) have been advanced to account for the 20-40% increment in average cellular content of rRNA in the rodent kidney remaining in situ 2-28 days after contralateral nephrectomy (Halliburton & Thomson, 1965;Threlfall et al, 1967;Malt & Lemaitre, 1968;Bucher & Malt, 1971;Dicker & Shirley, 1971;Shirley, 1976). Our view is that a decreased rate of degradation (conservation) of mature rRNA is sufficient to account for most of the increment during the first 2 days and that an increased rate of synthesis becomes progressively more important thereafter Melvin et al, 1976).…”