Parathyroid hormone alteration of free and tRNA-bound proline specific activities in cultured mouse osteoblast-like cells

Source of amino acids for tRNA acylation. Implications for measurement of protein synthesis

Estimates of protein-synthesis rates using radioisotopes require accurate measurement of the specific radioactivity of the label in protein and in the precursor pool over time. Although the extracellular and intracellular pools of amino acids are easiest to sample, the tRNA pool is the direct precursor and is the appropriate pool for sampling. To test if the intracellular or extracellular pools reflect the tRNA specific radioactivity, a chicken macrophage cell line was incubated in medium containing either 0.23 mM-leucine and 14.5 microCi of [3H]leucine (tracer dose) or 2.3 microM-leucine plus 145.0 microCi of [3H]leucine (flooding dose). At both leucine levels, the tRNA specific radioactivity reached a plateau quickly, but did not equilibrate with either the extracellular or intracellular specific radioactivity within 30 min, and remained closer to that of protein. In a second experiment, proteins in chicken macrophages were labelled with [3H]leucine for 2 days. Labelling medium was removed, and the cells were washed free of residual free [3H]leucine and incubated with medium containing either 0.23 mM- or 2.3 mM-leucine (unlabelled). The specific radioactivity of leucyl-tRNA leucine reached a plateau within 2 min and remained considerably closer to that in the protein than that in intracellular or extracellular pools for at least 60 min. These results suggest that amino acids from protein degradation are a primary source for charging tRNA. When protein-synthesis rates are estimated by label incorporation, use of extracellular or intracellular specific-radioactivity values result in a marked underestimation.

Physiological and pharmacological regulation of biological calcification

Biological calcification is a highly regulated process which occurs in diverse species of microorganisms, plants, and animals. Calcification provides tissues with structural rigidity to function in support and protection, supplies the organism with a reservoir for physiologically important ions, and also serves in a variety of specialized functions. In the vertebrate skeleton, hydroxyapatite crystals are laid down on a backbone of type I collagen, with the process being controlled by a wide range of noncollagenous proteins present in the local surroundings. In bone, cells of the osteoblast lineage are responsible for the synthesis of the bone matrix and many of these regulatory proteins. Osteoclasts, on the other hand, are continually resorbing bone to both produce changes in bone shape and maintain skeletal integrity, and to establish the ionic environment needed by the organism. The proliferation, differentiation, and activity of these cells is regulated by a number of growth factors and hormones. While much has already been discovered over the past few years about the involvement of various regulators in the process of mineralization, the identification and functional characterization of these factors remains an area of intense investigation. As with any complex, biological system that is in a finely tuned equilibrium under normal conditions, problems can occur. An imbalance in the processes of formation and resorption can lead to calcification disorders, and the resultant diseases of the skeletal system have a major impact on human health. A number of pharmacological agents have been, and are being, investigated for their therapeutic potential to correct these defects.(ABSTRACT TRUNCATED AT 250 WORDS)

Minocycline-treatment of diabetic rats normalizes skin collagen production and mass: possible causative mechanisms

Daily minocycline-treatment of streptozotocin-induced diabetic rats not only prevented a diabetes-caused atrophy of skin collagen mass (10-mos old rats), but also normalized skin collagen mass to match that of growing (ca. 1%/d) non-diabetic controls (4- and 5-mos old rats). The causative mechanism by which minocycline-treatment normalizes skin collagen mass must, in part, be related to a general anabolic effect on growth (body weight) because the effect on skin collagen mass correlates strongly to that on body weight. Consequently, a minocycline-stimulated increase of a systemic factor (such as insulin-like growth factor) is not unlikely. The anabolic effect of minocycline-treatment of diabetic rats is also expressed as a normalized cellular ribosome mass (an index of total protein synthetic capacity) and a normalized absolute rate of collagen production. (Calculation of an absolute rate was justified by an apparent maximum saturation of the prolyl-tRNA pool(s) of skin, maximum saturation obtained by the pool-flooding approach). The normalized skin ribosome amount does not, however, explain a selective effect of minocycline-treatment on collagen production as opposed to that for non-collagen protein, this selective effect measured as relative collagen production. To explain such selectivity, the inhibition of diabetes-induced excess skin collagenase activity seems unlikely. (This inference is based on results from a preliminary study indicating that recently [less than 2 h] synthesized collagen is not degraded by the excess collagenase in skin of diabetic rats). Thus, the principal collagen fraction acted on by pathologically excess collagenase might be collagen at a later stage (greater than 2 h after synthesis) in its life cycle. (Another possibility for the selective effect of minocycline on collagen production, as yet untested, is reduced intracellular procollagen degradation.) Overall, this is the first study aimed at discerning the mechanism(s) by which minocycline-treatment enhances the rate of collagen production in tissues of a diabetic rat. For future studies, the extent to which the positive effect on growth, ribosome mass, and rate of collagen production contributes to the change of collagen mass must, along with the known minocycline-inhibition of collagenase activity, be quantified. Such quantification is a prerequisite for evaluating the chemotherapeutic efficacy of minocycline-treatment on collagen-degradative diseases.