Bayard D. Catherwood

1,25(OH)2D3 and cAMP Synergistically Induce Complement 5a Receptor Messenger RNA

Complement 5a receptor (C5aR) mediates both acute and chronic participation of monocytes in the immune response. In the human U937 monoblast, C5aR is maximally expressed 4 days after treatment with 1,25(OH)2D3 (or cycloheximide) and prostaglandin E2 combined. The authors asked whether these agents altered expression of C5aR messenger RNA (mRNA). Unstimulated U937 cells expressed neither C5aR mRNA nor C5a binding. Complement 5aR mRNA rose 3 hours after prostaglandin E2 application and fell to basal levels by 12 hours. This early rise in C5aR mRNA did not cause an acute rise in C5a binding, which gradually increased between 1 and 4 days. Neither 1,25(OH)2D3 nor cycloheximide induced expression of C5aR mRNA in the absence of prostaglandin E2 but did enhance prostaglandin E2-stimulated C5aR mRNA expression and C5a binding. The authors observed a late increase in C5aR mRNA at day 3 in treated cells. Inhibition of this late rise in mRNA with 5,6-dichlorobenzimidazole riboside attenuated C5a binding by 65%, indicating its importance in the generation of C5a binding sites. The expression of functional C5aR is, therefore, a complex process involving regulation at transcriptional and posttranscriptional levels.

Rat osteoblasts and ROS 17/2.8 cells contain a similar protein tyrosine phosphatase

Tyrosine phosphorylation plays a central role in intracellular signaling by many hormones and growth factors. Termination of the signal is thought to involve dephosphorylation of target proteins by phosphotyrosine phosphatases (PTPase). Soluble protein PTPases from neonatal rat osteoblasts (ROBs) and rat osteosarcoma (ROS 17/2.8) cells were chromatographically distinguished and characterized using 32P-labelled glutamate/tyrosine co-polymer as substrate. Two activities from both cell types were chromatographically separable. The dominant PTPase activity in the presence of 60-125 mM salt (E1), was eluted from phosphocellulose by 180-280 mM NaCl, bound weakly to a strong anion exchange column (QAE-trisacryl), had an apparent Km for [32P]glutamate/tyrosine copolymer of 52 micrograms/ml, was enhanced (5-10-fold, ROS; 1.5-3-fold, ROB) by assay in 125 mM NaCl, had no significant alkaline, acid, or serine phosphatase activity and had an M(r) of 53,000. A second activity (E2) was not retained by phosphocellulose but eluted from QAE-trisacryl in a single peak at 90-130 mM NaCl. It had an apparent Km for [32P]glutamate/tyrosine copolymer of 30 micrograms/ml (ROS) and its activity was not enhanced by NaCl in the assay. Activity E1 from both cells was 50% inhibited by 0.05 microM Na3VO4, 20 microM ZnCl2, or 5-10 microM CoCl2, but not by 1 mM NaF; activity E2 had a similar inhibition profile, but was more sensitive to ZnCl2 (IC50, 5 microM). Co2+ is a relatively non-toxic metal which may be a useful tool for investigating the role of phosphotyrosine in osteoblast proliferation and function. The similarity between the E1 activity from ROS cells and ROBs suggests that ROS cells may be useful in studying PTPase regulation by hormones, but molecular approaches will be required to establish the identity of PTPases in ROBs and ROS cells.

Premenopausal osteoporosis associated with vitamin D-responsive calcium malabsorption: A case report

A premenopausal woman with severe osteoporosis was found to have impaired calcium absorption, without other evidence of intestinal malabsorption. Although circulating levels of 25-OH-vitamin D and 1,25-(OH)2-vitamin D were normal, calcium absorption improved markedly following two weeks of treatment with synthetic 1,25-(OH)2-vitamin D. This suggests that a partial intestinal resistance to the actions of 1,25-(OH)2-vitamin D contributed to the development of her osteoporosis. This case report demonstrates the feasibility of using the calciuric response to a standard oral calcium load to screen for impaired calcium absorption in osteoporotic patients.