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N S Hawa, J L H O'Riordan, and S M Farrow


Incubation of bovine parathyroid cells for 48 h in 0·4 mmol calcium/l had no significant effect on steady-state preproparathyroid hormone (preproPTH) mRNA levels when compared with cells incubated in 1·0 mmol calcium/l, but low calcium concentrations increased the membrane-bound polysomal content of preproPTH mRNA by 200±16% (mean±s.d.). No preproPTH mRNA was detected on free polysomes. Actinomycin D (5 and 10 μg/ml) had no effect on steady-state preproPTH mRNA levels measured in dot-blot assays after 24 h, but reduced levels in cells incubated in 1·0 mmol calcium/l to 54±16% and 39±12% of control values respectively after 48 h of incubation. Similarly, in cells incubated in 0·4 mmol calcium/l, actinomycin D (5 and 10μg/ml) reduced steady-state preproPTH mRNA levels to 57±13% and 45±5% of control values respectively. Actinomycin D did not prevent the rise in polysomal content of preproPTH mRNA induced in cells by incubation in 0·4 mmol calcium/l, but increased polysomal content in cells incubated in 0·4 and 1·0mmol calcium/l by 159±9% and 164±13% respectively after 48 h. These results demonstrate post-transcriptional regulation of PTH synthesis in cultured bovine parathyroid cells, and suggest that this control involves a protein which may be calcium-sensitive.

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R. Karmali, A. K. Bhalla, S. M. Farrow, M. M. Williams, S. Lal, P. M. Lydyard, and J. L. H. O'Riordan


The effects of 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3; 10 nmol/1) on the human monomyelocytic cell line U937 were investigated. Addition of 1,25-(OH)2D3 led to a decrease in cell proliferation which fell at 72 h to 67·8 ± 4·3 % (mean±s.e.m.) of control values. The presence of CD14, a surface marker found on mature monocytes/macrophages but not on U937 cells, was detectable as early as 18 h and peaked at 48 h, when 63·6 ± 4·2% of the cells were positive. However, changes in c-myc mRNA levels were detected earlier, starting within 4 h of exposure to the hormone and being reduced to 38±8·2% of control values of 24 h. These effects were reversible after removal of the hormone, with the same sequence of events seen following addition of the hormone. There was first an increase in c-myc mRNA levels, starting within 2 h and reaching control values by 24 h. These changes were followed by loss of CD14 which became undetectable after 72 h. Proliferation recovered slowly and incompletely, since it was 81·7 ± 0·7% of control after 72 h. A constant reciprocal relationship between c-myc mRNA and CD14 levels was found both in the presence and after removal of 1,25-(OH)2D3.

Regulation of U937 cell proliferation and maturation by 1,25-(OH)2D3 is thus preceded by early modulation of c-myc mRNA.