1,25(OH)2D3 regulates c-myc mRNA levels in tonsillar T lymphocytes

The effects of 1,25(OH)2D3 on proliferation, c-myc mRNA levels and 1,25(OH)2D3 receptor expression in activated tonsillar T lymphocytes were studied. Activation of resting T cells with phytohaemagglutinin (PHA) for 72 hr led to an increase in proliferation, c-myc mRNA levels and to induction of 1,25(OH)2D3 receptor expression. However, when activation was carried out in the presence of 1,25(OH)2D3, there was inhibition of PHA-stimulated proliferation and c-myc mRNA levels. Increased cell proliferation, c-myc mRNA expression and 1,25(OH)2D3 receptor number were also observed, albeit to a lesser extent, when T cells were stimulated by phorbol myristate acetate (PMA), anti-CD3 antibody or A23187. However, in these cases 1,25(OH)2D3 was unable to prevent increased proliferation or c-myc mRNA expression. PMA and anti-CD3 used in combination produced similar or greater changes in proliferation, c-myc mRNA levels, 1,25(OH)2D3 receptor expression and responsiveness to the hormone when compared to PHA alone. Thus the inhibition of c-myc expression in activated T lymphocytes by 1,25(OH)2D3 can be related to its anti-proliferative effects. Moreover this inhibition seems to be dependent on the level of 1,25(OH)2D3 receptor expression, which in turn appears to be related to the degree of cell activation.