Intracellular signalling by binding sites for the antipsoriatic agent monomethylfumarate on human granulocytes

Abstract

Monomethylfumarate (MMF), the most active metabolite of the new antipsoriasis drug Fumaderm, stimulates an anti-inflammatory mediator profile in human leucocytes and inhibits the proliferation of keratinocytes. These effects of MMF on cells in vitro may in part explain the beneficial action of Fumaderm in patients. In addition, we have reported that MMF stimulates an increase in the intracellular free Ca2+ concentration ([Ca2+]i) and the cyclic adenosine monophosphate (cAMP) concentration in granulocytes and keratinocytes. Because Ca2+ and cAMP control many physiological cellular responses, including cell proliferation and inflammatory mediator production, the present study focused on the intracellular signal transduction pathway which links interaction between MMF and granulocytes with increases in [Ca2+]i and the cAMP concentration. The increase in [Ca2+]i in granulocytes after MMF depended both on extracellular Ca2+ and Ca2+ from intracellular stores. Ca2+ is essential for the increase in the cAMP concentration after stimulation with MMF. The results found for pharmacological inhibitors of various protein kinases suggest that a staurosporine-sensitive protein kinase different from protein kinase C (PKC) and protein kinase A is involved in the MMF-induced increase in [Ca2+]i in granulocytes. As MMF activated protein tyrosine kinase (PTK), and inhibition of this protein kinase partially reduced the increase in [Ca2+]i in granulocytes, PTK activity most likely is involved. In addition, activation of protein kinase histone 4 (PKH4) probably plays a part in the MMF-stimulated increase in [Ca2+]i in granulocytes as well. As MMF stimulated an increase in the GTP-ase activity of membranes and pertussis toxin (PTX) inhibited the increase in the [Ca2+]i and PKH4 activity of granulocytes stimulated by this compound, it is concluded that MMF activates PTX-sensitive G proteins. Competition binding studies with radiolabelled dimethylfumarate (DMF) and unlabelled DMF and MMF revealed the presence of specific binding sites for methylated fumarates on granulocytes. In summary, MMF binds to specific sites on the plasma membrane of cells. This interaction activates pertussis toxin-sensitive G proteins which then stimulate an increase in PTK and PKH4 activity. These protein kinases may regulate the rise in [Ca2+]i and the intracellular cAMP concentration. Elevated [Ca2+]i and intracellular cAMP concentration stimulate protein kinases that regulate transcription factors. Activation of these factors results in induction of downstream gene expression and thus controls cell functions, e.g. cell proliferation and production of inflammatory mediators, as has been found for cells incubated with MMF.