Regulation of nucleoside transport regulation by lipopolysaccharide, phorbol esters and TNF in human B lymphocytes

Abstract

Nucleoside transport systems and their regulation in human B-lymphocytes have been characterized using the cell lines Raji and Bare lymphoma syndrome-1 (BLS-1) as experimental models. These cells express at least three different nucleoside transport systems as follows: a nitrobenzylthioinosine-sensitive equilibrative transport system of the es-type, which appears to be associated with hENT1 expression, and two Na+-dependent transport systems that may correspond to N1 and to the recently characterized N5-type, which is nitrobenzylthioinosine-sensitive and guanosine-preferring. B cell activators such as phorbol 12-myristate 13-acetate and lipopolysaccharide (LPS) up-regulate both concentrative transport systems but down-regulate the equilibrative es-type transporter, which correlates with lower hENT1 mRNA levels. These effects are dependent on protein kinase C activity. Phorbol 12-myristate 13-acetate and LPS also induce an increase in tumor necrosis factor-alpha (TNF-alpha) mRNA levels, which suggest that this cytokine may mediate some of the effects triggered by these agents, since addition of TNF-alpha alone can increase N1 and N5 transport activities by a mechanism that also depends on protein kinase C activation. Interestingly, TNF-alpha down-regulates es activity, but this effect cannot be abolished by inhibiting protein kinase C. This study reveals differential regulation of nucleoside transport systems following activation of human B-lymphocyte cell lines by agents of physiological relevance such as TNF-alpha and LPS. Moreover, it indicates that the recently characterized N5 transport system can also be regulated following B cell activation, which may be relevant to lymphocyte physiology and to the treatment of lymphocyte malignancies.