Transmembrane signalling associated with ganglioside-induced CD4 modulation

Ganglioside (GM₁) treatment of CD4⁺ human CEM ltmphoma cells stimulated transient phosphoinositide (PI) breakdown, production of inositol phosphates (IP), protein phosphorylation and rapid decrease of CD4 surface expression. A comparison between the actions of GM₁ and other agents that affect these signal transduction pathways demonstrated a distinct mechanism for GM₁-induced decrease of CD4. GM₁ stimulated both phospholipase C activity and protein phosphorylation but had no effect on either cellular cAMP levels or tyrosine kinase activity. Phorbol myristate acetate (PMA) stimulated protein phosphorylation and caused a significant decrease in surface display of CD4. Both of these processes were blocked by pretreating cells with the protein kinase C (PKC) inhibitor H7. These results demonstrate that GM₁ stimulates PI turnover and induces a rapid decrease of CD4 surface expression by processes that do not activate adenylate cyclase or tyrosine kinase. They further demonstrate that the mechanism for GM₁-induced decrease of CD4 is distinct from the CD4 internalization processes mediated by PKC activity.