Signaling through CD5 involves acidic sphingomyelinase, protein kinase C-zeta, mitogen-activated protein kinase kinase, and c-Jun NH2-terminal kinase.

The CD5 lymphocyte surface glycoprotein is a coreceptor involved in the modulation of Ag-specific receptor-mediated activation and differentiation signals. The molecular basis for its modulatory properties is not yet well understood. In the present study we describe early biochemical events triggered by CD5 stimulation, which include the phosphatidylcholine-specific phospholipase C (PC-PLC)-dependent activation of acidic sphingomyelinase (A-SMase) in normal and lymphoblastoid T and B cells. The functional coupling of PC-PLC and A-SMase is demonstrated by the abrogation of A-SMase activation by 1) xanthogenate tricyclodecan-9-yl (D609), a selective inhibitor of PC-PLC, and 2) replacement of several C-terminal serine residues (S458, S459, and S461) present in the cytoplasmic tail of CD5 that are known to be critical for PC-PLC activation. Additionally, we demonstrate that activation of protein kinase C-zeta (PKC-zeta) and members of the mitogen-activated protein kinase (MAPK) cascade (MAPK kinase and c-Jun NH2-terminal kinase), but not the NF-kappaB, are downstream events of the CD5 signaling pathway. A-SMase, PKC-zeta, and MAPK family members are key mediators of cell responses as diverse as proliferation, differentiation, and growth arrest and may contribute to CD5-mediated modulation of TCR or BCR signaling.