TYROSINE KINASE-DEPENDENT REGULATION OF L-SELECTIN EXPRESSION THROUGHTHE LEU-13 SIGNAL-TRANSDUCTION MOLECULE EVIDENCE FOR A PROTEIN-KINASEC-INDEPENDENT MECHANISM OF L-SELECTIN SHEDDING

The L-selectin adhesion molecule mediates lymphocyte extravasation in peripheral lymph nodes, and has also been implicated in directing leuk ocyte recruitment to inflammatory tissues and metastasis of lymphoid m alignancies, In this study, we demonstrate a novel level of regulation of L-selectin expression that involves the 16-kDa Leu-13 signal trans duction molecule. Leu-13 is a member of a multimeric cell surface comp lex in lymphocytes that includes TAPA-1 (target of antiproliferative A b-1, CD81) as well as lineage-specific proteins. In the present study, mAb-induced ligation of Leu-13 was shown to rapidly down-regulate L-s electin surface density on normal and malignant human lymphocytes, and to markedly inhibit L-selectin-mediated adhesion of lymphocytes to so luble carbohydrate ligands (i.e., PPME, phosphomonoester core polysacc haride) and to lymph node high endothelial venules. Through the use of genistein and staurosporine, potent inhibitors of tyrosine kinases (T K) and protein kinase C (PKC), respectively, Leu-13-induced L-selectin down-modulation was demonstrated to involve a TK-dependent, PKC-indep endent pathway, and was attributed to increased L-selectin shedding fr om surface membranes. Notably, direct L-selectin ligation, modeling cr oss-linking interactions with endothelial cell ligands, similarly down -regulates L-selectin surface expression through a TK-dependent, PKC-i ndependent mechanism. In sharp contrast, PMA and anti-CD3 mAb down-reg ulate L-selectin via a staurosporine-sensitive, genistein-resistant pa thway that is closely linked to lymphocyte proliferation. Taken togeth er, these results demonstrate a novel role for Leu-13- and L-selectin- induced TK activity in control of L-selectin expression, thus providin g insight into the complex molecular mechanisms that potentially regul ate L-selectin-dependent lymphocyte homing in vivo.