A very late activating antigen-alpha4 (CD49d) monoclonal antibody,BU49 induces phosphorylation of a cAMP response element-binding protein (CREB), resulting in induction of homotypic cell aggregation and enhancement of interleukin-8 (IL-8) production

A very late activating antigen-alpha4 (CD49d) monoclonal antibody (mAb), BU49 was found to induce phosphorylation of a cAMP response element-binding protein (CREB) in the human monocyte-like cell line, U937. This phosphorylation of CREB was completely inhibited by a protein kinase A (PKA) inhibitor H-89 with the optimum concentration (completely inhibits PKA). Furthermore, BU49 strongly and rapidly (within 5 hr) induced homotypic cell aggregation in the U937 cells accompanied by CREB phosphorylation. This cell aggregation was also completely inhibited by the addition of H-89. Interestingly, both of two mAbs (mAb13 and 4B4) recognizing different epitopes on the CD29 (beta1 integrin) completely inhibited this aggregation at the late phase (18 to 24 hr) but not at the early phase (5 hr) after cultured with BU49. On the other hand, BU49 significantly enhanced interleukin-8 (IL-8) production from the U937 cells into the culture supernatant. In addition, this IL-8 production was significantly blocked in the presence of H-89 with the optimum concentration. However, a CD29 mAb which inhibits homotypic cell aggregation could not block this IL-8 production. Taken together, these findings indicate that BU49 induces CREB phosphorylation mainly mediated by PKA, which finally results in the induction of homotypic cell aggregation and the enhancement of IL-8 production. Furthermore, these findings also indicate that the enhancement of IL-8 production from the U937 cells induced by BU49 partially depends on CREB phosphorylation mainly mediated by PKA.