A protein kinase C/Ras/ERK signaling pathway activates myeloid fibronectin receptors by altering β1 integrin sialylation

Here we report that myeloid cells differentiating along the monocyte/ macrophage lineage down-regulate the ST6Gal-I sialyltransferase via a protein kinase C/Ras/ERK signaling cascade. In consequence, the beta 1 integrin subunit becomes hyposialylated, which stimulates the ligand binding activity of alpha 5 beta 1 fibronectin receptors. Pharmacologic inhibitors of protein kinase C, Ras, and MEK, but not phosphoinositide 3-kinase, block ST6Gal-I down-regulation, integrin hyposialylation, and fibronectin binding. In contrast, constitutively active MEK stimulates these same events, indicating that ERK is both a necessary and sufficient activator of hyposialylation-dependent integrin activation. Consistent with the enhanced activity of hyposialylated cell surface integrins, purified alpha 5 beta 1 receptors bind fibronectin more strongly upon enzymatic desialylation, an effect completely reversed by resialylation of these integrins with recombinant ST6Gal-I. Finally, we have mapped the N-glycosylation sites on the beta 1 integrin to better understand the potential effects of differential sialylation on integrin structure/function. Notably, there are three N-glycosylated sites within the beta 1 I-like domain, a region that plays a crucial role in ligand binding. Our collective results suggest that variant sialylation, induced by a specific signaling cascade, mediates the sustained increase in cell adhesiveness associated with monocytic differentiation.