R5 gp120 and MIP-1β activate ERK-1/2 in human macrophages. MDMs were stimulated with (A) R5 gp120 (40 nM) or (B) MIP-1β (25 nM). Cell lysates were harvested at the indicated time points and subjected to Western blot with phospho-specific ERK-1/2 (P-ERK) antibodies. Membranes were stripped and reblotted with antibodies to determine total ERK-1/2 protein (T-ERK). ΔHI indicates protein subjected to heat denaturation.