Can polyurethane be sprayed over an item painted with acrylic paint?

Yes, both oil and water-based polyurethanes will adhere well to an item coated with acrylic paint. If it is a polyurethane paint, dull the surface prior to painting. If it is a clear. realize that that oil based polyurethane will impart an amber hue while a water based polyurethane is crystal clear and will not affect the color.