The worker honeybees of Apis mellifera L. age-dependently change from nurse bees which take care of their brood to forager bees which collect for nectar and convert it into honey during their lifespan of 30-40 days. A 56-kDa protein (p56kP-4) was shown to be present only in the nurse-bee hypopharyngeal gland. Here, we investigated the physiological effects of p56kP-4 on primary-cultured rat hepatocytes in the absence of serum. The p56kP-4 recombinant protein expressed in Escherichia coli significantly stimulated hepatocyte DNA synthesis and protected cells from apoptosis induced by serum deprivation. It also activated mitogen-activated protein kinase, as well as protein kinase B, which is a key regulator of cell survival. These findings suggest that p56kP-4 has a growth factor-like action on primary-cultured rat hepatocytes. (c) 2005 Elsevier Inc. All rights reserved.