Prolactin (PRL) is a potent mitogen in cultured astrocytes. Because one of the major effects of astrocyte proliferation is the expression of inflammatory cytokines, we examined the effect of PRL-induced mitogenesis on the expression of interleukin-1 (IL-I a), tumor necrosis factor-a (TNF-a), and transforming growth factor-a (TGF-a) in cultured astrocytes. Astrocytes were stimulated with PRL or growth hormone (GH), and the expression of cytokines was determined by immunohistochemistry and Western blot analysis. Following incubation of astrocytes with 1 nM PRL for 6 h, strong positive staining of IL-la and TNF-a, but not TGF-a, was found. No detectable staining for the above cytokines was found in vehicle, or GH treated astrocytes. When astrocytes were incubated in the presence of 1 nM PRL for 18 h, strong positive staining for IL-la and TGF-a was found. lmmunocytochemical analysis of the expression of TNF-a and IL-I a in PRL stimulated astrocytes suggested that the expression of IL-I a preceded the expression of TNF-a.
To confirm this observation, Western blot analyses were performed on extracts from astrocytes incubated with 1 nM PRL. In unstimulated astrocytes, IL-I a levels were not detectable. In astrocytes stimulated with 1 nM PRL, expression of I L -l a was clearly detected after 1 h of incubation, and I L -l a levels continued to increase during the course of the experiment (6 h). In contrast, in astrocytes stimulated with 1 nM PRL, an increase in the expression of TNF-a was first apparent after 2 h of incubation. TNF-a levels peaked 3 to 4 h after the addition of PRL, and returned to near control levels after 6 h. Finally, injection of PRL into a wound site in female rats increased the expression of glial fibrillary acid protein (GFAP), an astrocyte specific protein. These data suggest that PRL can stimulate astrogliosis at the wound site in vivo. These data clearly indicate that PRL can stimulate the expression of TNF-a and I L -l a in cultured astrocytes and suggest that PRL may play a role in the regulation of the neuroimmune response in vivo.