Identification of a novel keratinocyte mitogen derived from bovine pituitary glands

Bovine pituitary glands contain one or more factors that are important for keratinocyte proliferation in serum-free culture medium. We used a tissue culture system in which the growth of keratinocytes in basal medium (KBM) containing insulin was dependent upon exogenous growth factors. Using this experimental system, we began to purify and characterize the pituitary factor(s) necessary for clonal growth of human keratinocytes in serum-free medium. Proteins of approximately 150 kDa and 95 kDa bound specifically to living keratinocytes, and we suggest that the 95 kDa protein is a likely novel mitogen. Although prolactin has been previously identified as a pituitary hormone that may act as an in vitro mitogen for keratinocytes, imunoblots indicated that the 95 kDa protein was unrelated to prolactin. Furthermore, the 95 kDa protein showed high homology with a bovine 90 kDa heat shock protein in the limited sequencing of an internal peptide.