Enhancement by neuropeptide γ of DNA synthesis stimulated with EGF or TGFα in primary cultures of adult rat hepatocytes

DNA synthesis was stimulated by primary (complete) mitogens such as epidermal growth factor (EGF) or transforming growth factor α (TGFα) in adult rat hepatocytes in primary culture. Addition of neuropeptide γ (10^-9 M) to the hepatocyte culture amplified the stimulatory effect of EGF or TGFα by 5- to 6-fold. Neuropeptide γ-amplified DNA synthesis was observed in a dose-dependent manner with maximal effect at 10^-9 to 10^-7 M, although it effect was significantly observed at as low as 10^-10 M. Neuropeptide γ by itself did not stimulate DNA synthesis in the absence of EGF. Amplification of DNA synthesis by neuropeptide γ was observed when hepatocytes were cultured in Williams' medium E, but not in Leibovitz L-15 medium, suggesting that culture medium enriched with nutrients is required for hepatocytes to respond fully to neuropeptide γ. Neuropeptide γ amplified DNA synthesis in hepatocytes from both normal rat and partially hepatectomized rat, although its effect was stronger in normal hepatocytes. Neuropeptide K or substance P which are translated from mRNAs of the same gene as that of neuropeptide γ did not have amplifying effect on DNA synthesis at all concentrations tested even in the presence of EGF. Insulin (500 ng/ml), one of the secondary mitogens (comitogens), interacted additively with EGF. When EGF was present in the culture medium, neuropeptide γ interacted additively with insulin as well as EGF. From these results, neuropeptide γ can be regarded as a new secondary mitogen.