The effects of epidermal growth factor and the state of confluence on enzymatic activities of cultured rat liver epithelial cells

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Abstract
In cultured normal rat liver epithelial cells, the specific activity and/or isozyme expression of NADH‐diaphorase (NADH‐D), pyruvate kinase (PK), glucose‐6 phosphate dehydrogenase (G6PD), gamma‐glutamyl transpeptidase (GGT), and alkaline phosphatase (AP) were markedly dependent on the growth state of the cultures. Proliferating, preconfluent cells had higher specific activities of PK, NADH‐D, and G6PD but lower activities of GGT and AP than did the more stationary confluent cells. Addition of epidermal growth factor [EGF] to the media of proliferating cells enhanced the specific activities of PK, NADH‐D, G6PD, GGT, and lactate dehydrogenase (LDH) of these cells, but the specific activity of AP was markedly depressed. The increase in activity of PK and GGT by EGF appeared to involve new protein synthesis, whereas the effect of EGF on AP appeared to involve the EGF‐directed suppression of the synthesis of a form of AP that is produced exclusively by cells in confluent cultures. Furthermore, the preconfluent cells were more responsive to the action of EGF on AP than were confluent cells, i.e., the EGF‐mediated decrease in AP activity was seen at lower concentration in preconfluent than in confluent cells. Paradoxically, confluent cells exhibited a two‐to threefold higher capacity to bind [125I]EGF because of an increase in surface receptor number. The results of this study indicate that enzymatic or other biochemical studies performed on cultured cells must take into account the growth‐state of the cultures. EGF can modulate enzyme activity in growing and nongrowing cells; one effect of EGF is to maintain higher activity of glycolytic enzymes, suggesting that EGF or EGF‐like factors may contribute to the high rate of glycolysis in certain neoplasms.