Association of cellular thiol redox status with mitogen‐induced calcium mobilization and cell cycle progression in human fibroblasts

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Abstract
Human gingival fibroblast cultures were used to investigate the role of cellular thiol redox status in the mitogenic response. Increases in intracellular Ca2+ and cell cycle progression beyond G1 were followed as parameters of cellular mitogen‐induced responses. Ethionine provided a G1 stage synchronization and altered the cellular redox poise as measured by the ratio NAD(P)H/NAD(P)+. Cultures harvested immediately after the 6 day ethionine low‐serum synchronization showed a significant oxidation of their redox poise. Synchronized cultures, which were also glutathione (GSH) depleted, still showed an oxidized redox poise and significantly reduced GSH levels following a 24 hr incubation in drug‐free, rich medium. Cellular reduced nicotinamide nucleotide levels correlated strongly (r = 0.995) with capacity to mobilize intracellular Ca2+ in response to basic fibroblast growth factor (bFGF). The sustained mitogenic response, as determined by cell cycle progression beyond G1, was also found to be interrelated with the cellular thiol redox status. Following a 24 hr recovery incubation in serum‐rich medium, formerly synchronized cultures showed a rebound of their redox poise to a more reduced state and significant cell cycle progression beyond G1. In contrast, synchronized, GSH‐depleted cultures did not progress and showed population distributions similar to those of cultures harvested immediately postsynchronization. Upon recovery of cellular GSH and reduced nicotinamide nucleotide levels, formerly GSH‐depleted, growth‐arrested cultures resumed cell cycle progression. The results suggest that the cellular response to specific mitogens is interrelated with the cellular thiol redox status. Cells that posses a thiol redox status below a threshold response point may have compromised Ca2+ sequestration and/or mobilization and therefore may be incapable of initiating the mitogen induced response cascade that culminates in cell cycle progression.

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