title: Up-regulation of c-myc in a transformed cell line approaching stationary phase growth in culture.
abstract: The present report describes a transformed cell line (AKR-MCA) in which the c-myc proto-oncogene is up-regulated by as much as 14-fold as cultures approach stationary phase growth. The untransformed counterpart AKR-2B cells did not exhibit such an increase in c-myc expression at high cell densities, nor did chemically transformed derivatives of another murine fibroblast cell line (C3H 10T1/2). N,N-Dimethylformamide and retinoic acid reduced c-myc levels in confluent AKR-MCA cells in association with a loss of transformed morphology, a reduction in saturation density, and the formation of a contact-inhibited monolayer at confluency. These findings suggest that the high levels of c-myc in confluent AKR-MCA cells may interfere with the normal signals involved in density-dependent growth regulation in this cell system. The effects of N,N-dimethylformamide and retinoic acid were reversible and dose-related. The half-time for the early, rapid decline in c-myc mRNA was approximately 26 min in response to N,N-dimethylformamide and 38 min in response to retinoic acid, effects which preceded the alterations in morphology and saturation densities. Activation of the latent transforming growth factor-beta in serum-free medium conditioned by confluent AKR-MCA cells, followed by its addition to preconfluent AKR-MCA cells, resulted in an up-regulation of c-myc mRNA. However, addition of serum-containing conditioned medium under similar conditions did not require prior acidification to up-regulate c-myc. Thus, active transforming growth factor-beta may be present in conditioned medium from confluent AKR-MCA cells grown in serum-containing medium, or autocrine factors other than TGF-beta may produce the confluency-associated up-regulation of c-myc and the altered density-dependent growth regulation in AKR-MCA cells.
authors.affiliation: Department of Pharmacology, Baylor College of Medicine, Houston, Texas 77030.
authors.forename: K M
authors.initials: KM
authors.lastname: Mulder
authors.forename: A E
authors.initials: AE
authors.lastname: Levine
authors.forename: X H
authors.initials: XH
authors.lastname: Hinshaw
mesh_terms: D002455:Cell Division; D002461:Cell Line, Transformed; D004126:Dimethylformamide / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D011519:Proto-Oncogenes; D012333:RNA, Messenger / Q000032:analysis; D013997:Time Factors; D015291:Transforming Growth Factors / Q000494:pharmacology; D014212:Tretinoin / Q000494:pharmacology
chemical_list: D012333:RNA, Messenger; D014212:Tretinoin; D015291:Transforming Growth Factors; D004126:Dimethylformamide