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Ca2+-dependent cell surface protein phosphorylation may be involved in the initiation of DNA synthesis
Abstract:
Incubating T51B rat liver cells in Ca2+-deficient, serum-rich medium containing only 0.02 mM Ca2+ strikingly decreased the phosphorylation of several trypsin-removable cell surface proteins and arrested the cells in late G1 phase. Raising the Ca2+ concentration in the Ca2+-deficient medium from 0.02 mM to 0.5 mM or adding 80 nM TPA (12-O-tetradecanoyl-phorbol-13-acetate), a protein kinase C activator, stimulated the phosphorylation of a certain set of surface proteins within 5 min and the initiation of DNA replication within the next 2 hr. By contrast, incubation in the same Ca2+-deficient medium, which does not affect the proliferation of neoplastic T51B-261B cells, did not reduce the phosphorylation of cell surface proteins. These observations suggest that the stimulation of a Ca2+-dependent protein kinase (possibly protein kinase C) directly or indirectly phosphorylates certain cell surface proteins that might be part of the mechanism that triggers the Ca2+-dependent G1----S transition of normal cells. They also suggest that an alteration of this Ca2+-dependent protein kinase might be the reason for neoplastic cells being able to proliferate in the face of an external Ca2+ shortage that would stop the proliferation of normal cells.
Insights
Calcium levels regulate cell cycle progression in normal rat liver cells by influencing cell surface protein phosphorylation. Cancer cells bypass this calcium-dependent G1-S transition, suggesting altered protein kinase activity.
Area of Science:
- Cell Biology
- Biochemistry
- Cancer Research
Background:
- Cell proliferation is tightly regulated by extracellular signals, including calcium ions (Ca2+).
- Calcium-dependent protein kinases play crucial roles in cellular signaling pathways.
- Disruptions in cell cycle regulation are a hallmark of neoplastic (cancerous) cells.
Purpose of the Study:
- To investigate the role of extracellular calcium concentration in regulating cell surface protein phosphorylation and cell cycle progression in T51B rat liver cells.
- To explore the potential involvement of calcium-dependent protein kinases, such as protein kinase C, in these processes.
- To compare the response of normal cells to neoplastic cells under conditions of calcium deficiency.
Main Methods:
- Incubation of T51B rat liver cells in varying calcium concentrations (Ca2+-deficient medium).
- Treatment with 12-O-tetradecanoyl-phorbol-13-acetate (TPA) as a protein kinase C activator.
- Analysis of cell surface protein phosphorylation and DNA replication initiation.
Main Results:
- Low extracellular Ca2+ (0.02 mM) arrested normal cells in late G1 phase and decreased cell surface protein phosphorylation.
- Restoring Ca2+ to 0.5 mM or adding TPA stimulated phosphorylation and initiated DNA replication in normal cells.
- Neoplastic T51B-261B cells proliferated in Ca2+-deficient medium without reduced surface protein phosphorylation.
Conclusions:
- Ca2+-dependent protein kinase activation, possibly protein kinase C, phosphorylates cell surface proteins, triggering the G1-S transition in normal cells.
- Altered Ca2+-dependent protein kinase activity may explain the proliferation of neoplastic cells despite calcium deficiency.
- This highlights a critical difference in cell cycle regulation between normal and cancerous liver cells related to calcium signaling.