Growth factors control a network of interacting messengers

Role of rat liver plasma membrane phospholipids in regulation of protein kinase activities

The role of rat liver plasma membrane phospholipids in the regulation of protein kinase A, protein kinase C and tyrosine kinase activities was investigated. Plasma membrane composition was modified by phospholipase A2, phospholipase C and phospholipase D treatment and subsequent incorporation of various phospholipids. Phospholipase A2 deactivated the three types of protein kinases, while phospholipase C and D affected the enzymes in a different manner. Phosphatidylcholine and sphingomyelin were found to be the most effective activators of protein kinase A and tyrosine kinase. Incorporation of sphingomyelin and phosphatidylserine into partially delipidated plasma membranes resulted in a significant stimulation of protein kinase C activity. Since sphingomyelin appeared to be a specific effector of the three types of protein kinases under investigation, one might suggest that its role in cellular signaling could be manifested via regulation of protein kinase C as well as via modulation of protein kinase A and tyrosine kinase activities.

Effect of membrane phospholipid composition and fluidity on rat liver plasma membrane tyrosine kinase activity

1. The effect of membrane phospholipid composition and fluidity on tyrosine kinase activity was investigated in rat liver plasma membranes. 2. The phospholipid composition has been modified by in vitro enrichment of plasma membranes with different phospholipids in the presence of lipid transfer proteins and by partial delipidation with exogenous phospholipases A2, C and D and subsequent enrichment with phosphatidylglycerol. 3. Phosphatidylglycerol and dioleoylglycerophosphocholine caused dramatic elevation of this activity, while phosphatidylserine and phosphatidylethanolamine were less effective. Enrichment with dipalmitoylglycerophosphocholine and sphingomyeline reduced tyrosine kinase activity.

FGF and EGF act synergistically to induce proliferation in BC3H1 myoblasts

BC3H1 muscle cells proliferate when grown in high concentrations of FBS (20%). Lowering the FBS concentration to 0.5% causes the cells to stop proliferating and is permissive for the morphological and biochemical differentiation of BC3H1 cells. Exposure of differentiated BC3H1 myocytes to high concentrations of serum or to the purified growth factors FGF or TGF-b induced a shutdown of this differentiation program but did not induce cell proliferation (Olson et al., J. Cell Biol., 103:1799-1805, 1986; Lathrop et al., J. Cell Biol., 100:1540-1547, 1985, and J. Cell Biol., 101:2194-2198, 1985). We explored the possibility that BC3H1 cells require factors to act synergistically to induce proliferation. We found that EGF and FGF function in a synergistic fashion to stimulate BC3H1 proliferation. Moreover, the temporal requirement for these growth factors suggest that they are functioning as competence and progression factors for BC3H1 cell proliferation.