Involvement of phosphoprotein phosphatase 1 in collagen-platelet interaction

Protein phosphatase 1 in tumorigenesis: is it worth a closer look?

Cancer cells take advantage of signaling cascades to meet their requirements for sustained growth and survival. Cell signaling is tightly controlled by reversible protein phosphorylation mechanisms, which require the counterbalanced action of protein kinases and protein phosphatases. Imbalances on this system are associated with cancer development and progression. Protein phosphatase 1 (PP1) is one of the most relevant protein phosphatases in eukaryotic cells. Despite the widely recognized involvement of PP1 in key biological processes, both in health and disease, its relevance in cancer has been largely neglected. Here, we provide compelling evidence that support major roles for PP1 in tumorigenesis.

Phosphorylation-dephosphorylation states at different sites affect phosphoprotein phosphatase 1 activity

We have previously reported that the binding of type I collagen to its receptor initiates platelet aggregation involving phosphoprotein phosphatase 1 (PP 1), which coprecipitates with the 65-kDa platelet type I collagen receptor. Phosphorylation of the anti-PP1 precipitation PP1 decreases its enzyme activity. In the present investigation, the mechanism of the decreased enzyme activity was studied by examining the phosphorylation of PP 1 on serine/threonine or tyrosine residues. Phosphoamino acid analysis of the PP 1 indicates that serine, threonine, and tyrosine can all be phosphorylated. We find that the activity of PP 1 decreases with serine/threonine phosphorylation but that phosphorylation of tyrosine residue activates enzyme activity. These results indicate that the activity of platelet phosphoprotein phosphatase 1 is controlled by phosphorylation and dephosphorylation states at multiple, different site(s).

Collagen-induced platelet aggregation and release reaction: role of ras protein

The relationship between the platelet collagen receptor and ras protein was examined by immunoprecipitation of control and collagen stimulated platelets with both antibodies. Both ras protein and receptor are coprecipitated by both antibodies. The coprecipitated samples contained GTP and GDP which were separated by thin layer chromatography. The effect of guanine nucleotide binding protein (G-protein, 21ras) on platelet aggregation was examined by using a guanine triphosphate analogue (GTP-gamma-S). Results of streptolysin O permeabilized experiments show that the addition of the analogue to permeabilized platelet-rich plasma causes platelet aggregation and release of ATP in a dose-dependent fashion. The aggregation induced by GTP-gamma-S could not be obtained with GDP, GDP-beta-S, or 5'-GMP, suggesting that the effect of GTP-gamma-S is specific. The platelet aggregation induced by the analogue was inhibited in a dose-dependent manner by phenylmethanesulfonyl fluoride (PMSF) but not apyrase. GTP-gamma-S induced thromboxane B2 formation was also decreased by PMSF. Formation of thromboxane B2 was also blocked by the addition of PMSF and ethanol suggesting that GTP-gamma-S increases arachidonic acid release from permeabilized platelets. These results suggest that both GTP-gamma-S and GTP enhance phospholipase A2 activity which releases arachidonic acid in permeabilized platelets and subsequently causes platelets to aggregate.