Platelets promote angiogenesis by releasing angiogenesis-regulating factors from their α-granules upon aggregation. This effect has both physiological and pathological significance as it may contribute to carcinogenesis. Platelet α-granule release and aggregation are regulated, in part, via PKCα and β signaling. Therefore the purpose of our study was to investigate the effects of PKC inhibition on aggregation, angiogenesis-regulator secretion from α-granules and platelet-stimulated angiogenesis. We hypothesized that selective PKCα inhibition may preferentially suppress angiogenesis-regulator secretion from α-granules, but not aggregation, limiting platelet-stimulated angiogenesis. Human platelets were aggregated in the presence conventional PKC inhibitors myr-FARKGALRQ and Ro 32-0432. Immunofluorescence microscopy of PKC translocation was used to determine the specificity of PKC-inhibitor targeting. ELISA was used to measure vascular endothelial growth factor (VEGF) and thrombospondin-1 (TSP-1) release from platelets. Platelet effects on angiogenesis were tested using a capillary-formation assay. Ro 32-0432, but not the peptide inhibitor myr-FARKGALRQ, inhibited aggregation in a concentration-dependent manner, while both Ro 32-0432 and myr-FARKGALRQ preferentially suppressed VEGF over TSP-1 secretion. Suppression of angiogenesis-regulator release occurred at inhibitor concentrations that did not signifcantly affect aggregation. Immunofluorescence microscopy revealed that PKCα targeting to α-granules is inhibited when angiogenesis-regulator secretion is uncoupled from aggregation. At concentrations that uncoupled α-granule release from aggregation, Ro 32-0432 and myr-FARKGALRQ inhibited platelet-stimulated angiogenesis. Hence, selective PKCα inhibition suppresses angiogenesis-regulator release from platelet α-granules with minimal effects on aggregation. Thus, selective PKCα inhibitors may have pharmacological significance to regulate platelet-promoted angiogenesis.
- Received October 3, 2012.
- Revision received February 1, 2013.
- Accepted February 4, 2013.
- The American Society for Pharmacology and Experimental Therapeutics