Megakaryocytes endocytose insulin-like growth factor (IGF) I and IGF-binding protein-3: a novel mechanism directing them into alpha granules of platelets.
Lysis of platelets releases the contents of the alpha-granules, which contain growth factors, including insulin-like growth factor I (IGF-I) and IGF-binding protein-3 (IGFBP-3). We investigated the mechanism by which IGF-I and IGFBP-3 appeared in the alpha-granules with a goal of modulating their levels in platelets to affect platelet functions. Reverse transcription-PCR was initially used to test whether megakaryocytes contained IGFBP-3 and IGF-I messenger RNA transcripts. We found that megakaryocytes did not express the IGFBP-3 gene, but did have IGF-I messenger RNA. We subsequently investigated whether they incorporated IGFBP-3 and IGF-I by the process of endocytosis and packaged them into the alpha-granules. This hypothesis was tested in two ways. 1) We examined whether during pregnancy in the rat the alpha-granule content for IGFBP-3 paralleled the changes in plasma IGFBP-3 levels caused by the pregnancy-induced IGFBP-3 protease. The alpha-granule contents of both IGFBP-3 and IGF-I declined in parallel to the plasma changes in pregnant rats and returned to normal postpartum. As the binding protein protease acts extracellularly, endocytosis of the IGF-I:IGFBP-3 complex from the extracellular fluid by megakaryocytes was suggested. 2) We tested whether an IGF-I:IGFBP-3 complex comprised of human IGF-I and IGFBP-3 (recombinant 28.7 kDa) injected i.v. appeared in rat platelet alpha-granules. Hypophysectomized rats were injected i.v. with 5.24 mg of a 1:1 complex of IGF-I:IGFBP-3. After 24 h, platelet lysates were prepared and analyzed for IGFBP-3 by Western ligand blotting, and IGF-I was determined by RIA. Platelet lysates of the treated animals showed a prominent new band at approximately 28 kDa, whereas control rats were negative. In addition, the alpha-granule IGF-I concentration increased from 0.38 to 1.9 ng/1 x 10(9) platelets. These results indicate that the IGF-I:IGFBP-3 complex is taken up by megakaryocytes and packaged into the alpha-granules of platelets and demonstrate how the contents of IGF-I and IGFBP-3 in platelets can be modulated by their plasma concentrations. As reverse transcription-PCR has shown that the IGF-I, but not the IGFBP-3, gene is expressed by megakaryocytes, there may be two mechanisms for directing IGF-I into the alpha-granules of platelets.
Chan
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Department of Growth and Development, Davies Medical Center, San Francisco, California 94114, USA. igf@itsa.ucsf.edu
Spencer
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D000818:Animals; D001792:Blood Platelets / Q000378:metabolism*; D003594:Cytoplasmic Granules / Q000378:metabolism; D004705:Endocytosis / Q000502:physiology*; D005260:Female; D006801:Humans; D018972:Insulin-Like Growth Factor Binding Protein 3 / Q000235:genetics / Q000378:metabolism*; D007334:Insulin-Like Growth Factor I / Q000235:genetics / Q000378:metabolism*; D008533:Megakaryocytes / Q000378:metabolism / Q000502:physiology*; D010978:Platelet Factor 4 / Q000235:genetics; D011247:Pregnancy; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D011916:Rats, Inbred F344; D011919:Rats, Inbred Strains; D011994:Recombinant Proteins
D018972:Insulin-Like Growth Factor Binding Protein 3; D012333:RNA, Messenger; D011994:Recombinant Proteins; D010978:Platelet Factor 4; D007334:Insulin-Like Growth Factor I