title: Unconjugated Bilirubin exerts Pro-Apoptotic Effect on Platelets via p38-MAPK activation.
abstract: Thrombocytopenia is one of the most frequently observed secondary complications in many pathological conditions including liver diseases, where hyperbilirubinemia is very common. The present study sought to find the cause of thrombocytopenia in unconjugated hyperbilirubinemic conditions. Unconjugated bilirubin (UCB), an end-product of heme catabolism, is known to have pro-oxidative and cytotoxic effects at high serum concentration. We investigated the molecular mechanism underlying the pro-apoptotic effect of UCB on human platelets in vitro, and followed it up with studies in phenylhydrazine-induced hyperbilirubinemic rat model and hyperbilirubinemic human subjects. UCB is indeed found to significantly induce platelet apoptotic events including elevated endogenous reactive oxygen species generation, mitochondrial membrane depolarization, increased intracellular calcium levels, cardiolipin peroxidation and phosphatidylserine externalization (p < 0.001) as evident by FACS analysis. The immunoblots show the elevated levels of cytosolic cytochrome c and caspase activation in UCB-treated platelets. Further, UCB is found to induce mitochondrial ROS generation leading to p38 activation, followed by downstream activation of p53, ultimately resulting in altered expression of Bcl-2 and Bax proteins as evident from immunoblotting. All these parameters conclude that elevated unconjugated bilirubin causes thrombocytopenia by stimulating platelet apoptosis via mitochondrial ROS-induced p38 and p53 activation.
authors.affiliation: DOS in Biochemistry, University of Mysore, Manasagangothri, Mysuru-570 006, India.
authors.forename: Somanathapura K
authors.initials: SK
authors.lastname: NaveenKumar
authors.affiliation: DOS in Biochemistry, University of Mysore, Manasagangothri, Mysuru-570 006, India.
authors.forename: Ram M
authors.initials: RM
authors.lastname: Thushara
authors.affiliation: DOS in Biochemistry, University of Mysore, Manasagangothri, Mysuru-570 006, India.
authors.forename: Mahalingam S
authors.initials: MS
authors.lastname: Sundaram
authors.affiliation: DOS in Biochemistry, University of Mysore, Manasagangothri, Mysuru-570 006, India.|Department of Internal Medicine, Manitoba Centre for Proteomics and Systems Biology, University of Manitoba, Winnipeg- R3E3P4, Canada.
authors.forename: Mahadevappa
authors.initials: M
authors.lastname: Hemshekhar
authors.affiliation: DOS in Biochemistry, University of Mysore, Manasagangothri, Mysuru-570 006, India.
authors.forename: Manoj
authors.initials: M
authors.lastname: Paul
authors.affiliation: Department of Biochemistry and Molecular Biology, Pondicherry Central University, Pondicherry-605 014, India.
authors.forename: Chinnasamy
authors.initials: C
authors.lastname: Thirunavukkarasu
authors.affiliation: Laboratory of Chemical Biology, Department of Chemistry, Bangalore University, Bengaluru-560 056, India.
authors.lastname: Basappa
authors.affiliation: Department of Biochemistry, Indian Institute of Science, Bengaluru-560 012, India.
authors.forename: Ganesh
authors.initials: G
authors.lastname: Nagaraju
authors.affiliation: Department of Biochemistry, Indian Institute of Science, Bengaluru-560 012, India.
authors.forename: Sathees C
authors.initials: SC
authors.lastname: Raghavan
authors.affiliation: DOS in Biochemistry, University of Mysore, Manasagangothri, Mysuru-570 006, India.|Department of Studies and Research in Biochemistry, Tumkur University, Tumkur-572 103, India.
authors.forename: Kesturu S
authors.initials: KS
authors.lastname: Girish
authors.affiliation: DOS in Biochemistry, University of Mysore, Manasagangothri, Mysuru-570 006, India.
authors.forename: Kempaiah
authors.initials: K
authors.lastname: Kemparaju
authors.affiliation: DOS in Chemistry, University of Mysore, Manasagangothri, Mysuru-570 006, India.
authors.forename: Kanchugarakoppal S
authors.initials: KS
authors.lastname: Rangappa
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000187:drug effects; D001663:Bilirubin / Q000378:metabolism* / Q000494:pharmacology; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism; D002308:Cardiolipins / Q000378:metabolism; D004195:Disease Models, Animal; D006801:Humans; D006932:Hyperbilirubinemia / Q000378:metabolism; D053078:Membrane Potential, Mitochondrial; D008928:Mitochondria / Q000187:drug effects / Q000378:metabolism; D018384:Oxidative Stress / Q000187:drug effects; D010766:Phosphorylation; D051381:Rats; D015398:Signal Transduction / Q000187:drug effects; D016159:Tumor Suppressor Protein p53 / Q000378:metabolism; D048051:p38 Mitogen-Activated Protein Kinases / Q000378:metabolism*
chemical_list: D002308:Cardiolipins; D016159:Tumor Suppressor Protein p53; D048051:p38 Mitogen-Activated Protein Kinases; D001663:Bilirubin; D002118:Calcium