POTHANA SAIKUMAR, Ph.D.
Associate Professor
Phone (voice): 210 567 6597
Fax: (210) 567-2367
E-mail: saikumar@uthscsa.edu

Degrees

1985, Ph.D., Indian Institute of Science, India
1979, D.I.I.Sc., Indian Institute of Science, India
1978, M.S., Banaras Hindu University, India
1976, B.S., Andhra University, Waltair, India

Postdoctoral Training

1988-91, Postdoctoral Fellow, The Wistar Institute

Research Interests:

Dr. Saikumar and colleagues are investigating the molecular basis for necrotic and apoptotic cell death during hypoxia and re-oxygenation.

Role of Bcl-2 family Proteins in cell death during hypoxia-reoxygenation:
   Hypoxia of cultured proximal tubule cells leads to translocation of the pro-apoptotic Bcl-2 family protein Bax from the cytosol to mitochondria, release of cytochrome c from mitochondria into the cytosol and activation of a subset of cysteine proteases namely caspases (caspases 3, 8 and 9). When reoxygenated, the cells which leaked cytochrome c die by apoptosis. Although apoptosis is prevented by pharmacological inhibition of caspases, the cells are not viable and die by necrosis due to irreversible loss of mitochondrial cytochrome c in these cells. Transfection of the anti-apoptotic protein Bcl-2 does not inhibit Bax translocation, but inhibits release of cytochrome c, caspase activation and apoptosis, and confers viability. Protein cross-linking studies suggest that Bax permeabilizes mitochondrial outer membranes by forming homo-oligomers, and that Bcl-2 inhibits the membrane permeabilization activity of Bax by suppressing Bax oligomerization. Involvement of chymotryptic serine proteases in the initiation of caspase activation following hypoxia-associated release of cytochrome c is also noted. Currently studies are in progress investigating the possible role of IAPs (Inhibitor of Apoptosis Proteins) and hypoxia mediated induction of apoptotic and non-apoptotic proteins in regulating caspase activation and apoptosis in hypoxia-reoxygenation models.

Cell death by necrosis:
   Plasma membranes of cells become permeable to macromolecules during hypoxia induced ATP depletion. These porous defects are prevented by glycine, an amino acid that is normally present in cells at high levels (> 10 mM), but is lost to the extracellular milieu during ATP depletion. The beneficial effects of glycine are unrelated to its metabolism or energy turnover. Related pharmacological studies have suggested that the protective effects are mediated by low affinity interactions of glycine with a membrane protein that is related to the inhibitory glycine receptor in the central nervous system. Ongoing studies are aimed at investigating the polarity of the binding site on membranes and developing approaches to characterizing the receptor protein. Glycine was found not only to prevent porous lesions in the plasma membrane in hypoxic cells, but also to preserve intracellular structure despite complete loss of ATP. In studies to assess whether glycine can prevent abnormal molecular interactions that alter the physical state of the cytoplasm, specific cytosolic proteins were found to translocate from detergent soluble to insoluble fractions in the form of irreversible aggregates. Consisting of glyceraldehyde 3-phosphate dehydrogenase (GAPDH), actin and pyruvate kinase, the aggregates developed concurrently with porous plasma membrane defects, and both lesions were completely suppressed by glycine. Control studies showed that protein aggregation was not a passive, secondary effect of porous plasma membrane damage, and that the two lesions are closely coupled. Further studies showed that protein aggregation could be attributed in part to dephosphorylation of GAPDH. Ongoing investigation is focused on further characterizing the dephosphorylation, and possibly identifying the underlying phosphatase(s).

Unique Technical and Research Capabilities/Instrumentation:
We use standard and advanced biochemical and molecular biology techniques along with immunocytochemistry to understand fundamental mechanisms of cell death.

Selected Publications:

Saikumar P, Dong Z, Denton M, Weinberg JM, Venkatachalam MA. Death Paradigms in cellular hypoxia. Apoptosis in Health and Disease (Ruffolo R and Walsh F, ed). IPD, Singapore. 2000, pp 215-242

Dong Z, Saikumar P, Patel Y, Weinberg JM, Venkatachalam MA. Serine protease inhibitors suppress cytochrome c mediated caspase-9 activation and apoptosis during hypoxia-reoxygenation. Biochem J 347:669-677, 2000.

Saikumar P, Dong Z, Mikhailov V, Denton M, Weinberg JM, Venkatachalam MA. Apoptosis: Definition, Mechanisms, and Relevance to Disease. Am J Med 107:489-506, 1999.

Saikumar P, Dong Z, Patel Y, Hall K, Hopfer U, Weinberg JM, Venkatachalam MA. Role of hypoxia-induced Bax translocation and cytochrome c release in reoxygenation injury. Oncogene 17:3401-3415, 1998.

Saikumar P, Dong Z, Weinberg JM, Venkatachalam MA. Mechanisms of cell death in hypoxia/reoxygenation injury. Oncogene 17:3341-3349, 1998.

Dong Z, Saikumar P, Griess GA, Weinberg JM, Venkatachalam MA. Intracellular Ca2+ thresholds that determine survival or death of energy deprived cells. Am J Pathol 152:231-240, 1998.

Dong Z, Patel Y, Saikumar P, Weinberg JM, Venkatachalam MA. Development of porous defects in plasma membranes of ATP depleted Madin-Darby Canine Kidney cells and its inhibition by glycine. Lab Invest 78:657-668, 1998.

Dong Z, Saikumar P, Weinberg JM, Venkatachalam MA. Internucleosomal DNA cleavage triggered by plasma membrane damage during necrotic cell death: involvement of serine but not cysteine proteases. Am J Pathol 151:1205-1213, 1997.

Key Words:
Cell Death, Apoptosis, Hypoxia, Hypoxia / Reoxygenation, Glycine