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1.  Shifting the Paradigm: The Putative Mitochondrial Protein ABCB6 Resides in the Lysosomes of Cells and in the Plasma Membrane of Erythrocytes 
PLoS ONE  2012;7(5):e37378.
ABCB6, a member of the adenosine triphosphate–binding cassette (ABC) transporter family, has been proposed to be responsible for the mitochondrial uptake of porphyrins. Here we show that ABCB6 is a glycoprotein present in the membrane of mature erythrocytes and in exosomes released from reticulocytes during the final steps of erythroid maturation. Consistent with its presence in exosomes, endogenous ABCB6 is localized to the endo/lysosomal compartment, and is absent from the mitochondria of cells. Knock-down studies demonstrate that ABCB6 function is not required for de novo heme biosynthesis in differentiating K562 cells, excluding this ABC transporter as a key regulator of porphyrin synthesis. We confirm the mitochondrial localization of ABCB7, ABCB8 and ABCB10, suggesting that only three ABC transporters should be classified as mitochondrial proteins. Taken together, our results challenge the current paradigm linking the expression and function of ABCB6 to mitochondria.
doi:10.1371/journal.pone.0037378
PMCID: PMC3360040  PMID: 22655043
2.  Centrosomal PKCβII and pericentrin are critical for human prostate cancer growth and angiogenesis 
Cancer research  2008;68(16):6831-6839.
Angiogenesis is critical in the progression of prostate cancer. However, the interplay between the proliferation kinetics of tumor endothelial cells (angiogenesis) and tumor cells has not been investigated. Also, protein kinase C (PKC) regulates various aspects of tumor cell growth but its role in prostate cancer has not been investigated in detail. Here, we found that the proliferation rates of endothelial and tumor cells oscillate asynchronously during the growth of human prostate cancer xenografts. Furthermore, our analyses suggest that PKCβII was activated during increased angiogenesis and that PKCβII plays a key role in the proliferation of endothelial cells and tumor cells in human prostate cancer; treatment with a PKCβII-selective inhibitor, βIIV5-3, reduced angiogenesis and tumor cell proliferation. We also find a unique effect of PKCβII inhibition on normalizing pericentrin (a protein regulating cytokinesis), especially in endothelial cells as well as in tumor cells. PKCβII inhibition reduced the level and mislocalization of pericentrin and normalized microtubule organization in the tumor endothelial cells. Although pericentrin has been known to be upregulated in epithelial cells of prostate cancers, its level in tumor endothelium has not been studied in detail. We found that pericentrin is upregulated in human tumor endothelium compared with endothelium adjacent to normal glands in tissues from prostate cancer patients. Our results suggest that a PKCβII inhibitor such as βIIV5-3 may be used to reduce prostate cancer growth by targeting both angiogenesis and tumor cell growth.
doi:10.1158/0008-5472.CAN-07-6195
PMCID: PMC2597632  PMID: 18701509
3.  δPKC Participates in the Endoplasmic Reticulum Stress-Induced Response in Cultured Cardiac Myocytes and Ischemic Heart 
The cellular response to excessive endoplasmic reticulum (ER) stress includes the activation of signaling pathways, which lead to apoptotic cell death. Here we show that treatment of cultured cardiac myocytes with tunicamycin, an agent that induces ER stress, causes the rapid translocation of δPKC to the ER. We further demonstrate that inhibition of δPKC using the δPKC-specific antagonist peptide, δV1-1, reduces tunicamycin-induced apoptotic cell death, and inhibits expression of specific ER stress response markers such as CHOP, GRP78 and phosphorylation of JNK. The physiological importance of δPKC in this event is further supported by our findings that the ER stress response is also induced in hearts subjected to ischemia and reperfusion injury and that this response also involves δPKC translocation to the ER. We found that the levels of the ER chaperone, GRP78, the spliced XBP-1 and the phosphorylation of JNK are all increased following ischemia and reperfusion and that δPKC inhibition by δV1-1 blocks these events. Therefore, ischemia-reperfusion injury induces ER stress in the myocardium in a mechanism that requires δPKC activity. Taken together, our data show for the first time that δPKC activation plays a critical role in the ER stress-mediated response and the resultant cell death.
doi:10.1016/j.yjmcc.2007.07.061
PMCID: PMC2185772  PMID: 17825316
4.  Rational Design of A Selective Antagonist of ε Protein Kinase C Derived From the Selective Allosteric Agonist, Pseudo-Rack Peptide 
We have previously shown that domains involved in binding of protein kinase C (PKC1) isozymes to their respective anchoring proteins (RACKs2) and short peptides derived from these domains are PKC isozyme-selective antagonists. We also identified PKC isozyme-selective agonists, named ψRACK3 peptides, derived from a sequence within each PKC with high homology to its respective RACK. We noted that all the ψRACK sequences within each PKC isozyme have at least one non-homologous amino acid difference from their corresponding RACK that constitutes a charge change. Based on this information, we have devised here a new approach to design an isozyme-selective PKC antagonist, derived from the ψRACK sequence. We focused on εPKC ψRACK peptide, where the pseudo-εRACK sequence (ψεRACK; HDAPIGYD; corresponding to εPKC85-92) is different in charge from the homologous RACK-derived sequence (NNVALGYD; corresponding to εRACK285-292) in the second amino acid. Here we show that changing the charge of the ψεRACK peptide through a substitution of only one amino acid (aspartate to asparagine) resulted in a peptide with an opposite activity on the same cell function and a substitution for aspartate with an alanine resulted in an inactive peptide. These data support our hypothesis regarding the mechanism by which pseudo-RACK peptide activates PKC in heart cells and suggest that this approach is applicable to other signaling proteins with inducible protein-protein interactions.
doi:10.1016/j.yjmcc.2007.01.007
PMCID: PMC1978508  PMID: 17337000
PKC (protein kinase C); RACK (receptor for activated C-kinase); ψRACK (pseudo RACK); intramolecular interaction; carrier peptide
5.  A Single Point Mutation in the V3 Region Affects Protein Kinase Cα Targeting and Accumulation at Cell-Cell Contacts 
Molecular and Cellular Biology  2001;21(10):3351-3363.
Given the importance of intercellular adhesion for many regulatory processes, we have investigated the control of protein kinase Cα (PKCα) targeting to the cell-cell contacts. We have previously shown that, upon treatment of the pituitary cell line GH3B6 with thyrotropin-releasing hormone (TRH) or phorbol 12-myristate 13-acetate (PMA), human PKCα (hPKCα) is selectively targeted to the cell-cell contacts (42). Here we show that the D294G mutation of hPKCα, previously identified in a subpopulation of human tumors, induces the loss of this selective targeting. The D294G mutant is instead targeted to the entire plasma membrane, including the cell-cell contacts, and the duration of the first rapid and transient translocation induced by TRH (42) is longer than that of the wild-type enzyme (93.3 versus 22.5 s), coinciding with the duration of the [Ca2+]i increase. We found that in the presence or absence of PMA, RACK1 is never localized at the cell-cell contacts nor was it coimmunoprecipitated with hPKCα wild type or the D294G mutant. In contrast, PMA treatment or long-term TRH stimulation resulted in the presence of F-actin and β-catenin at the cell-cell contacts and their exclusion from the rest of the plasma membrane. Upon disruption of the F-actin network with phalloidin or cytochalasin D, wild-type hPKCα translocates but did not accumulate at the plasma membrane and β-catenin did not accumulate at the cell-cell contacts. In contrast, the disruption of the F-actin network affected neither translocation nor accumulation of the D294G mutant. These results show that the presence of PKCα at the cell-cell contacts is a regulated process which depends upon the integrity of both PKCα and the actin microfilament network.
doi:10.1128/MCB.21.10.3351-3363.2001
PMCID: PMC100257  PMID: 11313461

Results 1-5 (5)