Boyd K. Carr

A Functional Role for Mitochondrial Protein Kinase Cα in Bcl2 Phosphorylation and Suppression of Apoptosis

Phosphorylation of Bcl2 at serine 70 may result from activation of a classic protein kinase C (PKC) isoform and is required for functional suppression of apoptosis by Bcl2 in murine growth factor-dependent cell lines (Ito, T., Deng, X., Carr, B., and May, W. S. (1997) J. Biol. Chem. 272, 11671–11673). Human pre-B REH cells express high levels of Bcl2 yet remain sensitive to the chemotherapeutic agents etoposide, cytosine arabinoside, and Adriamycin. In contrast, myeloid leukemia-derived HL60 cells express less than half the level of Bcl-2 but are >10-fold more resistant to apoptosis induced by these drugs. The mechanism responsible for this apparent dichotomy appears to involve a deficiency of mitochondrial PKCα since 1) HL60 but not REH cells contain highly phosphorylated Bcl2; 2) PKCα is the only classical isoform co-localized with Bcl2 in HL60 but not REH mitochondrial membranes; 3) the natural product and potent PKC activator bryostatin-1 induces mitochondrial localization of PKCα in association with Bcl2 phosphorylation and increased REH cell resistance to drug-induced apoptosis; 4) PKCα can directly phosphorylate wild-type but not phosphorylation-negative and loss of function S70A Bcl2 in vitro; 5) stable, forced expression of exogenous PKCα induces mitochondrial localization of PKCα, increased Bcl2 phosphorylation and a >10-fold increase in resistance to drug-induced cell death; and (6) PKCα-transduced cells remain highly sensitive to staurosporine, a potent PKC inhibitor. Furthermore, treatment of the PKCα transformants with bryostatin-1 leads to even higher levels of mitochondrial PKCα, Bcl2 phosphorylation, and REH cell survival following chemotherapy. While these findings strongly support a role for PKCα as a functional Bcl2 kinase that can enhance cell resistance to antileukemic chemotherapy, they do not exclude the possibility that another Bcl2 kinase(s) may also exist. Collectively, these findings identify a functional role for PKCα in Bcl2 phosphorylation and in resistance to chemotherapy and suggest a novel target for antileukemic strategies.

CERAMIDE INDUCES BCL2 DEPHOSPHORYLATION VIA A MECHANISM INVOLVING MITOCHONDRIAL PP2A

Phosphorylation of Bcl2 at serine 70 is required for its potent anti-apoptotic function. We have recently shown that Bcl2 phosphorylation is a dynamic process that involves the protein kinase C α and protein phosphatase 2A (PP2A) (Ruvolo, P. P., Deng, X., Carr, B. K., and May, W. S. (1998) J. Biol. Chem. 273, 25436–25442; and Deng, X., Ito, T., Carr, B. K., Mumby, M. C., and May, W. S. (1998) J. Biol. Chem. 273, 34157–34163). The potent apoptotic agent ceramide can activate a PP2A, suggesting that one potential component of the ceramide-induced death signal may involve the inactivation of Bcl2. Results indicate that C2-ceramide but not inactive C2-dihydroceramide, was found to specifically activate a mitochondrial PP2A, which rapidly and completely induced Bcl2 dephosphorylation and correlated closely with ceramide-induced cell death. Using a genetic approach, the gain-of-function S70E Bcl2 mutation, which mimics phosphorylation, fails to undergo apoptosis even with the addition of high doses of ceramide (IC50 > 50 μm). In contrast, cells overexpressing exogenous wild-type Bcl2 were sensitive to ceramide at dosages where PP2A is fully active and Bcl2 would be expected to be dephosphorylated (IC50 = 14 μm). These findings indicate that in cells expressing functional Bcl2, the mechanism of death action for ceramide may involve, at least in part, a mitochondrial PP2A that dephosphorylates and inactivates Bcl2.

Long-term stable expression of human apolipoprotein A-I mediated by helper-dependent adenovirus gene transfer inhibits atherosclerosis progression and remodels atherosclerotic plaques in a mouse model of familial hypercholesterolemia

Background—Epidemiologic studies and transgenic mouse experiments indicate that high plasma HDL and apolipoprotein (apo) A-I protect against atherosclerosis. We used helper-dependent adenovirus (HD-Ad) gene transfer to examine the effect of long-term hepatic apoA-I expression on atherosclerotic lesion progression and remodeling in a mouse model of familial hypercholesterolemia. Methods and Results—We treated LDL receptor–deficient (LDLR−/−) mice maintained on a high-cholesterol diet for 6 weeks with either a HD-Ad containing human apoA-I gene (HD-Ad-AI) or saline (control). HD-Ad-AI treatment did not affect plasma liver enzymes but induced the appearance of plasma human apoA-I at or above human levels for the duration of the study. Substantial amounts of human apoA-I existed in lipid-free plasma. Compared with controls, HDLs from treated mice were larger and had a greater inhibitory effect on tumor necrosis factor-&agr;–induced vascular cellular adhesion molecule-1 expression in cultured endothelial cells. Twenty-four weeks after injection, aortic atherosclerotic lesion area in saline-treated mice progressed ≈700%; the rate of progression was reduced by >50% by HD-Ad-AI treatment. The lesions in HD-Ad-AI–treated mice contained human apoA-I that colocalized mainly with macrophages; they also contained less lipid, fewer macrophages, and less vascular cellular adhesion molecule-1 immunostaining but more smooth muscle cells (&agr;-actin staining) and collagen. Conclusions—HD-Ad-AI treatment of LDLR−/− mice leads to long-term overexpression of apoA-I, retards atherosclerosis progression, and remodels the lesions to a more stable-appearing phenotype. HD-Ad–mediated transfer of apoA-I may be a useful clinical approach for protecting against atherosclerosis progression and stabilizing atherosclerotic lesions associated with dyslipidemia in human patients.

Aldosterone synthase gene regulation by angiotensin.

Excessive aldosterone secretion in some hypertensive patients may result from abnormal aldosterone synthase (AS) gene regulation in response to changes in dietary sodium intake. We have utilized NCI-H295 cells, which exhibit stable angiotensin-induced aldosterone secretion, for transient transfections with murine AS/human growth hormone reporter constructs. An angiotensin response element increasing AS gene transcription during angiotensin stimulation appears to reside within the initial 425 nt of the murine AS promoter. We also noted the possible presence of a negatively-acting cis element between nt -425 and -1500. These studies provide an initial step toward characterizing molecular mechanisms by which angiotensin regulates AS gene transcription.

Inability of avian myeloblastosis virus RNA-directed DNA polymerase to transcribe poly(rU) due to instability of the oligo(dA): Poly(rU) complex

Abstract Avian myeloblastosis virus DNA polymerase is unable to transcribe poly(rU). When the random copolymer, poly(rI,rU), is used as template and oligo(dC)12–18 is used as primer, there is good incorporation of both dCMP and dAMP into product. In contrast, when oligo(dA)12–18 is used as primer, transcription is poor. This demonstrates that the problem with oligo(dA) as primer is in the initiation of DNA synthesis. This result, together with the observation of Martin and Tinoco that (dA:rU) duplexes are very unstable, suggests that it is the instability of the primer: template complex which results in the lack of transcription of poly (rU).

RNA-Directed DNA Polymerase Activity in Human Breast Cancer Biopsy Specimens

The expression of a mouse mammary tumor virus is inducible by hormones, and the virus contains a hormone-responsive element. Viral particles and RNA-directed DNA polymerase (RDDP, EC 2.7.7.7; reverse transcriptase) are both detectable in human breast tumors but the frequency and significance of these findings are unknown. Breast tumor biopsy specimens (from either the primary site or a metastasis), frozen in liquid nitrogen at the time of surgery, were routinely obtained to determine estrogen receptor (ERP) concentration. A sample of the tissue was pulverized, homogenized and centrifuged at low speed to remove nuclei and mitochondria. The supernate was then centrifuged at 225,000 g to obtain the cytosol fraction for estrogen and progestin receptor (PgR) assays. Partially purified membranes for the RDDP assays were prepared from the high-speed pellet by discontinuous sucrose density gradient centrifugation. The RDDP assay involved measuring primer-dependent poly(dT) synthesis in the presence of poly(A) as template and oligo-(dT)12-18 as primer. To date, we have studied biopsy specimens from 46 patients with breast cancer. 27 (59%) had ERP and 23 (50%) were RDDP-positive. There was no significant correlation between ERP concentration and RDDP activity. PgR data were available on 36 of the patients; 17 (47%) were positive. No correlation between RDDP and PgR was apparent. Similarly, there was no correlation between RDDP and clinical stage of the disease.

Template Specificities of a RNA-Directed DNA Polymerase from a Human Homologous Mixed Mesodermal Sarcoma

A RNA-directed DNA polymerase was partially purified from a human homologous, mixed mesodermal sarcoma by DEAE-cellulose chromatography after sucrose density centrifugation. The enzyme transcribed poly(rA) most effectively but also transcribed poly(rI), poly(dA) and poly(rG) and to a lesser extent, poly(rmC). It was unable to transcribe poly(rU). The product with poly(rA) as template contained large material (greater than 28S) in addition to some proper size product demonstrating a slippage reaction. This pattern of transcription, while similar to avian myeloblastosis virus DNA polymerase, reveals qualitative differences making direct extrapolation from studies with animal oncornaviruses to human cancer difficult. In this paper, the detection and purification of RNA-directed DNA polymerase from a patient with an uncommon uterine sarcoma is reported along with the template specificities of the enzyme.