Linda D. Martin, Ph.D.
Assistant Professor of Cell Biology
Department of Molecular Biomedical Sciences

B.A.: Goshen College, Goshen, IN
Ph.D.: Indiana University
Predoctoral fellowship: Laboratoire de Génétique Moléculaire du CNRS, Paris, France
Postdoctoral study: Iowa State University; North Carolina State University, College of Veterinary Medicine

Phone: (919) 515-7393
Fax: (919) 515-4237
E-mail: linda_martin@ncsu.edu

Curriculum Vitae

Research Area:

Inflammatory airway disease; Mechanisms of epithelial cell differentiation, injury and repair; Effects of diabetes on the respiratory system; Signal transduction and intracellular oxidants.

Current Research:

My research program is focused on the elucidation of molecular and biochemical mechanisms that affect human airway epithelial cell functions during inflammatory respiratory diseases such as asthma and chronic bronchitis. These investigations emphasize the study of regulated cytokine production by the airway epithelium, and the response of the epithelium to cytokine inflammatory mediators. Current research efforts are concentrated on articulating molecular mechanisms and signaling pathways regulating 1) Interleukin-13 induced mucous cell hyperplasia, and 2) Oxidant-mediated interleukin-6 expression. The main model used in this research has been an in vitro cell culture system that maintains normal human bronchial epithelial (NHBE) cells in their differentiated state (ciliated, mucous, and basal cells). These cells retain functional characteristics similar to those found in vivo while allowing manipulation in vitro. Efforts are also under way to expand the models used in this research program to include complimentary in vivo and in vitro approaches using mice, and to establish methods for studying complex signaling pathways via genetic manipulation of airway epithelial cells. The latter effort involves the use of in vitro cultures of airway epithelial cells derived from knockout mice, and the development of efficient methods for the delivery of DNA to primary epithelial cells.

1) Interleukin-13 Induced Mucous Cell Hyperplasia. Mucous cell hyperplasia is a common lesion associated with inflammatory airway diseases such as asthma, COPD, bronchiectasis and cystic fibrosis. To study the molecular mechanisms and signaling pathways that result in this pathological lesion, we have developed an in vitro mucous cell hyperplasia model by exposing differentiated NHBE cells continuously to interleukin-13 (IL-13) throughout the course of mucociliary differentiation. (IL-13 has previously been shown to be a central cytokine mediating a variety of asthmatic effects in the airways.) This method provides a powerful model that mimics the effects of inflammatory mediators during the injury and repair process that occurs due to environmental insult or inflammation in the airway. Similar to in vivo hyperplasia, continuous IL-13 treatment causes an increase in total secreted mucus and proliferation of the epithelial cells in this model system.

Hypothetical Pathway - Click image to enlarge
Our current studies focus on elucidating the mechanisms of IL-13-induced proliferation of the epithelial cells, and, to date, have led us to the conclusion that IL-13 induces proliferation of airway epithelial cells via activation of the epidermal growth factor receptor (EGF-R) due to the autocrine/paracrine production of transforming growth factor alpha (TGFa). Proliferation ensues through a pathway mediated by phosphatidylinositol 3'-kinase (PI 3'-kinase). In addition, IL-13 also modulates phosphorylation of insulin-receptor substrate 2, an upstream mediator of a number of PI 3' kinase-mediated signaling pathways.

In collaboration with Dr. Kenneth Adler in our department, we have also recently established an in vitro murine tracheal epithelial (MTE) cell culture system allowing us to use "genetically-altered" epithelial cells from knockout or transgenic mice to directly establish the relevance of specific parts of the IL-13-mediated pathway. An additional benefit of the MTE cell system is its ability to be complimented by in vivo experiments.

2) Oxidant-mediated Interleukin-6 Expression. Airway epithelial cells are continuously exposed to oxidative species from the air, pollutants, and the release of exogenous oxidants (ROS) from neutrophils and eosinophils during inflammatory airway diseases. To counter this, these cells have evolved numerous mechanisms that use oxidant-mediated pathways to control cellular processes. We have found that expression of interleukin-6 (IL-6), a proinflammatory cytokine present in the airways of patients with inflammatory respiratory diseases, is regulated by these oxidant-mediated pathways in human airway epithelial cells. Our research in this area focuses on elucidating the molecular mechanisms regulating IL-6 expression in differentiated human airway epithelial cells exposed to exogenous superoxide or TNFa.

Our studies demonstrate that both exogenous oxidants and TNFa can modulate IL-6 expression in NHBE cells via oxidant-mediated pathways. These pathways differ in response to the two stimuli; TNFa causes an increase in IL-6 production, while exogenous superoxide mediates a decrease. While these two responses are both mediated by oxidants, they exhibit temporal differences in the elevation of IL-6 steady-state mRNA levels and the activation of oxidant-sensitive transcription factors NFkB and NFIL-6. Through the use of real-time fluorescence monitoring, we have been able to quantify changes in the very small, but important, amounts of intracellular oxidative species as they react with an intracellular dye following exposure to superoxide or TNFa. The opposite effects on the production of measurable intracellular oxidants by the two stimuli suggest that the two pathways mediating IL-6 expression may involve the production of different amounts or types of oxidative species.

Click image to enlarge
To examine the functional effects of TNFa and superoxide on IL-6 gene expression at the molecular level, we have begun to standardize methods to support the use of IL-6 promoter/reporter gene constructs in primary bronchial epithelial cells. Recently, we have shown that primary NHBE cells, sufficiently differentiated as to mimic the TNFa and superoxide-induced effects on IL-6 expression observed in fully differentiated cultures, can be sufficiently transfected to allow expression of the promoter construct in response to TNFa. Inhibitors of intracellular oxidants were also found to inhibit this TNFa-induced expression. We have also initiated collaborative efforts with Dr. Dan Feldheim and his research group (Department of Chemistry, NCSU) to design and use nanoparticles coated with DNA and various cellular signaling molecules for transfer of nucleic acids into differentiated cells.

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Equipment and Skills:

Students in my laboratory have the opportunity to learn and use a variety of molecular biology techniques (plasmid construction, reporter gene assays, eukaryotic cell transfections, Northern analysis, RT-PCR), biochemical techniques (gel electrophoresis, immunoblotting, enzyme assays, zymography, immunoprecipitation, ELISA) and cell biology techniques (primary tissue culture, immunohistochemistry, cell proliferation assays). Trainees also benefit from interactions with the many respiratory researchers located in Research Triangle area, not only at NCState, but also at Duke University Medical Center, University of North Carolina at Chapel Hill, National Institute for Environmental Health Sciences (NIEHS), Environmental Protection Agency (EPA), CIIT, and numerous pharmaceutical companies. Special events sponsored by these groups include the monthly Visiting Pulmonary Scholars series showcasing research talks by internationally recognized respiratory scientists and local researchers.

Representative Publications:

Martin LD , Macchione M, Bonner JC, Booth BW, Akley NJ, Adler KB: Interleukin-13 induced mucous cell hyperplasia in airway epithelium. In: Salathe M, Satir P, Adler K, Boucher R (eds), "Mucous, Cilia and Mucociliary Interactions." Marcel Dekker, Inc., New York, 2001, pp. 253-263.

Booth BW, Adler KB, Bonner JC, Tournier F, Martin LD : Interleukin-13 induces proliferation of human airway epithelial cells in vitro via a mechanism mediated by transforming growth factor-alpha. Am J Respir Cell Mol Biol 25:739-743, 2001.

Little D, Dean RA, Young KM, McKane SA, Martin LD, Jones SL, Blikslager AT: Phosphatidylinositol-3-kinase (PI3'K) signaling is required for prostaglandin-induced mucosal recovery in ischemia-injured porcine ileum. Am J Physiol Gastrointest Physiol, 248:G46-G56, 2003.

Deshmukh HS, Case LM, Wesselkamper SC, Borchers MT, Martin LD, Shertzer HG, Nadel JA, Leikauf GD: Metalloproteinases mediate Mucin5AC expression by epidermal growth factor receptor activation. Am J Respir Crit Care Med, 171:305-314, 2005.

Willens S, Stoskopf MK, Martin LD, Lewbart GA: Viability of glycerol preserved and cryopreserved anuran skin. In Vitro Cell Dev Biol – Animal, 41:258-263, 2005.

Fischer BM, Cuellar JG, Byrd AS, Rice AB, Bonner JC, Martin LD, Voynow JA. ErbB2 activity is required for airway epithelial repair following neutrophil elastase exposure. FASEB J, 19:1374-1376, 2005.

Lankford SM, Macchione M, Crews AL, McKane SA, Akley NJ, Martin LD: Modeling the Airway Epithelium in Allergic Asthma: IL-13-induced Effects in Differentiated Murine Tracheal Epithelial Cells. In Vitro Cell Dev Biol – Animal, 41:217-224, 2005.

Mexas AM, Hess RS, Hawkins EC, Martin LD: Pulmonary Pathology Associated with Diabetes Mellitus in Cats. J Vet Int Med, 20:47-51, 2006.

Brian N. Chorley, Anne L. Crews, Yuehua Li, Kenneth B. Adler, Michael Minnicozzi and Linda D. Martin. Differential Muc2 and Muc5ac secretion by stimulated guinea pig tracheal epithelial cells in vitro. Respiratory Research 2006, 7:35.
http://respiratory-research.com/content/7/1/35

Booth BW , Sandifer T , Martin EL, Martin LD : IL-13-induced proliferation of airway epithelial cells: mediation by intracellular growth factor mobilization and ADAM17.  Respir Res, 8:51, 2007. [Published as provisional PDF ahead of final publication.]    http://respiratory-research.com/content/8/1/51

Links:

Martin Lab Personnel

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