1. ¹Laboratory of Immunology, Gerontology Research Center, National Institute on Aging, Baltimore, MD, USA
2. ²VA Healthcare System, Boston, MA, USA
3. ³MARC scholar, University of Maryland Eastern Shore, Princess Anne, MD, USA
4. ⁴Department of Biology and Marine Biology, University of North Carolina, Wilmington, NC, USA
5. ⁵Research Resources Branch, Gerontology Research Center, National Institute on Aging, Baltimore, MD, USA
6. ⁶Department of Pathology, Loyola University Medical Center, Maywood, IL, USA
7. ⁷Medical Biotechnology Department, VTT Technical Research Centre of Finland, University of Turku, Turku, Finland
8. ⁸Cancer Genetics Branch, National Human Genome Research Institute, Bethesda, MD, USA
9. ⁹Laboratory of Cellular and Molecular Biology, Gerontology Research Center, National Institute on Aging, Baltimore, MD, USA

Correspondence: Dr AT Weeraratna, Laboratory of Immunology, National Institute on Aging, National Institutes of Health, Gerontology Research Center, 5600 Nathan Shock Drive, Box 21, Baltimore, MD 21224, USA. E-mail: weerarat@grc.nia.nih.gov

¹⁰Current address: Nucleonics, Delaware Valley College, DE, USA.

¹¹Current address: Purdue University, West Lafayette, IN, USA.

Received 21 June 2006; Revised 21 August 2006; Accepted 25 August 2006; Published online 11 December 2006.

Abstract

Serial analysis of gene expression followed by pathway analysis implicated the tight junction protein claudin-1 (CLDN1) in melanoma progression. Tight junction proteins regulate the paracellular transport of molecules, but staining of a tissue microarray revealed that claudin-1 was overexpressed in melanoma, and aberrantly expressed in the cytoplasm of malignant cells, suggesting a role other than transport. Indeed, melanoma cells in culture demonstrate no tight junction function. It has been shown that protein kinase C (PKC) can affect expression of claudin-1 in rat choroid plexus cells, and we observed a correlation between levels of activated PKC and claudin expression in our melanoma cells. To determine if PKC could affect the expression of CLDN1 in human melanoma, cells lacking endogenous claudin-1 were treated with 200 nM phorbol myristic acid (PMA). PKC activation by PMA caused an increase in CLDN1 transcription in 30 min, and an increase in claudin-1 protein by 12 h. Inhibition of PKC signaling in cells with high claudin-1 expression resulted in decreased claudin-1 expression. CLDN1 appears to contribute to melanoma cell invasion, as transient transfection of melanoma cells with CLDN1 increased metalloproteinase 2 (MMP-2) secretion and activation, and subsequently, motility of melanoma cells as demonstrated by wound-healing assays. Conversely, knockdown of CLDN1 by siRNA resulted in the inhibition of motility, as well as decreases in MMP-2 secretion and activation. These data implicate claudin-1 in melanoma progression.