Nuclear Receptor Expression and Characterization of the Major Retinoic Acid Target Gene in Melanoma

Malignant melanoma is the primary cause of death from all diseases arising from the skin and it is one of the most resistant tumors to standard treatment options. The increasing incidence of melanoma and its poor prognosis makes the understanding of melanoma cell biology very important. Melanoma progression is a complex, multi-step process, and the molecular events leading to melanoma tumor cell proliferation and invasion still need further examination. This study is focused on measuring the expression and activity of two specific genes and their protein counterparts that are altered during melanoma progression. It also examines the mechanism through which Vitamin A induces melanoma cell differentiation and inhibits tumor cell growth. The first part of this research focused on a family of ligand activated transcription factors, the PPARs (Peroxisome Proliferator-Activated Receptors). I examined the expression and activity of PPARs and the effects of PPARα and γ agonists on the growth of mouse melanocytes and melanoma cells. I found increased expression of PPARα and γ protein levels in B16 mouse melanoma cells compared to mouse melan-a melanocytes. PPARγ-selective agonists, prostaglandin J2, troglitazone and ciglitazone, were able to inhibit growth of both melan-a and B16 cells. The only effective PPARα agonist was leukotriene B4 and it was only able to inhibit melan-a cell growth. PPAR transcriptional activity was examined by reporter gene activity and I found a correlation between activation of the receptor and its ability to inhibit cell growth. The second part of my thesis work examines the expression and function of an indirect target gene of atRA stimulation, AKAP12. This protein functions as a scaffold, assembling a multi-protein complex containing enzymes involved in regulating the activity of various signaling pathways. Results show atRA treatment of mouse and human melanocytes and melanoma cells increased AKAP12 expression, and this change was modulated by the transcription factor AP-1. I found a significant correlation between atRA induction of AKAP12 mRNA and the ability of atRA to inhibit growth. Functional studies using AKAP12 siRNA to decrease the level of this protein indicate that AKAP12 contributes to the ability of melanoma cells to invade and grow in an anchorage-independent manner