abstract

Fibropapillomatosis (FP) is a neoplastic disease that induces tumors on soft tissues of marine turtles. Tumors can impede a turtle's ability to see, forage, swim, and internal tumors can interfere with organ function. All species of hard-shelled marine turtles are afflicted by FP, however it is most often found in juvenile green turtles (Chelonia mydas). The etiology of the disease is thought to be complex and multifactorial. Transmission experiments implicate a viral agent and a novel herpesvirus has been detected from tumor tissue, but a number of cofactors are thought to contribute to the development of the disease. Mortality rates are not fully understood, however recapture studies have shown that tumor regression occurs in some instances. Morphological characteristics of tumors range from cauliflower-like, thought to be actively growing; to tumors with a smooth surface, thought to be regressing. To better understand molecular mechanisms involved in FP, this study explores stress protein and anti-apoptotic protein expression in healthy and tumor tissue from juvenile green turtles.

Stress proteins, also known as heat shock proteins (HSPs), act as chaperons and housekeepers in a cell. They assure proper folding and assist in repair or degradation of damaged proteins. Certain HSPs are upregulated by physiological stressors (such as heat, pollutants, and viral infections) to protect a cell from increased protein damage, and some HSPs act as immune stimulants, presenting antigenic peptides from virally infected cells or tumor cells to the immune system.

Apoptosis is defined as programmed cell death occurring at the end of a cell's natural lifespan or may be induced by physiological stress. Under stress, cell survival vs. cell death depends on the balance between pro- and anti-apoptotic proteins. An upregulation of anti-apoptotic proteins has been implicated in some mammalian cancers and contributes to uncontrolled tumor cell growth and differentiation.

This study examines one anti-apoptotic protein, Bcl-2, and two inducible HSPs, HSP72 and GRP96. Samples of healthy and tumor tissue were collected from two locations in Florida, the highly polluted Indian River Lagoon (IRL) and more pristine Trident Submarine Basin (TSB). Turtles with a range of tumor severity and morphologies were included. Tissue proteins were analyzed using Western blots.

Upregulation in HSP72, GRP96, and Bcl-2 was found in tumor tissue. This pattern indicates tumor tissue is more protected from protein damage and apoptosis than healthy tissue. This also implies that at a molecular level FP tumors resemble tumor growth in mammalian systems, revealing potential avenues for future treatment, as both Bcl-2 and HSPs are targets of investigation in human cancer treatment.

There was higher concentration of HSP72 and GRP96 in cauliflower-like tumors compared to healthy tissue and no difference between smooth tumors and healthy tissue. This suggests that cauliflower-like tumors are more protected from apoptosis than smooth tumors. The lower levels of these protective proteins in smooth tumors may contribute to their regression by causing a build-up of damaged proteins, resulting in cell death. Bcl-2 was found in higher concentrations in both smooth and cauliflower-like tumors which would suggest that regression is not a result of the down-regulation of Bcl-2 but as a result of some other mechanism. We speculate that the immunostimulatory characteristics of HSPs may play a role in inducing regression. In order for HSPs to induce an immune response they must be released extracellularly. Necrosis of tumor tissue is often observed and would result in intracelllar HSPs conjugated with tumor peptides to be released into the extracellular matrix. This would allow antigen-presenting cells to recognize the HSPs and initiate a T-cell mediated immune response, resulting in regression (or clearance of genetically similar tumors).