Correspondence to: Allan L. Goldstein, PhD, Department of Biochemistry and Molecular Biology, The George Washington University School of Medicine and Health Sciences, 2300 I St., NW, Rm. 530, Washington, DC 20037 (e-mail: bcmalg{at}gwumc.edu)
Until recently, little attention has been paid to the nuclear activities of actin, a housekeeping protein, or the actin-sequestering molecule thymosin 4, or to their use as potential targets for antitumor strategies. This lack of attention has been due, in part, to the fact that these molecules are associated primarily with the structural activities of the cytoskeleton and with basic physiologic functions associated with differentiation, adhesion, wound healing, and cell motility. It now appears that actin and some of the molecules that regulate actin are bona fide residents of the nucleus and may play previously unrecognized roles in tumor metastasis and angiogenesis (1).
The findings reported by Cha et al. (2) in this issue of the Journal demonstrate important new activities for thymosin 4, a molecule found in the nucleus as well as the cytoplasm (3), and suggest the need for a paradigm shift in our thinking about its role in metastasis. The article provides compelling evidence that thymosin
4, the major actin-sequestering molecule in eukaryotic cells, plays a key role in facilitating tumor metastasis and angiogenesis. Thymosin
4 thus provides a new target of opportunity for cancer researchers seeking to understand how cancer cells subvert host responses to facilitate metastasis. Thymosin
4 expression is elevated in metastasis, as shown by Cha et al. (2) and others (46) and, as shown by Cha et al., it is associated with an increase in the number of metastatic tumors. Of particular interest is the finding by Cha et al. that overexpression of thymosin
4 is associated with an increase in the expression of a known angiogenic factor, vascular endothelial growth factor (VEGF) (2).
Results of recent studies (1) have firmly established that actin plays an important role in the nucleus in a number of chromatin-modifying complexes and in mRNA processing and export. -actin, for example, is a key component of a mammalian SWI/SNF-like BAF chromatin remodeling complex involved in critical aspects of cellular proliferation, differentiation, and genomic stability. [The first complex of this type was found in yeast, and its name is the acronym for its mating type switching (SWI) and sucrose nonfermenting (SNF) roles (7). BAF is the acronym for BRG1 or BRM-associated factors. BRG1 and BRM are two mammalian genes closely related to SWI. BRG1 binds to zinc finger proteins, and BRM interacts with two ankyrin repeat proteins (8).] The observation that this BAF complex functions as a suppressor complex necessary for the actions of the retinoblastoma protein (1) suggests that actin and actin-sequestering molecules may have important physiologic roles as transcription factors that are vital for angiogenesis and tumorigenesis. Also of interest is the recent study by Wang et al. (4), which reported that overexpression of thymosin
4 in human SW48 colon cancer cells is associated with the activation of E-cadherin/
-catenin complexes, resulting in accumulation of
-catenin in the nucleus and binding of T-cell factor/lymphoid enhancer factor molecules to the c-myc and cyclin D1 promoters.
Thymosin 4 plays a key role in the actin dynamics of the cell by sequestering the bulk of monomeric G-actin (9). Thymosin
4 is a small (43 amino acids) acidic peptide found in most cells (10). It stimulates tissue remodeling, cell differentiation, and cell and tissue healing after injury by mechanisms that have not been well defined (1114). Thymosin
4 has potent chemotactic properties for endothelial cells, and its expression is associated with a decrease in the expression of a number of inflammatory chemokines and cytokines (15). The data reported by Cha et al. (2) suggest that thymosin
4 may also act by increasing VEGF expression in wounds. Topical or systemic administration of thymosin
4 accelerates wound healing and stimulates angiogenesis in rodent models (12,13) and the active site of the peptide associated with its angiogenic properties resides in the actin-binding domain (amino acid residues KTTLET) of the thymosin
4 peptide (14).
The actin-sequestering properties of thymosin 4 were first demonstrated by Safer et al. (16). Subsequently, Huff et al. (17) reported that human platelets activated by thrombin release large quantities of thymosin
4 and factor XIIIa, a transglutaminase that covalently couples thymosin
4 through its lysine groups to a fibrin clot. In terms of angiogenesis, this covalent coupling of thymosin
4 is important because accumulation of this small soluble peptide triggers the migration of endothelial cells to facilitate angiogenesis (18).
To date, more than 20 isoforms of -thymosin have been identified in different species (18). In humans, increases in the levels of three isoformsthymosin
4, thymosin
10, and thymosin
15have been associated with malignancy. Thymosin
15, the newest member of the
-thymosin family, was first identified by Bao et al. (19) in patients with highly metastatic prostate cancer. Thymosin
15 may prove to be a more useful marker than prostate-specific antigen for identifying patients at high risk of prostate cancer metastasis because, unlike prostate-specific antigen, it is not expressed in prostate tissue from noncancerous prostate glands or in tissue obtained from patients without metastatic tumors (20). Elevations of thymosin
4, thymosin
10, and thymosin
15 levels have also been reported in a number of other cancers, including melanoma, breast cancer, thyroid cancer, and fibrosarcomas (46,2022).
The results reported by Cha et al. (2) are potentially clinically significant because they provide new insights into the way malignant cells may use thymosin 4 to stimulate angiogenesis and metastasis. What is clear from this study is that there is a marked increase in the number of blood vessels growing into solid tumors derived from injected melanoma B16-F10 cells that are induced to overexpress thymosin
4 compared with tumors derived from B16-F10 cells that do not overexpress thymosin
4. Moreover, injected melanoma cells that overexpress thymosin
4 produced larger tumors and more metastatic lung nodules than injected melanoma cells that did not overexpress thymosin
4.
It is important to note that thymosin 4 overexpression had no effect on cell growth, invasion, or proliferation or on matrix metalloproteinase activity (2). These results confirm those of an earlier study (11) that suggested that increased levels of thymosin
4 cannot convert a normal cell into a cancer cell. Cha et al. also clearly demonstrated that the increased expression of VEGF, a protein responsible for migration, motility, adhesion, signaling, and other processes needed by normal cells, can be subverted by cells with malignant phenotypes and used to more efficiently carry out specific activities (i.e., angiogenesis and cell migration) associated with malignant progression and growth. However, the study by Cha et al. (2) did not establish whether thymosin
4 alone is angiogenic or whether the angiogenic potential associated with thymosin
4 is actually mediated by VEGF.
The findings of Cha et al. (2), coupled with the newly described role of nuclear actin and actin-related proteins in chromatin remodeling, suggest that thymosin 4 and perhaps other actin-sequestering molecules have a dynamic role in regulating cellular processes associated with tumorigenesis. Thus, these observations are important for cancer researchers and are also far-reaching because they provide molecular oncologists with a potential new pathway and target that could be used to develop novel anticancer therapies to treat tumors with a high metastatic potential.
The results reported by Cha et al. (2) provide a window of opportunity for cancer researchers searching for new targets for antitumor strategies. By using second- or third-generation antisense technologies, including small interfering RNAs (siRNAs), or even antibodies targeted to thymosin 4 isoforms, it should be possible to target cancers with a high metastatic potential and for which angiogenesis may play a vital role. In addition, the study by Cha et al. is an important and novel contribution because it provides cancer researchers with a new view of molecules that regulate actin.
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