Thrombospondin I
(TSP1, TSP-1)

THBS1 (gene locus)

Gene: THBS1 maps to 15q11-qter. The THBS1 message is encoded by 21 exons (Wolf F.W. et al., 1990).
mRNA: size: 5.8 kb. The long 3' UTR of 2166 nucleotides is extremely A/T-rich, particularly in the latter half. It contains 37 TATT or ATTT(A) sequences. Another unusual feature of the 3' UTR of TS mRNA is a stretch of 42 nucleotides of which 40 are thymidines (uridine in the mRNA) including an uninterrupted sequence of 26 thymidines. This region is flanked by two sets of direct repeats suggesting that it may be an insertion element of retrotranscriptional origin (Hennessy S.W. et al., 1989).
Protein: a homotrimeric extracellular glycoprotein with disulfide-linked subunits of MW 180,000. THBS binds thrombin, heparin, sulfatides, fibrinogen, fibronectin, plasminogen, laminin, and type V collagen. It functions in many cell adhesion and migration events including platelet aggregation. The protein is notably involved in normal lung homeostasis (Lawler J. et al., 1998). TSP-1 inhibits angiogenesis, endothelial cell growth, motility and adhesion. Peptides from the type I repeats of TSP1 mimic the adhesive and growth inhibitory activities of the intact protein and specifically interact with heparin and transforming growth factor-beta (TGF beta).

Cell lines:
- It has been shown that breast cancer cells (BCC) use TSP to form aggregates and to attach to human endothelial cells (Incardona F. et al., 1995).

- In MDA-MB-231 BCC, thrombospondin and TGF-beta 1 increased cell-associated uPA and cell-secreted PAI-1, suggesting that thrombospondin and TGF-beta 1 could promote metastasis by increasing uPA-mediated cell invasion, whereas through the action of PAI-1, also protect blood-born tumor emboli from destruction by host fibrinolytic enzymes (Arnoletti J.P. et al., 1995).

- In the BT549 BCC line, the reintroduction of a wild type p53 tumor suppressor gene resulted in the stimulation of the secretion of TSP-1, and as a result the cells lost their angiogenic phenotype and became able to suppress angiogenesis induced by the parental tumor line (Volpert O.V. et al., 1995).

- TSP-1 was shown to promote an increase in tumor cell invasion for MDA-MB-231, SKBR-3, and MCF-7 BCC. TSP-1 had no effect on the invasiveness of the benign cell type MCF-10A. Anti-TSP-1 antibody inhibited TSP-1 promoted invasion (Boyden chamber assay with collagen-coated membranes) in the MDA-MB-231, SKBR-3, and MCF-7 cell lines (Wang T.N. et al., 1996).

- Thrombospondin 1 and type I repeat peptides of thrombospondin 1 were shown to specifically induce apoptosis of endothelial cells, but not of MDA-MB-435S BCC (Guo N. et al., 1997).

- In MDA-MB-231 BCC, uPAR expression (ELISA and Western-blot analysis) was up-regulated by both TSP-1 and TGF-beta 1. The effect of TSP-1 involved its receptor and the activation of TGF-beta 1 by TSP-1. Breast tumor cell invasion (Boyden chamber assay) was up-regulated by both TSP-1 and TGF-beta 1 compared with the control group. Antibodies against uPA or uPAR neutralized the TSP-1- and TGF-beta 1-promoted breast tumor cell invasion (Albo D. et al., 1997).

- In vitro adhesion of BCC to thrombospondin was found to be inhibited by GRGDS peptide (van der Pluijm G. et al., 1997).

Tumors:
- In a study of in 48 fresh specimens of breast carcinoma, TSP stained strongly in the desmoplastic stroma or at the basement membrane associated with the malignant ductal epithelium. Tumor cells abutting these tissues revealed cytoplasmic staining for TSP. Stronger TSP staining was seen in the poorly differentiated tumors. Normal and benign breast tissue showed no TSP staining apart from reactivity in the large distended cysts of fibrocystic disease and faint staining in the stroma of fibroadenomas (Wong S.Y. et al., 1992).

- It has been suggested that high affinity hepatocyte growth factor (HGF)-TSP-1 interactions may mediate the binding of HGF to the breast cancer matrix, and that HGF-TSP-1 interaction may contribute to modulation of angiogenesis (Lamszus K. et al., 1996).

- In a study of >200 samples, high IL-1beta content in invasive carcinomas was significantly associated with higher contents of HGF, VWF, and TSP1, but not TNF alpha (Jin L. et al., 1997).

- Using RT-PCR, it was found that TSP2, like TSP1, was expressed in human breast tissues, and that TSP1 and TSP2 mRNA expression in invasive breast carcinoma not otherwise specified (NOS) was significantly increased compared to that observed in normal and benign tissues. The expression of TSP1 and TSP2 in invasive breast ductal carcinoma NOS did not significantly correlate with any of the prognostic factors studied (tumor size, lymph node status, morphology, and hormone receptor status). However, when the study population was divided according to the quantity of tumor stroma, TSP1 (and possibly TSP2) mRNA expression and microvessel counts in desmoplastic-rich stroma of breast carcinoma NOS were significantly increased compared to those observed in desmoplastic-poor stroma (Bertin N. et al., 1997).

- Sera from patients with breast cancers were evaluated for their capacity to selectively modulate the proliferation of human umbilical vein endothelial cells (ECs); sera from 15 of 78 (19%) breast cancer patients induced human umbilical vein EC growth, whereas sera from 4 of 78 (5%) breast cancer patients inhibited EC proliferation. Growth-stimulatory sera were significantly more frequent among postmenopausal (14 of 53) than premenopausal (1 of 25) breast cancer patients; inhibitory activity was observed in 3 of 25 premenopausal patients versus 1 of 53 postmenopausal individuals. The levels of vascular endothelial growth factor were elevated in about 45% of patients with growth-stimulatory sera, whereas the serum inhibition of EC growth was found to be due, at least in part, to high levels of soluble thrombospondin-1 (Morelli D. et al., 1998).

- Expression of the angiogenic factor VEGF; the VEGF receptors flt-1 and KDR; thrombospondin-1, which has been reported to inhibit angiogenesis; and the stromal components collagen type I, total fibronectin, ED-A+ fibronectin, versican, and decorin was investigated by mRNA in situ hybridization on frozen sections of 113 blocks of breast tissue from 68 patients including 28 sections of breast tissue without malignancy, 18 with in situ carcinomas, 56 with invasive carcinomas, and 8 with metastatic carcinomas. A characteristic expression profile emerged that was remarkably similar in invasive carcinoma, carcinoma in situ, and metastatic carcinoma, with the following characteristics: strong tumor cell expression of VEGF; strong endothelial cell expression of VEGF receptors; strong expression of thrombospondin-1 by stromal cells and occasionally by tumor cells; and strong stromal cell expression of collagen type I, total fibronectin, ED-A+ fibronectin, versican, and decorin. The formation of vascular stroma preceded invasion, raising the possibility that tumor cells invade not into normal breast stroma but rather into a richly vascular stroma that they have induced. Similarly, tumor cells at sites of metastasis appear to induce the vascular stroma in which they grow (Brown L.F. et al., 1999).

Albo D. et al. (1997) Thrombospondin-1 and transforming growth factor-beta l promote breast tumor cell invasion through up-regulation of the plasminogen/plasmin system. Surgery 122, 493-499.
Arnoletti J.P. et al. (1995) Thrombospondin and transforming growth factor-beta 1 increase expression of urokinase-type plasminogen activator and plasminogen activator inhibitor-1 in human MDA-MB-231 breast cancer cells. Cancer 76, 998-1005.
Bertin N. et al. (1997) Thrombospondin-1 and -2 messenger RNA expression in normal, benign, and neoplastic human breast tissues: correlation with prognostic factors, tumor angiogenesis, and fibroblastic desmoplasia. Cancer Res. 57, 396-399.
Brown L.F. et al. (1999) Vascular stroma formation in carcinoma in situ, invasive carcinoma, and metastatic carcinoma of the breast. Clin. Cancer Res. 5, 1041-1056.
Dixit V.M. et al. (1986) Characterization of a cDNA encoding the heparin and collagen binding domains of human thrombospondin. Proc. Natl. Acad. Sci. USA 83, 5449-5453.
Frazier W.A. (1987) Thrombospondin: a modular adhesive glycoprotein of platelets and nucleated cells. J. Cell Biol. 105, 625-632.
Guo N. et al. (1997) Thrombospondin 1 and type I repeat peptides of thrombospondin 1 specifically induce apoptosis of endothelial cells. Cancer Res. 57, 1735-1742.
Hennessy S.W. et al. (1989) Complete thrombospondin mRNA sequence includes potential regulatory sites in the 3' untranslated region. J. Cell. Biol. 108, 729-736.
Incardona F. et al. (1995) Thrombospondin modulates human breast adenocarcinoma cell adhesion to human vascular endothelial cells. Cancer Res. 55, 166-173.
Jin L. et al. (1997) Expression of interleukin-1beta in human breast carcinoma. Cancer 80, 421-434.
Lamszus K. et al. (1996) Scatter factor binds to thrombospondin and other extracellular matrix components. Am. J. Pathol. 149, 805-819.
Lawler J. et al. (1998) Thrombospondin-1 is required for normal murine pulmonary homeostasis and its absence causes pneumonia. J. Clin. Invest. 101, 982-992.
Morelli D. et al. (1998) Evaluation of the balance between angiogenic and antiangiogenic circulating factors in patients with breast and gastrointestinal cancers. Clin. Cancer Res. 4, 1221-1225.
van der Pluijm G. et al. (1997) Attachment characteristics and involvement of integrins in adhesion of breast cancer cell lines to extracellular bone matrix components. Lab. Invest. 77, 665-675.
Volpert O.V. et al. (1995) The modulation of thrombospondin and other naturally occurring inhibitors of angiogenesis during tumor progression. Breast Cancer Res. Treat. 36, 119-126.
Wang T.N. et al. (1996) Thrombospondin-1 (TSP-1) promotes the invasive properties of human breast cancer. J. Surg. Res. 63, 39-43.
Wolf F.W. et al. (1990) Structure and chromosomal localization of the human thrombospondin gene. Genomics 6, 685-691.
Wong S.Y. et al. (1992) Thrombospondin and other possible related matrix proteins in malignant and benign breast disease. An immunohistochemical study. Am. J. Pathol. 140, 1473-1482.