The timing of angiogenic switch during cervical carcinogenesis remains controversial. A debate exists regarding the ability of CIN to induce angiogenesis (Abulafia O et al. 1996; Abulafia O et al. 1999; Smith-McCune KK and Weidner N 1994). Smith-McCune and Weidner found a significant increase of MVD in the CIN III lesions compared with those underlying low grade lesions such as condyloma and CIN I (Smith-McCune KK and Weidner N 1994). On the contrary, reports from Abulafia el al. showing that microinvasive squamous cell carcinoma is angiogenic, but not carcinoma in situ (Abulafia O et al. 1996). In the present study, we intend to examine the slides that contain different severities of cervical lesions in the same slide, so that every lesion can be used as an internal control for each other. Thus, the heterogeneity in the inborn characters of angiogenesis can be eliminated. Our data showed that the angiogenic switch in cervical carcinogenesis occurred during the transition from LSIL to HSIL, and the neovascularization was largely confined to a narrow zone immediately underneath the dysplastic epithelium. This is in concordance with the results from Smith-McCune and Weidner (Smith-McCune KK and Weidner N 1994). and further suggests that cervical carcinogenesis is angiogenesis-dependent.
Our results also showed that TSP-1 was mainly localized on basal cervical epithelial cells, and arrayed like a barrier. We therefore name it as the ‘TSP-1 fence’.
TSP-1 decreases significantly during the transition from LSIL to HSIL, which is concomitant with the increase of MVD counts. The temporal and spatial concordance of TSP-1 down-regulation and the emergence of angiogenic imply that the “TSP-1 fence” may act as an angiogenic barrier to inhibit angiogenesis which occurred in early phase of cervical carcinogenesis. The disappearance of the angiogenic barrier may induce a vigorous angiogenic response for tumor growth
and perhaps tumor metastasis (Sheibani N and Frazier WA 1999). Evidence from Kodama et al. showing that TSP-1 mRNA expression was significantly lower in advanced-stage cervical cancer, and its expression is of value as a prognostic factor in cervical cancer (Kodama J et al. 2001). The origin of TSP-1 is currently unknown, and two possible origins including tumor cells themselves and host cells (endothelial cells) may be responsible for the production (Folkman J 1992). Our data suggest that the basal epithelial cells contribute to the production of TSP-1 under physiological condition, and lose the ability to secrete TSP-1 during the transformation from LSIL to HSIL.
Argument has been raised that our statistical comparisons were made between different lesions, irrespective of the patients. Since more than one category of cervical epithelium may apply to a block and two blocks were taken from each patient, we also analyzed the data as dependent samples. Pairwise comparisons with Wilcoxon signed-rank test further confirm the significant decrease of TSP-1 and elevation of MVD during the transition from LSIL to HSIL.
Accumulating evidence indicates that for most tumors, the switch to the angiogenic phenotype depends upon the outcome of a balance between angiogenic stimulators and angiogenic inhibitors (Folkman J 1992). Up-regulation of angiogenesis activators alone may not be enough for the emergence of angiogenic switch; it accompanies with down-regulation of some angiogenesis inhibitors in the same time (Folkman J 1995b). TSP-1 and VEGF appear to be the constituents of a "switch" that regulates in concert many components of the angiogenic and differentiated phenotypes of endothelial cells (Sheibani N and Frazier WA 1999). In skin cancer model, down-regulation of TSP-1 and up-regulation of VEGF happened coincidently and had spatial correlation throughout the consecutive stages of tumorigenesis (Hawighorst T et al. 2001). In
contrast, down-regulation of TSP-1 secretion is a key event in the switch from anti-angiogenic to an angiogenic phenotype, while VEGF seems to play little role in bladder cancer (Campbell SC et al. 1998). VEGF has been found to increase significantly in high-grade cervical intraepithelial lesions as compared with low-grade intraepithelial lesions and benign epithelium (Guidi AJ et al. 1995). We here suggest that TSP-1, as one of the endogenous angiogenic inhibitors, may play the comparable role to other angiogenic activators such as VEGF in the angiogenesis balance during cervical carcinogenesis.
The angiogenic response, which is induced by the disappearance of “TSP-1 fence”, modulates the peri-cellular environment, and can potentially change the cell-matrix interactions associated with cell movement and further progression.
TSP-1 does not appear to contribute directly to the structural integrity of connective tissue elements. Instead, TSP-1 acts by modulating the activity and bioavailability of protease and growth factors and by interaction with cell-surface receptors (Bornstein P et al. 2000b; Stetler-Stevenson WG 1999). MMPs have been shown to play an active role in the neovascularization of tumors through their ability to degrade the extracellular matrix (Liotta LA et al. 1982; Zetter BR 1990). Bergers et al. showed that the switch from vascular quiescence to angiogenesis involves MMP-9, which is upregulated in angiogenic islets and tumors, rendering VEGF more available to its receptors (Bergers G et al. 2000). Notably, MMP-9 is negatively modulated by TSP-1. Thus, TSP-1 acts as a multifunctional modulator of angiogenesis by modulating through the activity and bioavailability of MMP-9. How does TSP-1 modulate MMPs deserves for further studies.
Acknowledgement:
This work was supported by a grant from Chi Mei Foundation Hospital (CMFHR No.
9107). We thank Miss Chin-Li Lu, Section of Statistic Consultation, Department of Medical Research, Chi Mei Foundation Hospital, for statistic analytic assistance.
Chapter 4 A novel role of TSP-1 in cervical carcinogenesis: Inhibit stroma