Overcoming resistance to chemotherapy and radiation therapy has been a difficult but important goal in the effort to cure cancer. We used gene-expression microarrays to identify differentially expressed genes involved in colorectal cancer resistance to chemotherapy and identified secreted protein, acidic and rich in cysteine (osteonectin) (SPARC) as a putative resistance-reversal gene by demonstrating low SPARC expression in refractory human MIP101 colon cancer cells. We were able to achieve restoration of their radiosensitivity and sensitivity to 5-fluorouracil and irinotecan by reexpression of SPARC in tumor xenografts. Moreover, treatment of mice with SPARC conferred increased sensitivity to chemotherapy and led to significant regression of xenografted tumors. The results show that modulation of SPARC expression affects colorectal cancer sensitivity to radiation and chemotherapy. SPARC-based gene or protein therapy may ameliorate the emergence of resistant clones and eradicate existing refractory clones and offers a novel approach to treating cancer.
Isabella T. Tai, Meiru Dai, David A. Owen, Lan Bo Chen
Submitter: Isabella T. Tai | firstname.lastname@example.org
University of British Columbia
Published June 22, 2005
We thank Dr. Colombe for his interest in our manuscript and for providing us with an opportunity to clarify some of the points raised in his letter.
It was intriguing for us to find this novel role for SPARC in chemotherapy resistance from our Affymetrix microarray analysis. We observed that chemotherapy resistant colorectal cancer cell lines had decreased SPARC expression in comparison to chemotherapy sensitive cell lines and that this phenotype could be reversed by re-introducing SPARC in -vitro and in-vivo. Our observations imply that cancers with decreased SPARC expression following the development of resistance or malignant transformation would be more likely to benefit from higher SPARC levels (either through manipulation of gene expression or exposure to exogenous protein). This would in turn allow these cells to regain a level of SPARC that would provide a more robust response to chemotherapy. In our view, based on our observations, cancers with high levels of SPARC, such as breast cancer, melanoma and gliomas, are unlikely to gain additional benefit from SPARC therapy. However, malignancies with reported low levels of SPARC, such as lung, pancreatic and ovarian cancers are more likely to benefit from such manipulation. Although in our discussion we mentioned that variable SPARC expression exists in other cancers (although not explicitly listed in terms of high or low levels of SPARC), we limited a more extensive description of these studies to malignancies with low SPARC expression (ovarian, lung and pancreatic) as the role of SPARC in these systems would be more representative and similar to ours.In our paper, we also inferred that the tumor microenvironment and the interplay between stromal and tumor-derived SPARC to be important in promoting tumor regression in-vivo, as suggested by Dr. Colombe, and agree that additional studies are required to further evaluate the mechanisms involved in tumor regression in our mouse models.
Submitter: Mario P. Colombo | email@example.com
stituto Nazionale Tumori, Milano, Italy
Published June 7, 2005
I read with great interest the paper by Tai IT and colleagues describing the secreted protein acidic and rich in cysteine (SPARC) that was down regulated in chemo-resistant colon cancer and can revert such resistance if replenished as exogenous protein or through gene transfer(1). While all experiments concur to sustain their conclusion, we feel the findings cannot be generalized.
Considering the available literature, the paper is at least unbalanced in both introduction and discussion. The authors do not mention several reports showing opposite results in different types of tumors. The same gene profile approach that indicated SPARC down regulation in chemo-resistant colon cancers, when applied to breast carcinomas, identified SPARC up-regulation to correlate with poor prognosis (2) (3) (4). Moreover, a signature distinguishing patients with progressive disease from objective response to tamoxifen includes SPARC, among others, in the former group (5).
In general SPARC has been associated with advanced cancer of breast, head and neck(6), stomach(7), prostate(8), esophagus(9) and melanoma(10). Glioma cell lines transfected to express different amounts of SPARC acquire an increasingly malignant phenotype as a function of increased SPARC expression (11). The authors only mention results like those obtained in ovarian carcinomas where SPARC downregulation has been associated with tumor progression (since it is expressed in normal ovary cells and gradually lost as disease progresses) (12).
Moreover, restoration of SPARC expression in ovarian carcinoma cells by gene transduction suppresses tumorigenesis by inducing apoptosis (13). Not in their list, lung adenocarcinoma downregulates SPARC because of promoter methylation during progression (14).
The SPARC antiprolierative effect, called into question to explain why low protein level allow increased cell proliferation and tumor aggressiveness, was apparently lost when a colon carcinoma cell line was transfected to obtain clones over expressing SPARC. A critical point in the Tai et al. research stems from using cell lines selected in vitro for drug resistance as source material for gene profile. The use of cell lines then implies that only SPARC of tumor origin can be analyzed.
In vivo, in addition to the tumor, stromal cells can produce SPARC and, indeed, enhanced SPARC expressed by the stroma correlate with poor prognosis of non-small cell lung carcinoma(15); it might be possible that stroma-derived SPARC compensates for SPARC downregulation in colon cancer cells.
Of interest is the question on whether chemotherapy can down-modulate SPARC expression also in stroma cells. We have analyzed the respective roles of host- and tumor-derived SPARC in wild-type and congenic SPARC-/- mice injected into the mammary fat pad with SPARC-producing mammary carcinoma cells derived from c-erB2 transgenic mice. In contrast to 3LL lung carcinoma that grow faster when injected into SPARC-/- mice (16), we found in the same SPARC-/- mice, although on a different genetic background, reduced tumor growth and impaired vascularization associated with a defect in collagen type IV deposition in the stroma defining the lobular structure of the tumor (17). Chimeric mice expressing SPARC only in bone marrow–derived cells were able to organize peritumoral and perilobular stroma, whereas reciprocal chimeras transplanted with bone marrow from SPARC-/-mice developed tumors with less defined lobular structures, lacking assembled collagen type IV and with a parenchyma heavily infiltrated by leukocytes(17).
These contrasting results can be reconciled considering that 3LL tumors also had reduced collagen, albeit collagen type I, while type IV was the most affected in our model. Differences in the prominent type of collagen and its proteolysis can generate fragments favouring or inhibiting tumor angiogenesis. Indeed, trimmer carboxyl pro-peptide of collagen type I is chemotactic for endothelial cells(18), while collagen type IV degradation by MMP-9 generates tumstatin, which induces apoptosis of proliferating endothelial cells(19).
The bottom line of these mouse experiments not only indicate the important role of tumor stroma but also the need to clearly defining the origin, either from tumor or stroma, of factor that, like SPARC, can be produced by several cell types and that can be differently activated during tumor progression and differently modulated by therapeutic treatments. Not surprisingly, in the absence of SPARC, leukocytes infiltrate the tumor more easily(17) and the immune response is accelerated(20).
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