The major findings to emerge from this study are: (1) increased expression of SPARC by ovarian tumor cells inhibits cell growth, in vitro; (2) host-derived SPARC may reduce tumor volume and improve survival in the absence of therapy; (3) cisplatin therapy is more efficacious in tumor-bearing SPARC-deficient animals than wild-type mice.
SPARC is a matricellular glycoprotein that influences tumor cell interaction with the extracellular matrix.1,2
Differentiating the function of tumor-associated and host-derived SPARC is a challenge given that SPARC can be secreted by and can act on many different cells in the tumor 4microenvironment. Some studies have found tumor-associated SPARC to be an important modulator of tumor progression and metastasis,19
whereas others have found host-derived SPARC to have a greater influence.1,20
We found that increasing tumor-associated SPARC diminished ovarian cancer cell growth in vitro. Even in cell lines already expressing SPARC, forcing additional expression, the results were a decrease in cell growth. This is consistent with previous findings by Yiu et al.2
Cell lines overexpressing SPARC have significantly decreased growth with forced induction of SPARC, suggesting a potential for further SPARC induction specifically in SPARC producing ovarian cancers.
Attempts to block SPARC activity in the murine cell line OSEID8 with MAb30315
resulted in improved response to cisplatin treatment, but it had little effect on improving the cisplatin response in the 3 human cell lines. The mechanism(s) underlying the enhanced effect of MAb303 and cisplatin is unclear. Shi et al15
found that MAb303 reduced SPARC-induced activation of AKT in glioma cells. This, however, is counterintuitive, given that increased expression of SPARC by OSEID8 cells reduces cell number rather than promotes cell survival as would be expected if AKT activation was the primary driver of SPARC effects in ovarian cells.
Another intriguing possibility is SPARC-induced modulation of cell shape or adhesion via inhibition of integrin-collagen interaction. It is possible that MAb303, by binding to SPARC, perturbs OSEID8 cell adhesion, resulting in enhanced cisplatin-mediated cell killing. This is supported by in vitro studies demonstrating that MAb303 can induce cell rounding and deadhesion.14
The use of additional platinum-resistant human ovarian cancer cell lines and other anti-SPARC antibodies such as AON-1, MAb255, or MAb29314
may provide different effects.
To evaluate the effect of host-derived SPARC on chemoresistant ovarian tumor growth and response to chemotherapy, we compared the growth of OSEID8 cells in WT
animals. Our results are similar to the studies of Said and Motamed,3
the results of which showed increased tumor burden in the absence of SPARC. In particular, we found that SP−/−
animals had a decreased time to formation of ascites and a larger tumor burden.
Importantly, we show for the first time, in vivo, that platinum-resistant ovarian tumors have increased sensitivity to chemotherapy in the absence of host-derived SPARC. The mechanism by which SPARC affected the sensitivity of these tumor cells remains to be elucidated. Interestingly, because SPARC is known to participate in ECM deposition, it is postulated that the delivery of chemotherapeutic agents to tumor cells themselves may be improved in the absence of SPARC.
The use of cell lines is controversial because of molecular changes that may occur between generations. Because our goal did not depend on the parent cell lines’ molecular characteristics, we did not search for alterations from the original generation. Additionally, although SPARC is a protein often found in the stroma, we chose to study cell lines to examine cell associated and secreted levels of SPARC and to determine the comparative platinum resistance between the murine cell line, OSEID8, and known platinum-resistant human OSE cell lines.
Additional limitations of this study include the use of MAb303, which can bind bovine serum in the media to a small extent, and the small cohort of animals used for the in vivo experiment. The use of a transient transfection protocol for inducing SPARC expression prevented evaluation of the effect of altered endogenous SPARC on in vitro chemosensitivity, which would require a transfection that could persist several days.
Questions needing further study include examining whether forced expression or knockdown of SPARC is functionally equivalent to the natural expression pattern of SPARC in ovarian cancer and whether SPARC expression correlates with chemosensitivity. Future studies may include transfection with a stable intracellular modifier of SPARC, development of alternative SPARC modulators, and inoculation with human ovarian cancer cell lines for the in vivo model.
Cisplatin resistance in ovarian cancer may be related to alterations in excision repair proteins, decreased drug accumulation because of alteration in cellular transport, and increased capacity to tolerate DNA damage.21,22
Although our study does not elucidate mechanisms by which chemosensitivity is induced in the absence of host SPARC, it does underscore the importance of incorporating the interplay between tumor and host when studying ovarian cancer growth and response to chemotherapy. Our results and those of others reinforce the fact that SPARC is an important factor in regulating ovarian tumor growth.
Our study suggests that SPARC may also modulate the chemosensitivity of ovarian cancer, either as the primary modulator or an effector of an undiscovered modulator. It would be interesting to determine the effect SPARC has on those factors already known to alter chemoresistance, such as tumor suppressor gene products or the cellular matrix action on drug uptake and distribution. Future therapeutic approaches may include immunotherapy directed at inactivating host-SPARC. Further studies are warranted to determine the mechanisms by which SPARC acts. Selective inhibition of SPARC may provide an attractive strategy for increasing the efficacy of chemotherapy in platinum-resistant ovarian tumors.