Endocrine Abstracts

Background: Due to its cross-talk with estrogen receptor (ER) signalling, increased IGF-I signalling through the IGF-I receptor (IGF-IR) has been postulated as an important factor in the development of tamoxifen resistance in breast cancer.

Aims: To investigate the importance of IGF-I signalling in tamoxifen resistant (TR) and wild-type (WT) MCF-7 breast cancer cells by (i) assessing cell proliferation in response to IGF-I and tamoxifen and (ii) using small interfering RNA (siRNA) to silence IGF-IR expression.

Methods: We have previously established a TR cell line from a parent MCF-7 cell line. (i) WT & TR cells were cultured in media ±50 ng/ml IGF-I ±10⁻⁷ M 4-OH tamoxifen. Cell proliferation was assessed every 24 hours using the MTS assay. (ii) WT & TR cells were transfected with 200 nM IGF-IR-specific siRNA for 15 days. Cell proliferation was assessed every 3 days. Previous optimisation has verified >90% reduction in IGF-IR levels in siRNA transfected cells.

Results: (i) WT cell proliferation showed a significant increase when cultured with IGF-I and decrease when cultured with tamoxifen. In contrast TR cells showed no significant response to IGF-I or tamoxifen (mean cell number ± S.D. expressed as % of media only).

(ii) Silencing of the IGF-IR caused a rapid decline in WT cell number but significantly less effect was seen in the TR cells (47.2±10% vs. 93.4±7% at day 3**, 23.8±8% vs. 59.6±9% at day 9** and 14.0±5% vs. 49.2±7% at day 15**). (*P<0.0001; **P<0.0001).

Conclusions: In contrast to expectation, development of tamoxifen resistance appears to also involve resistance to the effects of IGF-I and IGF-IR signalling. This may reflect the close interaction between the ER and IGF-IR signalling pathways.