Tumor suppressor ING4 and estrogen response in ER+ breast cancer cells
Up to 70% of breast tumors express estrogen receptor and require estrogen for growth. Thus, antiestrogen agents such as tamoxifen are effective in treating patients with estrogen receptor-positive (ER+) breast tumors. Despite antiestrogen therapy, however, 30-40% of ER+ breast cancer patients relapse within 15 years, underscoring the necessity of improving therapeutic strategies. Low expression of inhibitor of growth 4 (ING4), a tumor suppressor downregulated in 34% of breast tumors, has been correlated with high rates of recurrence in ER+ breast cancer patients treated with tamoxifen. Additionally, ING4 overexpression has been shown to inhibit estrogen-mediated tumor cell growth. However, the effects of ING4 deletion in ER+ breast cancer cells have not directly been investigated. In order to address the role of ING4 in estrogen response, the T47D ER+ breast cancer cell line was genetically modified to overexpress or delete ING4. The growth rates of cells grown in full serum (FS), charcoal-stripped serum (CSS)(hormone deprivation), and CSS with estrogen (CSS+E2) were compared, and the estrogen dosage response of the ING4-deleted cells was determined across a range of estrogen concentrations (1 fM-1 μM). The results show that the growth rate of the ING4 overexpressor was 40% less than that of the vector control in CSS, characterizing ING4 as a suppressor of estrogen-independent growth, as reflected in previous literature. However, the ING4-deleted cells grew comparably to the vector control, suggesting that ING4 deletion does not enhance estrogen-independent growth. Taken together, our findings indicate that cells cannot surpass basal-level estrogen-independent growth regardless of ING4 expression. In the dosage response experiment, ING4-deleted cells expressed a tenfold greater maximum estrogen dosage response than the vector control, demonstrating that ING4 inhibits estrogen-dependent growth and that cells hyper-respond to estrogen when ING4 is deleted. Therefore, ING4-low ER+ tumors may experience more estrogen-dependent growth and may thus require higher dosage tamoxifen therapy to counter their enhanced estrogen response.