Induction of the CLOCK Gene by E2-ER[alpha] Signaling Promotes the Proliferation of Breast Cancer Cells

Growing genetic and epidemiological evidence suggests a direct connection between the disruption of circadian rhythm and breast cancer. Moreover, the expression of several molecular components constituting the circadian clock machinery has been found to be modulated by estrogen-estrogen receptor [alpha] (E2-ER[alpha]) signaling in ER[alpha]-positive breast cancer cells. In this study, we investigated the regulation of CLOCK expression by ER[alpha] and its roles in cell proliferation. Immunohistochemical analysis of human breast tumor samples revealed high expression of CLOCK in ER[alpha]-positive breast tumor samples. Subsequent experiments using ER[alpha]-positive human breast cancer cell lines showed that both protein and mRNA levels of CLOCK were up-regulated by E2 and ER[alpha]. In these cells, E2 promoted the binding of ER[alpha] to the EREs (estrogen-response elements) of CLOCK promoter, thereby up-regulating the transcription of CLOCK. Knockdown of CLOCK attenuated cell proliferation in ER[alpha]-positive breast cancer cells. Taken together, these results demonstrated that CLOCK could be an important gene that mediates cell proliferation in breast cancer cells.