Human Cytomegalovirus Suppresses Estrogen and Progesterone Receptor Expression in Hormone Receptor-Positive Breast Cancer Cells: Implications for Endocrine Resistance

Background: Hormone receptor-positive breast cancer depends on estrogen and progesterone signaling, yet factors that modulate hormone receptor expression within tumors remain incompletely understood. Human cytomegalovirus (HCMV), a widespread herpesvirus detected in breast tumors, has been associated with reduced expression of estrogen receptor-α (ERα) and progesterone receptor (PR), but a direct causal relationship has not been established. Methods: ER+/PR+ breast cancer cell lines MCF-7 and T47D were infected with HCMV in vitro. ERα and PR protein levels were assessed by immunoblotting, and transcript levels of ESR1 and PGR were quantified by qPCR. To determine whether virus replication was required, parallel experiments used UV-inactivated HCMV. Results: HCMV infection resulted in a marked reduction in ERα and PR protein levels in both cell lines, accompanied by decreased ESR1 and PGR transcript levels by 48 h post-infection. Notably, UV-inactivated HCMV produced a comparable suppression of hormone receptor expression, indicating that viral gene expression and productive replication are not required for this effect. Conclusions: These findings indicate that HCMV exposure suppresses ERα and PR expression in breast cancer cells through a replication-independent mechanism. This effect suggests that viral components or host responses to infection may alter hormone receptor signaling within tumors, with potential implications for hormone receptor signaling and endocrine therapy responsiveness that warrant further investigation. Together, these results identify HCMV as a previously underrecognized modulator of hormone receptor pathways in breast cancer.