Intracellular amphiregulin is critical for heterochromatin maintenance and genomic stability in response to replication stress in BRCA2mut/+mammary epithelial cells
The EGFR ligand, amphiregulin (AREG) is a key mammary ductal cell differentiation and growth factor. AREG has also been detected in the nucleus of some epithelial cancers although the physiological stimulus and nuclear role are not known. Using immortalized mammary epithelial cells (MECs), we have discovered that AREG undergoes retrograde trafficking to the nuclear membrane (nAREG) in close proximity with lamin A where it is required to both maintain constitutive heterochromatin and transiently increases H3K9me3 in response to replication stress (RS). RS resulted in an increase in AREG protein, enhanced nuclear membrane prelamin A and increased heterochromatin protein, HP1. In contrast, siRNA-mediated depletion of endogenous AREG reduced HP1 and SUV39h1 proteins accompanied by decompaction and reduction in H3K9me3 heterochromatin despite the presence of soluble AREG. The nuclear membrane (NM) was also impacted resulting in dissipation of the Ran-GTPase gradient, reduced matrix lamin A with increased invaginations. Moreover, AREG knockdown slowed replication fork speed, increased new replication origins and enhanced global transcription while promoting and exacerbating DNA damage in response to RS. DNA damage was most pronounced in AREG-depleted BRCA2mut/+ MECs which entered senescence following RS, indicating an important nAREG-dependent role in genomic stabilization in these cells. Overall, this study reveals a novel and fundamental role for nAREG in heterochromatin maintenance and the response to RS, that is most critical in BRCA2mut/+ MECs deficient in replication fork protection.