Novel roles of GRKs and arrestins in cell cycle regulation and breast cancer
G protein-coupled receptor kinases (GRKs) and arrestins are best known for their role in homologous desensitization of G protein-coupled receptors (GPCRs). However, it is now known that in addition to regulating GPCR activity, GRKs and arrestins also possess additional functions, such as the ability to regulate a number of non-receptor substrates and act as molecular signaling scaffolds, respectively.
Observations to uncover the endogenous localization of arrestin revealed that arrestins are constitutively associated with the centrosome, the principle microtubule organizing center of the cell, throughout all stages of mitosis. Through immunofluorescence microscopy, analysis of purified centrosome preparations, and in vitro binding assays, it was determined that arrestin2 and 3 colocalized with the centrosomal marker γ-tubulin and also directly interacted with γ-tubulin. Furthermore, through siRNA depletion studies and rescue experiments it was found that arrestin2 and arrestin3 were required for normal centrosome function. Additionally, overexpression of arrestin2 or arrestin3 in the breast cancer cell line MDA-MB-231, which is known to display high centrosomal abnormalities, partially rescued the centrosomal defect of multinucleation.
Centrosome amplification is a common occurrence in many human cancers, such as breast cancer, and is associated with the development of chromosomal instability and aneuploidy. In addition, it is now well recognized that cancer cells often manipulate normal GPCR physiology, such as to promote growth, avoid immune response, and metastasize. Therefore, changes in arrestin expression in cancer could promote tumor growth either through deregulation of normal centrosome function or contribute to aberrant GPCR signaling. Analysis of several breast cancer cell lines revealed that more aggressive basal-like lines expressed less arrestin2 than luminal lines, while arrestin3 expression was comparable. In addition, arrestin2 expression in primary breast tumors decreased with disease progression and was associated with decreased survival, increased tumor size and positive lymph node status. In contrast, arrestin3 expression increased during breast cancer progression and was associated with decreased survival. Arrestin3 was also an independent prognostic marker of breast cancer. Whether these alterations in arrestin expression reflect an increase in centrosomal abnormalities or aberrant GPCR signaling remains to be determined.
Since GRKs and arrestins work hand in hand to regulate GPCR function, we wanted to determine whether GRKs also localized to and regulated centrosome function. Both GRK2 and GRK5 colocalized with the core centrosomal components, such as γ-tubulin, and their centrosomal localization varied with the cell cycle. siRNA and shRNA depletion of either kinase resulted in a G2/M arrest and expression of wild type but not the catalytically inactive GRK2 or GRK5 into GRK2-shRNA and GRK5-shRNA cells respectively, partially rescued the G2/M delay. Centrosome duplication was not affected by loss of GRK2 or GRK5 expression, but loss of GRK5 did impair the microtubule nucleation capacity of the centrosome.
Taken together, these studies reveal that arrestins and GRKs are novel centrosomal components that regulate centrosome function and cell cycle progression and suggest that alteration in their expression, as with arrestin in breast cancer, may promote cancer development and/or progression.