GRK2 and thyroid gland carcinoma: On the other hand, GRK2 attenuates serum- or PDGF-induced proliferation of thyroid cancer cell lines [49] and smooth muscle cells [43], whereas its expression increases MAPK signaling in response to EGF in HEK-293 cells [52] and GRK2 kinase activity is required for IGF-1-triggered proliferation and mitogenic signaling in osteoblasts [53] (Figures 1 and 2).