A role for enhanced integrin and FAK expression in uremia-induced parathyroid hyperplasia

Parathyroid (PT) hyperplasia is a major feature of secondary hyperparathyroidism (SH) in uremia. The transforming growth factor-alpha (TGFalpha) / epidermal growth factor receptor (EGFR)Ethgrowth loop is the main contributor to uremia-induced PT hyperplasia. Since integrin beta1 and focal adhesion kinase (FAK) are known to directly activate cell growth and enhance EGFR-driven growth, these studies examined their contribution to PT hyperplasia in uremia. Western blot analysis was used to measure the expression of EGFR, integrin beta1, and the non-receptor integrin-sensitive FAK, in PT glands from 8 hemodialysis patients with various degrees of SH at the time of the surgery, and in a normal human PT gland. In all patients, PT EGFR expression was higher than in the normal control. Integrin beta1, a direct activator of EGFR-driven growth, was increased in 5 of the 8 hyperplastic glands, whereas 7 out of 8 PT glands showed a marked enhancement in FAK expression, an elevation unrelated to increases in integrin beta1, but directly associated to time in hemodialysis. Similar increases in PT FAK content were observed after 1 month after the onset of uremia by 5/6 nephrectomy in rats. These findings suggest that in kidney disease, the increased PT cell growth driven by enhanced EGFR could be further aggravated through elevations in integrin beta1 and FAK expression.