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Heparin-binding EGF-like growth factor expression increases selectively in bladder smooth muscle in response to lower
1Department of Urology, Children's Hospital, Harvard Medical School, Boston, Massachusetts, USA.
Abstract:
Heparin-binding epidermal growth factor-like growth factor (HB-EGF), an activating ligand for the epidermal growth factor receptor (ErbB1) tyrosine kinase and at least one isoform of the ErbB4 receptor tyrosine kinase, is synthesized by the smooth muscle of the human bladder wall. In this study we tested the hypothesis that HB-EGF plays a role in the bladder-wall thickening that occurs in response to obstructive syndromes affecting the lower urinary tract, possibly by acting as an autocrine smooth muscle cell (SMC) growth factor. HB-EGF was mitogenic for primary culture human bladder SMC, and cell growth in serum-containing medium was inhibited more than 70% by [Glu52]-diphtheria toxin/CRM197, a specific HB-EGF inhibitor, consistent with a physiologic role for HB-EGF as an autocrine bladder SMC mitogen. Human and mouse bladder SMC in vivo and cultured human bladder SMC expressed the primary HB-EGF receptor, ErbB1, but not mRNA for the secondary HB-EGF receptor, ErbB4, thereby identifying ErbB1 as the cognate HB-EGF receptor in the bladder wall. Reverse transcription-polymerase chain reaction analysis also demonstrated ErbB2 and ErbB3 expression in human bladder muscle tissue, suggesting the possibility of receptor cross-talk after ErbB1 activation. Urethral ligation in mice resulted in an increase in steady-state HB-EGF mRNA expression up to 24 hours in whole bladder tissue in comparison with ErbB1 and glyceraldehyde 3-phosphate dehydrogenase mRNA levels, which did not change in a demonstrable pattern. HB-EGF protein increased coordinately with HB-EGF mRNA levels. Dissection of bladder tissue into muscle and mucosal layers demonstrated that the increase in HB-EGF mRNA occurred predominantly in the muscle layer, with peak levels (13-fold higher than sham controls) occurring 12 hours after obstruction. These data support a physiologic role for HB-EGF as a mediator of hypertrophic bladder tissue growth.