MAP kinase 1=2 (Erk 1=2) and serine=threonine specific protein kinase Akt=PKB expression and activity in the human corpus cavernosum F Sommer 1 *, T Klotz 1 D Steinritz 2 , A Schmidt 2 , K Addicks 2 , U Engelmann 1 and W Bloch 2 1 Department of Urology, University Medical Centre of Cologne, Cologne, Germany; and 2 Institute I of Anatomy, University Medical Centre of Cologne, Cologne, Germany Nitric oxide (NO) is an important mediator in the cavernosal smooth muscle relaxation that causes erections. The purpose of this study was to examine the existence, distribution and phosphoryla- tion stage of two recently discovered key enzymes for NO regulation in human cavernosal tissue, the MAP Kinase 1=2 (Erk 1=2) and the serine=threonine specific protein kinase Akt=PKB. The expression of the enzymes was examined in corpus cavernosum specimens taken from both potent men and from patients with long-term impotence. There was a distinct difference in the activation stage of the MAP Kinase 1=2 (Erk 1=2) between endothelium and smooth muscle cells in potent patients. This finding gives evidence for a cell-type-specific regulation of the eNOS-dependent NO release. Furthermore, we found a higher basal level of active MAP Kinase 1=2 (Erk 1=2) in impotent patients. This finding gives the first evidence for an inhibitory influence of MAP Kinase 1=2 (Erk 1=2) on cavernosal eNOS activity. International Journal of Impotence Research (2002) 14, 217–225. doi:10.1038=sj.ijir.3900856 Keywords: corpus cavernosum; MAP Kinase 1=2 (Erk 1=2); serine=threonine specific protein kinase Akt=PKB Introduction Penile erection is a hemodynamic process, invol- ving increased arterial inflow and restricted venous outflow, in coordination with corpus cavernosum smooth muscle relaxation. 1 Although this process is generally accepted to be under neuroregulatory control, 2 biochemical mediators released locally from the endothelium and=or smooth muscle also participate in initiating and maintaining erection. 3 Nitric oxide (NO), which is produced both in cavernosal nerves and the endothelium, has been recognized as playing a key role in the physiology of penile erection. 2 Additionally, it has been shown that endothelial NO synthase (eNOS) is one of the main sources of NO in the cavernosal tissue, which is then available in endothelial cells and cavernosal smooth muscle cells. 4 Nitrinergic innervation and eNOS expression have shown a broad heterogeneity; up until now, no correlation between eNOS expres- sion and erectile function has been observed. 5 Recently it was shown that the NO release through eNOS is modulated by phosphorylation of the enzyme, mediated by MAP Kinase 1=2 (Erk 1=2) and serine=threonine specific protein kinase Akt=PKB. 6,7 It has been demonstrated that the serine=threonine specific protein kinase Akt=PKB enzyme plays an important role in the activation of eNOS. Mimicking the phosphorylation of Ser 1177 directly enhances enzyme activity and alters the sensitivity of the enzyme to Ca 2þ , rendering max- imum enzyme activity at sub-physiological concen- trations of Ca 2þ . Thus, phosphorylation of eNOS by serine=threonine specific protein kinase Akt=PKB represents a novel Ca 2þ -independent regulatory mechanism for the activation of eNOS. Constitu- tively active serine=threonine specific protein ki- nase Akt=PKB stimulates the phosphorylation of eNOS, but the inactive kinase does not. As only a few substrates for the serine=threonine specific protein kinase Akt=PKB have been described so far, the results reported identify eNOS as a novel Akt target. 6 While active Akt=PKB increases the eNOS- mediated NO release, the activation of MAP Kinase 1=2 (Erk 1=2) leads to a reduction of eNOS activity through an as yet unknown phosphorylation pro- cess. A direct interaction between these kinases seems unlikely because the inactivation of the putative Akt=PKB phosphorylation state does not *Correspondence: F Sommer, Klinik und Poliklinik fu ¨r Urologie der Universita ¨t zu Ko ¨ ln, Joseph-Stelzmann-Str. 9, D-50924 Ko ¨ ln, Germany. E-mail: Frank.Sommer@uni-koeln.de Received 1 June 2001; revised 21 November 2001; accepted 29 January 2002 International Journal of Impotence Research (2002) 14, 217–225 ß 2002 Nature Publishing Group All rights reserved 0955-9930/02 $25.00 www.nature.com/ijir