Regulation of Kv Channels by Anorexigens

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Voltage-gated K+ (Kv) channels in pulmonary arterial smooth muscle cells (PASMCs) have been implicated in the initiation of pulmonary hypertension: inhibition of these channels results in membrane depolarization and an increase in intracellular Ca2+ concentration, leading to vasoconstriction and cell growth / remodeling. The use of anorexic agents (phentermine, fenfluramine and their related drugs) is associated with an increased incidence of pulmonary hypertension. These drugs also decrease the activity and expression of Kv channels in PASMCs. Thus, understanding the mechanisms by which these identified stimuli produce alterations in the function and level of these channels may provide clues for prevention and treatment of primary pulmonary hypertension. The anorexic agents reduce Kv channel activity at multiple steps. They acutely inhibit 4-aminopyridine (4-AP)-sensitive Kv current in PASMCs. Furthermore, long-term treatment of PASMCs with fenfluramine leads to decreases in Kv current density and the expression of Kv1.5 mRNA. Lung tissues from patients with primary, but not secondary, pulmonary hypertension also exhibit reduced expression of Kv1.5 mRNA. These findings suggest acute and long-term exposures to these drugs influence 4-AP-sensitive Kv channels at plasma membrane and transcription of Kv channel subunit genes, respectively. Using Xenopus oocyte expression system, we found that fenfluramine and phentermine inhibit Kv1.5, Kv2.1 and Kv4.2, but not Kv3.1b, current. Using cultured rat PASMCs and heterologous expression systems, we have analyzed molecular mechanisms underlying the anorexigen-induced changes in the activity and expression of Kv channels. First, exposure to fenfluramine decreased endogenous Kv2.1 proteins in PASMCs. The drug also reduced heterologously expressed Kv2.1, but not Kv1.5 or Kv4.3, proteins in a mammalian cell line. In addition, the non-selective kinase inhibitor staurosporin mimicked and occluded the fenfluramine-induced decrease in the channel protein level in PASMCs. Second, the anorexic drugs caused significant decreases in the level of endogenous Kv1.5 mRNA and reporter gene expression driven by the Kv1.5 promoter in PASMCs. Reductions in the channel promoter activity were also seen in A7r5 smooth muscle cells, but not in CHO or HEK293 cells. Finally, fenfluramine and phentermine rapidly and reversibly inhibited Kv1.5, Kv2.1 and Kv4.2, but not Kv3.1b, currents in Xenopus oocytes. Thus, the anorexigen-induced pulmonary hypertension may be mediated by their multitude of actions to produce acute and long-term inhibition of PASMC Kv channels. Hence, this proposal is to identify molecular mechanisms for anorexigen-induced inhibition of Kv channels at the three levels: a slow decrease in Kv2.1 proteins, inhibition of Kv1.5 gene transcription and blockade of Kv currents at plasma membrane.