Regulation of CD38 expression in human airway smooth muscle (HASM) cells.

Abstract

CD38 is a multifunctional enzyme-cum-receptor expressed in a variety of mammalian tissues including airway smooth muscles (ASM). The ADP-ribosyl cyclase activity of CD38 generates cyclic ADP-ribose (cADPR), a Ca2+ mobilizing agent in ASM cells. In vivo studies in mice showed that CD38 plays an important role in the development of airway hyperresponsiveness (AHR) following exposure to cytokines. In vitro studies demonstrated that a variety of cytokines, including the inflammatory cytokine TNF-alpha, induce CD38 expression in human ASM (HASM) cells. Studies also showed that the TNF-alpha-induced CD38 expression in HASM cells is mediated through both the transcriptional and post-transcriptional mechanisms and involves activation of mitogen-activated protein kinases (MAPKs) and transcription factors NF-kappaB and AP-1. The role of CD38 in the pathogenesis of AHR in human asthmatics is not known. The current studies demonstrate that HASM cells isolated from asthmatic patients show differentially elevated CD38 expression following TNF-alpha exposure compared to non-asthmatic HASM cells. Basal and TNF-alpha-induced activation of extracellular signal- regulated kinase (ERK) and p38 MAPK are elevated in asthmatic HASM cells, whereas the TNF-alpha-induced activation of c-jun N terminal kinase (JNK) is elevated in non-asthmatic HASM cells compared to asthmatic cells. The TNF-alpha-induced NF-kappaB activation is elevated in asthmatic HASM cells, indicating that the differentially elevated CD38 expression in asthmatic HASM cells is mediated through transcriptional mechanisms. The role of cross-talk between ERK and phosphatidylinositol 3-kinase/Akt (PI3K/Akt) pathways in TNF-alpha-induced CD38 expression is investigated in the current studies. Two pharmacological inhibitors of PI3K, LY294002 and wortmannin, show differential effects on the TNF-alpha-induced CD38 expression in HASM cells. Transient expression of PI3K catalytic subunit (p110) elevates CD38 expression while transfection with phosphatase tensin homolog (PTEN) attenuates the TNF-alpha-induced CD38 expression, suggesting that PI3K/Akt pathway mediates CD38 expression in HASM cells in a non-ERK dependant manner. The role of adenylate-uridylate-rich elements (AREs) of the CD38 mRNA 3' untranslated region (3'UTR) in CD38 mRNA stability is currently being investigated. Preliminary findings show that, in HASM cells, the RNA-binding proteins HuR and TIA-1 selectively bind to an ARE of CD38 3'UTR in response to TNF-alpha-exposure, suggesting a potential role for the CD38 mRNA AREs in the post-transcriptional regulation of CD38 expression in HASM cells.