Stimulation of the chemosensory TRPA1 cation channel by volatile toxic substances promotes cell survival of small cell lung cancer cells.

TRPA1, a member of the transient receptor potential (TRP) family of cation channels, has mainly been characterized as a chemosensory protein in neuronal cells. TRPA1 is activated by toxic or irritating volatile agents like allyl isothiocyanate (AITC), tear gas, formalin, or cigarette smoke. To date, little is known about a function of TRPA1 in non-neuronal cells in the respiratory system and even less regarding a possible role in cancer biology. Here, we show that TRPA1 is expressed in a panel of human small cell lung cancer (SCLC) cell lines. Of note, TRPA1 mRNA was also significantly higher expressed in tumor samples of SCLC patients as compared to non-SCLC tumor samples or non-malignant lung tissue. Stimulation of SCLC cells with AITC led to a rise of the intracellular calcium concentration. This calcium response was inhibited by TRPA1 antagonists. Furthermore, AITC or formalin stimulated ERK1/2 in TRPA1-expressing HEK293 cells and in SCLC cells via a Src- and calcium-dependent mechanism. More importantly, TRPA1 activation in SCLC cells prevented apoptosis induced by serum starvation and thus promoted cell survival, an effect which could be blocked by inhibition of TRPA1 or ERK1/2. Vice versa, down-regulation of TRPA1 severely impaired anchorage-independent growth of SCLC cells. Since TRPA1 appears to play a pivotal role for cell survival in SCLC cells we propose that this channel could represent a promising target for therapeutic interventions. Furthermore, our data suggest that exogenous, inhalable activators of TRPA1 could be able to exert tumor promoting effects in SCLC cells.

Melanocortin-induced PKA activation inhibits AMPK activity via ERK-1/2 and LKB-1 in hypothalamic GT1-7 cells.

α-Melanocyte-stimulating hormone (α-MSH)-induced activation of the melanocortin-4 receptor in hypothalamic neurons increases energy expenditure and inhibits food intake. Active hypothalamic AMP-activated protein kinase (AMPK) has recently been reported to enhance food intake, and in vivo experiments suggested that intrahypothalamic injection of melanocortins decreased food intake due to the inhibition of AMPK activity. However, it is not clear whether α-MSH affects AMPK via direct intracellular signaling cascades or if the release of paracrine factors is involved. Here, we used a murine, hypothalamic cell line (GT1-7 cells) and monitored AMPK phosphorylation at Thr(172), which has been suggested to increase AMPK activity. We found that α-MSH dephosphorylated AMPK at Thr(172) and consequently decreased phosphorylation of the established AMPK substrate acetyl-coenzyme A-carboxylase at Ser(79). Inhibitory effects of α-MSH on AMPK were blocked by specific inhibitors of protein kinase A (PKA) or ERK-1/2, pointing to an important role of both kinases in this process. Because α-MSH-induced activation of ERK-1/2 was blunted by PKA inhibitors, we propose that ERK-1/2 serves as a link between PKA and AMPK in GT1-7 cells. Furthermore, down-regulation of liver kinase B-1, but not inhibition of calcium-calmodulin-dependent kinase kinase-ß or TGFß-activated kinase-1 decreased basal phosphorylation of AMPK and its dephosphorylation induced by α-MSH. Thus, we propose that α-MSH inhibits AMPK activity via a linear pathway, including PKA, ERK-1/2, and liver kinase B-1 in GT1-7 cells. Given the importance of the melanocortin system in the formation of adipositas, detailed knowledge about this pathway might help to develop drugs targeting obesity.