Transient Receptor Potential Channel TRPM8 Agonists Stimulate Calcium Influx and Neurotensin Secretion in Neuroendocrine Tumor Cells

TRPM8 is a member of the melastatin-type transient receptor potential ion channel family. Activation by cold or by agonists (menthol, icilin) induces a transient rise in intracellular free calcium concentration ([Ca²⁺]_i). Our previous study demonstrated that Ca²⁺-permeable cation channels play a role in IGF-1-induced secretion of chromogranin A in human neuroendocrine tumor (NET) cell line BON [Mergler et al.: Neuroendocrinology 2006;82:87–102]. Here, we extend our earlier study by investigating the expression of TRPM8 and characterizing its impact on [Ca²⁺]_i and the secretion of neurotensin (NT). We identified TRPM8 expression in NET BON cells by RT-PCR, Western blotting and immunofluorescence staining. Icilin increased [Ca²⁺]_i in TRPM8-transfected human embryonic kidney cells (HEK293) but not in mock-transfected cells. Icilin and menthol induced Ca²⁺ transients in BON cells as well as in primary NET cell cultures of two different pancreatic NETs as detected by single cell fluorescence imaging. Icilin increased non-selective cation channel currents in BON cells as detected by patch-clamp recordings. This activation was associated with increased NT secretion. Taken together, this study demonstrates for the first time the expression TRPM8 in NET cells and its role in regulating [Ca²⁺]_i and NT secretion. The regulation of NT secretion in NETs by TRPM8 may have a potential clinical implication in diagnosis or therapy.