Genotype analysis of prepro-vasopressin signal peptide in vasopressin-producing and -non-producing lung tumors

Inhibition of Ectopic Arginine Vasopressin Production by Phenytoin in the Small Cell Lung Cancer Cell Line Lu-165

Phenytoin, a voltage-gated sodium channel (Na_V channel) antagonist, reportedly inhibits arginine vasopressin (AVP) release from an isolated rat neurohypophysis. So far, it is uncertain whether phenytoin has a direct action on ectopic AVP-producing neuroendocrine tumors. We studied the effect of phenytoin on the release of copeptin, the C-terminal fragment of pro-AVP, and expression of AVP gene in the human small cell lung cancer cell line Lu-165. Cells were maintained in RPMI1640 medium with 10% fetal bovine serum and were used within the fifth passage. Copeptin was detected using a new sandwich immunoassay, and AVP mRNA levels were measured using real-time reverse transcription polymerase chain reaction. Treatment with phenytoin at a concentration of 25 µg/mL, but not at 5 or 10 µg/mL, had an inhibitory effect on copeptin levels in the medium at 48 h. At the same concentration, AVP mRNA levels in Lu-165 cells also decreased. Although a sodium challenge with added sodium at 20 mEq/L increased copeptin levels in the medium, a sodium challenge with added sodium at 10 and 20 mEq/L had no effect on AVP mRNA levels. Phenytoin at a concentration of 25 µg/mL suppressed copeptin levels in the medium under the sodium challenge with added sodium at 10 and 20 mEq/L. Phenytoin at a concentration of 25 µg/mL also decreased AVP mRNA levels in Lu-165 cells under the sodium challenge with added sodium at 10 mEq/L, but not at 20 mEq/L. Among five tested Na_V channel subunits, Na_V1.3 was highly expressed in Lu-165 cells. However, phenytoin significantly decreased Na_V1.3 mRNA levels under the sodium challenge with added sodium at 10 and 20 mEq/L. These results suggest that Lu-165 cells are sensitive to phenytoin and sodium to control of AVP release and its gene expression. Phenytoin might have a direct action on ectopic AVP-producing tumors, suggesting the importance of Na_V channels in AVP-producing neuroendocrine tumors.

Gene regulation of vasopressin and vasopressin receptors in cancer

It is proposed that neuropeptide production by tumours is an important part of a special process of oncogenic transformation rather than a pre-existing condition of progenitor cells; this concept is called Selective Tumour gene Expression of Peptides essential for Survival (STEPS). All small-cell lung cancers and breast cancers evidently express the vasopressin gene, and this gene seems to be structurally normal in all but exceptional cases. Vasopressin gene expression in cancer cells leads to the production of both normal and abnormal forms of tumour vasopressin mRNA and proteins. Although the necessary post-translational processing enzymes are expressed in these cells, most processing seems to be extragranular, and most of the protein products become components of the plasma membrane. Small-cell lung cancer and breast cancer cells also express normal genes for all vasopressin receptors and produce normal vasopressin receptor mRNAs and V1a and V1b receptor proteins, and the vasopressin-activated calcium mobilising (VACM) protein; plus both normal and abnormal forms of the V2 receptor. Through these receptors, vasopressin exercises multifaceted effects on tumour growth and metabolism. A normal protein vasopressin gene promoter seems to be present in small-cell lung cancer cells, and this promoter contains all of the transcriptional elements known to be involved in gene regulation within hypothalamic neurones. Since these elements largely account for regulation of tumour gene expression observed in vitro, it is likely that as yet unknown factors are selectively produced by tumours in vivo to account for the observed seemingly autonomous or unregulated production of hormone in tumour patients. Promoter elements thought to be responsible for selective vasopressin gene expression in small-cell lung cancer probably include an E-box and a neurone restrictive silencer element close to the transcription start site. It is possible that transcription factors acting at these same elements can explain selective vasopressin expression, not only in small-cell tumours, but also in all other tumours such as breast cancer. By extrapolation, similar mechanisms might also be responsible for the expression of additional features that characterize the 'neuroendocrine' profile of these cancers.