Expression of all known vasopressin receptor subtypes by small cell tumors implies a multifaceted role for this...

Vasopressin is one of several small neuropeptides that are reported to be autocrine growth factors for small cell carcinoma of the lung (SCCL). It has been assumed that this peptide exercises its mitogenic influences through the vasopressin V1a receptor, and we have previously demonstrated that this receptor is expressed by classical and variant SCCL. Activation of the vasopressin V1a receptor produces changes in phospholipases C, D, and A2, in protein kinase C, and in Ca2+ mobilization. This study demonstrates that SCCL cells express not only vasopressin V1a receptors but also mRNAs and proteins representing normal V1b receptors and V2 receptors. They were also shown to express mRNA for a human form of the putative receptor rabbit vasopressin-activated calcium-mobilizing receptor (VACM-1). Additionally, SCCL tumor cells were found to express mRNA and protein representing a possible nonfunctional, shortened, "diabetic" form of the vasopressin V2 receptor that is the product of incomplete posttranscriptional splicing. At least four of these five vasopressin receptors were produced by cell lines exemplifying classical and variant forms of SCCL. No differences in the sequences for the V1 receptors between classical and variant SCCL were found. However, although the nature and expression of both vasopressin V1 receptors and human VACM are apparently unaffected by dedifferentiation in SCCL, only the abnormal (and probably nonfunctional) form of the V2 receptor could be demonstrated in variant cell line NCI H82. Functional engagement of vasopressin V2 receptors is reported to produce rises in cAMP and activation of protein kinase A, whereas stimulation of V1b receptors is believed to produce similar changes to those produced by V1a receptors, i.e., activation of phospholipases and of protein kinase C. Stimulation of VACM receptors raises intracellular free Ca2+ through currently unknown but phosphoinositide-independent mechanisms. The presence of all known vasopressin receptors that are, together, potentially capable of inducing several different transduction cascades in small cell tumor cells suggests that this peptide serves a multifaceted role in tumor physiology.