Differential Signaling and Immediate-Early Gene Activation by Four Splice Variants of the Human Pituitary Adenylate CyclaseActivating Polypeptide Receptor (hPACAP-R)

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Abstract
Pituitary adenylate cyclase-activating polypeptide (PACAP), a neuropeptide belonging to the VIP/secretin/glucagon family, is present in the hypothalamus, anterior pituitary, and adrenal gland where it regulates hormone release, in the GI tract where it modulates motility, and in human tumoral cell lines where it shows a growth-promoting effect. It is now appreciated that alternative splicing of two exons of the rat PACAP-R gene generate four major rPACAP-R splice variants that are differentially expressed in tissues and variably coupled to intracellular second messengers. Because of the potential implications of these findings in human physiology, we cloned the hPACAP-R gene. Similar to the rat, two exons (SV-1 and SV-2) are alternatively spliced to account for four major hPACAP-R receptor splice variants. These splice variants (hPACAP-R-null, hPACAP-R-SV1, hPACAP-R-SV2, hPACAP-R-SV-3) were cloned from a human frontal cortex cDNA library, stably transfected in NIH/ 3T3 cells and each characterized for ligand affinity, stimulation of adenylate cyclase (AC) and phospholipase C (PLC), and ligand-induced expression of the proto-oncogenes, c-fos, and c-myc. Stably transfected NIH/3T3 cells expressing similar numbers of receptors of the four splice variants showed nearly identical responses for ligand affinity and potency for P-38- and P-27-stimulated increases in cAMP and total inositol phosphates. However, each receptor splice variant differed in their ligand-stimulated efficacy for total inositol phosphate stimulation. The hPACAP-R-SV2 showed the greatest efficacy for stimulating phospholipase C that was approximately seven-fold greater than the hPACAP-R-SV1, twofold greater than the hPACAP-R-Null, and 1.5-fold greater than the hPACAP-R-SV-3 splice variants. To determine whether the splice variants also differ in their ability to stimulate immediate early gene expression, c-fos and c-myc transcripts were assayed by Northern blot and quantified by densitometry. PACAP-38 increased c-fos and c-myc expression for all four of the receptor splice variants that paralleled the efficacy for PLC stimulation, with the the SV-2 splice variant showing the greatest stimulation. These results show that the hPACAP-R-SV2 exhibits enhanced efficacy for coupling to both PLC and activation of the protooncogenes, c-fos and c-myc suggesting a novel and potentially important mechanism for differentially activating signal transduction pathways that influence cellular growth and differentiation.

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