We have previously shown that sustained pCPT treatment
We have previously shown that sustained 8-pCPT treatment favored Ca2+-dependent arrhythmogenic activity in rat ventricular cardiomyocytes . Fig. 2 shows line-scan Ca2+ images of myocyte incubated without (Fig. 2A left) and with 8-pCPT (Fig. 2A right) subjected to a train of electrical stimulations at 1Hz. Most of cardiomyocytes incubated without 8-pCPT showed no or very little spontaneous Ca2+ waves after electric pacing interruption, whereas sustained EPAC activation leaded in ~60% Nifedipine to the appearance of multiple spontaneous Ca2+ waves (Fig. 2B). The co-incubation with EPAC inhibitors, CE3F4 or ESI-05, prevented Ca2+ waves in 8-pCPT-incubated myocytes (Fig. 2B). Moreover, the EPAC-induced Ca2+ waves were reduced in cells when SKF-96365 or the selective TRPC3 antagonist Pyr3 (10μM)  were added to the perfusion (Fig. 2C to F). These effects were reversible upon washout of the drugs (data not shown). This echoed recent data showing that TRPC3 through its dynamic modulation of NCX is able to trigger arrhythmic events .
Altogether, our results show that EPAC enhances SOCE-like activity, which is related to an increased amount of functional TRPC3/C4 channels. This additional Ca2+ entry pathway in the cardiomyocyte could contribute to a proarrhythmic effect of EPAC and to the physiopathological consequences that this entails.
Conflict of interest The following are the supplementary data related to this article.
Introduction Neuroendocrine tumors (NETs) are rare neoplasia showing a wide spectrum of clinical behavior, from the benign nature of pituitary adenomas, to the slow growth of most carcinoids and the very malignant phenotype of undifferentiated tumors , . Diagnosis is usually late, metastases being identified at presentation in 60–80% of gastroenteropancreatic NETs. Moreover, surgical resection is curative only in patients with early-stage disease . Though systemic treatment options for patients with advanced NETs are so far limited, treatment approaches with targeted therapy for different pathways involved in neuroendocrine oncogenesis could provide new therapeutic options. In this respect, cAMP could represent an interesting target since it is considered a key signaling molecule that plays a pivotal role in different biological events involved in endocrine tumorigenesis including cell proliferation . The ubiquitous second messenger cyclic adenosine 3′-5′-monophosphate (cAMP) represents a key signaling molecule implicated in the regulation of a wide variety of cell functions including cell proliferation  and modulation of integrin-mediated cell adhesion , , , . Interestingly, cAMP effects are characterized by cell-type specificity , since cAMP may either stimulate or inhibit cell growth and adhesion depending on the cell type. In this respect, it has been previously reported that cAMP may affect cell growth in some neuroendocrine tumor cells (i.e. pituitary adenomas and melanomas) , , , this either stimulatory on inhibitory effect being mediated by cAMP dependent protein kinase A (PKA) activation. cAMP physiologic effects are mediated by three ubiquitously expressed intracellular proteins: PKA, the exchange proteins directly activated by cAMP 1 and 2 (Epac1/2) and cyclic-nucleotide-gated ion channels. Similarly to PKA, Epac1/2 proteins contain an evolutionarily conserved cAMP-binding domain that activates downstream signaling molecules such as small GTPases Rap1 and Rap2 , , regulators of integrin-mediated , ,  and cadherin-mediated cell adhesion , , . The relative roles of PKA and Epac in mediating cAMP effects have been assessed in several cell systems by using cAMP analogs, such as 8-pCPT-2′-O-Me-cAMP or Sp-6-Bnz-cAMP that selectively activate Epac and PKA, respectively , , , . To date, the specific role of cAMP and its two main effectors, PKA and Epac, are still unknown in P-NET and bronchial carcinoid cells.