The putative physiological meaning of epinephrine related
The putative physiological meaning of epinephrine-related hepatic drug transporter regulation remains to be established. It is nevertheless noteworthy that the catecholamine rather decreases expression of sinusoidal SLC transporters like NTCP, OAT2, OAT7, OCT1, OATP1B1 and OATP2B1 whereas those of canalicular ABC transporters, excepted that of BSEP, were better preserved, i.e., both BCRP and MRP2 protein levels were unchanged whereas that of MDR1/P-glycoprotein was even induced. Taken together, transporter regulation in response to adenosine deaminase inhibitor may consequently be interpreted as a protective mechanism for hepatic cells, possibly leading to decreased intracellular accumulation of xenobiotics through reduction of their uptake and preservation or enhancement of their efflux. Such disturbance of hepatic transport may in fact be considered as a part of the physiological adaptive processes triggered by stress, that hugely stimulates secretion of epinephrine by adrenal medulla, and aimed at favoring the adrenergic “fight-or-flight” response (Tank and Lee Wong, 2015). Whether physiological activation of the adrenergic component of the autonomic nervous system, that directly innerves the liver (Jensen et al., 2013), may in vivo impairs drug transporter expression remains however to be determined. Effects of epinephrine towards hepatic drug transporters involve activation of the β2-ADR because (i) β2-ADR expression is notable in both cultured human hepatocytes and HepaRG cells, as in freshly isolated hepatocytes, (ii) the non-selective β-ADR antagonist carazolol and the selective β2-ADR blocker ICI-118,551, but not the α-ADR blocker phentolamine, prevented OATP1B1, OATP2B1, OAT2 and OAT7 repression due to epinephrine, (iii) fenoterol, a selective β2-ADR agonist, led to transporter changes similar to those caused by epinephrine, (iv) the α1-ADR agonist methoxamine failed to significantly alter expression of drug transporters, (v) Gö 6983, a potent inhibitor of the key protein kinase C-related signaling way activated by α1a-ADR (Zhong and Minneman, 1999), an ADR isoform highly expressed by hepatocytes and HepaRG cells, failed to inhibit OATP1B1, OATP2B1, OAT2 and OAT7 repression caused by epinephrine, and (vi) forskolin, a potent activator of the immediate downstream target of β2-ADR, i.e., adenylate cyclase, also mainly reproduced epinephrine effects towards transporters, as well as 8-Br-cAMP, used as an analogue of physiological cAMP generated by the β2-ADR/adenylate cyclase pathway. From cAMP generation, various signaling ways have to be considered for putative implication in epinephrine-mediated alterations of transporter expression (Gloerich and Bos, 2010, Kleppe et al., 2011). That linked to EPACs can most likely be discarded because the selective EPAC activator 8-pCPT-cAMP failed to affect transporter expression, whereas the cAMP analogue 6-Bnz-cAMP, which does not interact with EPACs, induced transporter changes close to those caused by epinephrine. PKA, although known to be potently activated by 6-Bnz-cAMP (Christensen et al., 2003), may also likely not be involved in epinephrine-mediated alterations of transporter expression because the PKA inhibitor H89 did not prevent transporter expression changes such as OATP1B1, OATP2B1, OAT2 and OAT7 mRNA repression caused by the catecholamine; the protein kinase inhibitor peptide (PKI), an endogenous inhibitor of PKA (Dalton and Dewey, 2006), similarly failed to counteract these epinephrine-mediated transporter changes (data not shown). PKA- and EPACs-independent ways may therefore be involved in epinephrine-mediated changes of transporters, as already reported for other epinephrine effects like induction of the defense factor REDD1 in macrophages (Yanagawa et al., 2014). In this context, the putative implication of CNGCs, that are expressed by hepatocytes (Kraus-Friedmann, 2000), has likely to be considered in future studies.