Patrick T. Ronaldson

Transforming Growth Factor-β Signaling Alters Substrate Permeability and Tight Junction Protein Expression at the Blood-Brain Barrier during Inflammatory Pain

Our laboratory has shown that peripheral inflammatory pain induced by λ-carrageenan (CIP) can increase blood-brain barrier (BBB) permeability and alter tight junction (TJ) protein expression leading to changes in BBB functional integrity. However, the intracellular signaling mechanisms involved in this pathophysiologic response have not been elucidated. Transforming growth factor (TGF)-β signaling pathways are known to regulate vascular integrity and permeability. Therefore, we examined the function of TGF-β signaling at the BBB in rats subjected to CIP. During CIP, serum TGF-β1 and protein expression of the TGF-β receptor activin receptor-like kinase-5 (ALK5) were reduced. Brain permeability to 14C-sucrose was increased and expression of TJ proteins (i.e., claudin-5, occludin, zonula occluden (ZO-1)) were also altered after 3 h CIP. Pharmacological inhibition of ALK5 with the selective inhibitor SB431542 further enhanced brain uptake of 14C-sucrose, increased TJ protein expression (i.e., claudin-3, claudin-5, occludin, ZO-1), and decreased nuclear expression of TGF-β/ALK5 signaling molecules (i.e., Smad2, Smad3), which suggests a role for TGF-β/ALK5 signaling in the regulation of BBB integrity. Interestingly, administration of exogenous TGF-β before CIP activated the TGF-β/ALK5 pathway and reduced BBB permeability to 14C-sucrose. Taken together, our data show that TGF-β/ALK5 signaling is, in part, involved in the regulation of BBB functional integrity.

Regulation of Multidrug Resistance Protein 1 by Tumor Necrosis Factor α in Cultured Glial Cells: Involvement of Nuclear Factor-κB and c-Jun N-Terminal Kinase Signaling Pathways

Pharmacotherapy of brain HIV-1 infection may be limited by ABC transporters [i.e., P-glycoprotein (P-gp), multidrug resistance protein 1 (Mrp1)] that export antiretroviral drugs from HIV-1 brain cellular targets (i.e., astrocytes, microglia). Using an in vitro astrocyte model of an HIV-1 associated inflammatory response, our laboratory has shown that cytokines [i.e., tumor necrosis factor α (TNF-α), interleukin (IL)-1β, IL-6], which are secreted in response to HIV-1 envelope glycoprotein gp120 exposure, can decrease P-gp functional expression; however, it is unknown whether these same cytokines can alter expression and/or activity of other ABC transporters (i.e., Mrp1). In primary cultures of rat astrocytes, Mrp1 expression was increased by TNF-α (2.7-fold) but was not altered by IL-1β or IL-6. Cellular retention of 2′,7′-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein, an Mrp substrate, was reduced in TNF-α-treated astrocytes, suggesting increased Mrp-mediated transport. Pharmacologic inhibition of nuclear factor-κB (NF-κB) signaling with SN50 prevented both TNF-α release and Mrp1 expression changes in astrocytes triggered with gp120; however, SN50 did not attenuate Mrp1 expression in cells triggered with TNF-α. In contrast, Mrp1 functional expression was not altered in the presence of gp120 or TNF-α when astrocyte cultures were pretreated with 1,9-pyrazoloanthrone (SP600125), an established c-Jun N-terminal kinase (JNK) inhibitor. SP600125 did not affect TNF-α release from cultured astrocytes triggered with gp120. Mrp1 mRNA expression was increased after treatment with gp120 (1.6-fold) or TNF-α (1.7-fold), suggesting altered Mrp1 gene transcription. These data suggest that gp120 and TNF-α can up-regulate Mrp1 expression in cultured astrocytes. Furthermore, our results imply that both NF-κB and JNK signaling are involved in Mrp1 regulation during an HIV-1 associated inflammatory response.

P-glycoprotein trafficking at the blood–brain barrier altered by peripheral inflammatory hyperalgesia

J. Neurochem. (2012) 122, 962–975.

Inflammatory Pain Signals an Increase in Functional Expression of Organic Anion Transporting Polypeptide 1a4 at the Blood-Brain Barrier

Pain is a dominant symptom associated with inflammatory conditions. Pharmacotherapy with opioids may be limited by poor blood-brain barrier (BBB) permeability. One approach that may improve central nervous system (CNS) delivery is to target endogenous BBB transporters such as organic anion-transporting polypeptide 1a4 (Oatp1a4). It is critical to identify and characterize biological mechanisms that enable peripheral pain/inflammation to “transmit” upstream signals and alter CNS drug transport processes. Our goal was to investigate, in vivo, BBB functional expression of Oatp1a4 in animals subjected to peripheral inflammatory pain. Inflammatory pain was induced in female Sprague-Dawley rats (200–250 g) by subcutaneous injection of 3% λ-carrageenan into the right hind paw; control animals were injected with 0.9% saline. In rat brain microvessels, Oatp1a4 expression was increased during acute pain/inflammation. Uptake of taurocholate and [d-penicillamine2,5]-enkephalin, two established Oatp substrates, was increased in animals subjected to peripheral pain, suggesting increased Oatp1a4-mediated transport. Inhibition of inflammatory pain with the anti-inflammatory drug diclofenac attenuated these changes in Oatp1a4 functional expression, suggesting that inflammation in the periphery can modulate BBB transporters. In addition, diclofenac prevented changes in the peripheral signaling cytokine transforming growth factor-β1 (TGF-β1) levels and brain microvascular TGF-β receptor expression induced by inflammatory pain. Pretreatment with the pharmacological TGF-β receptor inhibitor 4-[4-(1,3-benzodioxol-5-yl)-5-(2-pyridinyl)-1H-imidazol-2-yl]benzamide (SB431542) increased Oatp1a4 functional expression in λ-carrageenan-treated animals and saline controls, suggesting that TGF-β signaling is involved in Oatp1a4 regulation at the BBB. Our findings indicate that BBB transporters (i.e., Oatp1a4) can be targeted during drug development to improve CNS delivery of highly promising therapeutics.

Targeted drug delivery to treat pain and cerebral hypoxia

Limited drug penetration is an obstacle that is often encountered in treatment of central nervous system (CNS) diseases including pain and cerebral hypoxia. Over the past several years, biochemical characteristics of the brain (i.e., tight junction protein complexes at brain barrier sites, expression of influx and efflux transporters) have been shown to be directly involved in determining CNS permeation of therapeutic agents; however, the vast majority of these studies have focused on understanding those mechanisms that prevent drugs from entering the CNS. Recently, this paradigm has shifted toward identifying and characterizing brain targets that facilitate CNS drug delivery. Such targets include the organic anion–transporting polypeptides (OATPs in humans; Oatps in rodents), a family of sodium-independent transporters that are endogenously expressed in the brain and are involved in drug uptake. OATP/Oatp substrates include drugs that are efficacious in treatment of pain and/or cerebral hypoxia (i.e., opioid analgesic peptides, 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors). This clearly suggests that OATP/Oatp isoforms are viable transporter targets that can be exploited for optimization of drug delivery to the brain and, therefore, improved treatment of CNS diseases. This review summarizes recent knowledge in this area and emphasizes the potential that therapeutic targeting of OATP/Oatp isoforms may have in facilitating CNS drug delivery and distribution. Additionally, information presented in this review will point to novel strategies that can be used for treatment of pain and cerebral hypoxia.

Targeting blood–brain barrier changes during inflammatory pain: an opportunity for optimizing CNS drug delivery

The blood-brain barrier (BBB) is the most significant obstacle to effective CNS drug delivery. It possesses structural and biochemical features (i.e., tight-junction protein complexes and, influx and efflux transporters) that restrict xenobiotic permeation. Pathophysiological stressors (i.e., peripheral inflammatory pain) can alter BBB tight junctions and transporters, which leads to drug-permeation changes. This is especially critical for opioids, which require precise CNS concentrations to be safe and effective analgesics. Recent studies have identified molecular targets (i.e., endogenous transporters and intracellular signaling systems) that can be exploited for optimization of CNS drug delivery. This article summarizes current knowledge in this area and emphasizes those targets that present the greatest opportunity for controlling drug permeation and/or drug transport across the BBB in an effort to achieve optimal CNS opioid delivery.

Hypoxia/reoxygenation stress signals an increase in organic anion transporting polypeptide 1a4 (Oatp1a4) at the blood-brain barrier: relevance to CNS drug delivery.

Cerebral hypoxia and subsequent reoxygenation stress (H/R) is a component of several diseases. One approach that may enable neural tissue rescue after H/R is central nervous system (CNS) delivery of drugs with brain protective effects such as 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitors (i.e., statins). Our present in vivo data show that atorvastatin, a commonly prescribed statin, attenuates poly (ADP-ribose) polymerase (PARP) cleavage in the brain after H/R, suggesting neuroprotective efficacy. However, atorvastatin use as a CNS therapeutic is limited by poor blood–brain barrier (BBB) penetration. Therefore, we examined regulation and functional expression of the known statin transporter organic anion transporting polypeptide 1a4 (Oatp1a4) at the BBB under H/R conditions. In rat brain microvessels, H/R (6% O2, 60 minutes followed by 21% O2, 10 minutes) increased Oatp1a4 expression. Brain uptake of taurocholate (i.e., Oap1a4 probe substrate) and atorvastatin were reduced by Oatp inhibitors (i.e., estrone-3-sulfate and fexofenadine), suggesting involvement of Oatp1a4 in brain drug delivery. Pharmacological inhibition of transforming growth factor-β (TGF-β)/activin receptor-like kinase 5 (ALK5) signaling with the selective inhibitor SB431542 increased Oatp1a4 functional expression, suggesting a role for TGF-β/ALK5 signaling in Oatp1a4 regulation. Taken together, our novel data show that targeting an endogenous BBB drug uptake transporter (i.e., Oatp1a4) may be a viable approach for optimizing CNS drug delivery for treatment of diseases with an H/R component.

Acetaminophen modulates P-glycoprotein functional expression at the blood-brain barrier by a constitutive androstane receptor-dependent mechanism.

Effective pharmacologic treatment of pain with opioids requires that these drugs attain efficacious concentrations in the central nervous system (CNS). A primary determinant of CNS drug permeation is P-glycoprotein (P-gp), an endogenous blood-brain barrier (BBB) efflux transporter that is involved in brain-to-blood transport of opioid analgesics (i.e., morphine). Recently, the nuclear receptor constitutive androstane receptor (CAR) has been identified as a regulator of P-gp functional expression at the BBB. This is critical to pharmacotherapy of pain/inflammation, as patients are often administered acetaminophen (APAP), a CAR-activating ligand, in conjunction with an opioid. Our objective was to investigate, in vivo, the role of CAR in regulation of P-gp at the BBB. Following APAP treatment, P-gp protein expression was increased up to 1.4–1.6-fold in a concentration-dependent manner. Additionally, APAP increased P-gp transport of BODIPY-verapamil in freshly isolated rat brain capillaries. This APAP-induced increase in P-gp expression and activity was attenuated in the presence of CAR pathway inhibitor okadaic acid or transcriptional inhibitor actinomycin D, suggesting P-gp regulation is CAR-dependent. Furthermore, morphine brain accumulation was enhanced by P-gp inhibitors in APAP-treated animals, suggesting P-gp–mediated transport. A warm-water (50°C) tail-flick assay revealed a significant decrease in morphine analgesia in animals treated with morphine 3 or 6 hours after APAP treatment, as compared with animals treated concurrently. Taken together, our data imply that inclusion of APAP in a pain treatment regimen activates CAR at the BBB and increases P-gp functional expression, a clinically significant drug-drug interaction that modulates opioid analgesic efficacy.

P-glycoprotein Modulates Morphine Uptake into the CNS: A Role for the Non-steroidal Anti-inflammatory Drug Diclofenac

Our laboratory has previously demonstrated that peripheral inflammatory pain (PIP), induced by subcutaneous plantar injection of λ-carrageenan, results in increased expression and activity of the ATP-dependent efflux transporter P-glycoprotein (P-gp) that is endogenously expressed at the blood-brain barrier (BBB). The result of increased P-gp functional expression was a significant reduction in CNS uptake of morphine and, subsequently, reduced morphine analgesic efficacy. A major concern in the treatment of acute pain/inflammation is the potential for drug-drug interactions resulting from P-gp induction by therapeutic agents co-administered with opioids. Such effects on P-gp activity can profoundly modulate CNS distribution of opioid analgesics and alter analgesic efficacy. In this study, we examined the ability of diclofenac, a non-steroidal anti-inflammatory drug (NSAID) that is commonly administered in conjunction with the opioids during pain therapy, to alter BBB transport of morphine via P-gp and whether such changes in P-gp morphine transport could alter morphine analgesic efficacy. Administration of diclofenac reduced paw edema and thermal hyperalgesia in rats subjected to PIP, which is consistent with the known mechanism of action of this NSAID. Western blot analysis demonstrated an increase in P-gp expression in rat brain microvessels not only following PIP induction but also after diclofenac treatment alone. Additionally, in situ brain perfusion studies showed that both PIP and diclofenac treatment alone increased P-gp efflux activity resulting in decreased morphine brain uptake. Critically, morphine analgesia was significantly reduced in animals pretreated with diclofenac (3 h), as compared to animals administered diclofenac and morphine concurrently. These novel findings suggest that administration of diclofenac and P-gp substrate opioids during pain pharmacotherapy may result in a clinically significant drug-drug interaction.

Tempol modulates changes in xenobiotic permeability and occludin oligomeric assemblies at the blood-brain barrier during inflammatory pain.

Our laboratory has shown that λ-carrageenan-induced peripheral inflammatory pain (CIP) can alter tight junction (TJ) protein expression and/or assembly leading to changes in blood-brain barrier xenobiotic permeability. However, the role of reactive oxygen species (ROS) and subsequent oxidative stress during CIP is unknown. ROS (i.e., superoxide) are known to cause cellular damage in response to pain/inflammation. Therefore, we examined oxidative stress-associated effects at the blood-brain barrier (BBB) in CIP rats. During CIP, increased staining of nitrosylated proteins was detected in hind paw tissue and enhanced presence of protein adducts containing 3-nitrotyrosine occurred at two molecular weights (i.e., 85 and 44 kDa) in brain microvessels. Tempol, a pharmacological ROS scavenger, attenuated formation of 3-nitrotyrosine-containing proteins in both the hind paw and in brain microvessels when administered 10 min before footpad injection of λ-carrageenan. Similarly, CIP increased 4-hydroxynoneal staining in brain microvessels and this effect was reduced by tempol. Brain permeability to [(14)C]sucrose and [(3)H]codeine was increased, and oligomeric assemblies of occludin, a critical TJ protein, were altered after 3 h CIP. Tempol attenuated both [(14)C]sucrose and [(3)H]codeine brain uptake as well as protected occludin oligomers from disruption in CIP animals, suggesting that ROS production/oxidative stress is involved in modulating BBB functional integrity during pain/inflammation. Interestingly, tempol administration reduced codeine analgesia in CIP animals, indicating that oxidative stress during pain/inflammation may affect opioid delivery to the brain and subsequent efficacy. Taken together, our data show for the first time that ROS pharmacological scavenging is a viable approach for maintaining BBB integrity and controlling central nervous system drug delivery during acute inflammatory pain.