The COX-2/PGE₂/EP3/G_i/o/cAMP/GSIS Pathway in the Islet: The Beat Goes On

Interest in regulation of glucose-induced insulin secretion (GSIS) by arachidonic acid (AA) metabolites began in the early 1970s. The metabolite of primary interest has been prostaglandin E₂ (PGE₂) because it is one of several negative modulators of GSIS yet unique because it is not classified as a hormone, such as epinephrine or somatostatin. Rather, PGE₂ is considered an autocoid because it is a substance whose action takes place in the same cell as its synthesis, which in the case of the pancreatic β-cell suggests PGE₂ is an endogenous modulator of GSIS. This autocoid, produced via cleavage of AA from phospholipid and further metabolism by cyclooxygenase-2 (COX-2), is the most abundant AA metabolite in the β-cell. The article in this issue of Diabetes by Neuman et al. (1) takes a fresh look at this pathway by focusing on PGE₃, an alternate prostaglandin formed when eicosapentaenoic acid (EPA) rather than AA is used as the dominant dietary substrate. (Before reading further, it may be helpful to examine Table 1 of the article by Neuman et al., which identifies the players in this complex lexicon.) Clinical relevance of this consideration of the impact of a diet rich in EPA stems from the fact that this polyunsaturated fatty acid is proposed to be beneficial in chronic conditions such as diabetes.

Historically, long before the discovery of prostaglandins, Ebstein (2) reported in 1876 that treatment of his patients with diabetes with oral sodium salicylate reduced urinary glucose levels. It took nearly 100 years for the Nobel prize–winning work of Vane (3) and his colleagues to establish that nonsteroidal anti-inflammatory drugs, such as sodium salicylate, used inhibition of the COX-2 pathway as a major mechanism of action (4). At roughly the same time, several investigative groups were performing experiments in vitro with static incubation of isolated islets or in vivo by infusing PGE₂ in animals and humans (5–8) with variable and sometimes contrary results. The result of our own work found consistently that PGE₂ inhibited first-phase GSIS specifically and did not inhibit first-phase insulin release in response to nonglucose secretagogues (reviewed in Robertson [9]). The mechanism of action for this inhibitory effect was later discovered in 1987 to involve PGE₂-specific binding sites on the plasma membrane of isolated primary mammalian islets as well as the β-cell line HIT-T15, with its postreceptor (EP3) effect of pertussis toxin–sensitive G_i/o protein activation and a decline of cAMP levels (10). Eighteen years later, Kimple et al. (11) published work studying PGE₁ rather than PGE₂ action in the Ins-1(832/13) β-cell–derived line and reported that the PGE₁ effect to inhibit GSIS was not pertussis toxin–sensitive. They identified Gα_Z as the inhibitory G-protein involved in PGE₁ action in their cell line. More recent information using advanced molecular and genetics-based technology has confirmed the inhibitory effects of PGE₂ on β-cell structure and function (12,13).

The question asked by Neuman et al. (1) was whether shifting plasma membrane polyunsaturated fatty acid composition to favor EPA would alter prostaglandin production toward formation of PGE₃ and thereby diminish PGE₂ signaling and enhance β-cell function. The authors designed an extensive series of experiments featuring methodologies using BTBR wild-type mice, Leptin^ob/ob (Ob) mice, and NOD mice; EPA-enriched diets; islet isolation and GSIS; lipid extraction and gas chromatography; cDNA synthesis and gene expression; islet imaging; and PGE₂ ELISA. They observed that an AA-enriched diet accelerated the development of diabetes, whereas the EPA-enriched diet shifted prostaglandin production to favor PGE₃ as opposed to PGE₂ and enhanced GSIS. Notably, they reported that both PGE₂ and PGE₃ reduced GSIS from BTBR-Ob islets in a dose-dependent manner but that the IC₅₀ for this reduction was 10-fold weaker for PGE₃, the effects of both were fully competed by a specific EP3 antagonist, and the EPA-enriched diet also reduced EP3 gene expression fivefold. Other experiments examined the effects of interleukin-1β (IL-1β) on prostaglandin production in BTBR-Ob islets. The study by Neuman et al. confirmed earlier work that showed this cytokine increases COX-2 mRNA levels (14–16) but found uniquely that this effect was diminished by dietary EPA enrichment. The beauty of these experiments lies in the use of dietary manipulation to assess the pathophysiological role of PGE₂ in abnormal β-cell function in animal models of diabetes. This approach is more elegant than previously published work that was dependent on the use of pharmacological agents, such as nonsteroidal inflammatory drugs, which by their very nature are nonspecific tools. Dietary enrichment of EPA depends on substrate-driven enhancement of endogenous prostaglandins, which can be measured to document the magnitude of shift in product without concern for off-target effects of drugs that provide only nonspecific enzyme inhibition. In this sense, the work of Neuman et al. puts to rest any lingering doubt that PGE₂ is an endogenous inhibitor of GSIS.

Does this now mean that no work is left to be done in the research area of prostaglandins and islet function? Not at all. Quite the contrary. Development of therapeutic agents for humans with diabetes and abnormal β-cell function that will suppress inhibitory prostaglandins has not been robust. As illustrated in Fig. 1, there are many attack points that could be used to develop new β-cell–specific therapeutic agents that interrupt PGE₂ synthesis or antagonize the postreceptor actions of EP3, especially in states of inflammation that involve IL-1β and perhaps other cytokines that can harm islets. Further laboratory work with islets is needed to better characterize enzymatic pathways in the AA cascade to verify that the islet does or does not conform to conventional thinking about regulation of prostaglandin production. Very little is known about the final step in PGE₂ synthesis. In most cells, a set of proteins termed prostaglandin synthases are responsible for the final fine-tuning of the regulation of prostaglandins. Work in this area has indicated that generally regulation of PGE₂ in the basal state is regulated by the enzyme cPGES, whereas mPGES-1 regulates cytokine-stimulated PGE₂ production. However, we reported in a recent publication the unanticipated discovery that mPGES-1 mRNA and protein is absent in mouse, rat, and human islets (17). This leaves COX-2 itself as the sole regulator of PGE₂ synthesis, which carries the implication that drug discovery efforts designed to block PGE₂ synthesis should not focus on inhibiting mPGES-1 activity. Rather, pharmacological development of specific inhibitors of β-cell COX-2 that do not affect COX-2 in other cells is a goal that is much more likely to provide a drug that will benefit people with type 2 diabetes and intrinsically impaired GSIS as well as that induced by cytokines. So, I invite you to listen to this developing story as the beat goes on.