Differences between Human and Rodent Pancreatic Islets

Abstract

Anaplerosis, the net synthesis in mitochondria of citric acid cycle intermediates, and cataplerosis, their export to the cytosol, have been shown to be important for insulin secretion in rodent beta cells. However, human islets may be different. We observed that the enzyme activity, protein level, and relative mRNA level of the key anaplerotic enzyme pyruvate carboxylase (PC) were 80 -90% lower in human pancreatic islets compared with islets of rats and mice and the rat insulinoma cell line INS-1 832/13. Activity and protein of ATP citrate lyase, which uses anaplerotic products in the cytosol, were 60 -75% lower in human islets than in rodent islets or the cell line. In line with the lower PC, the percentage of glucose-derived pyruvate that entered mitochondrial metabolism via carboxylation in human islets was only 20 -30% that in rat islets. This suggests human islets depend less on pyruvate carboxylation than rodent models that were used to establish the role of PC in insulin secretion. Human islets possessed high levels of succinyl-CoA:3-ketoacid-CoA transferase, an enzyme that forms acetoacetate in the mitochondria, and acetoacetyl-CoA synthetase, which uses acetoacetate to form acyl-CoAs in the cytosol. Glucose-stimulated human islets released insulin similarly to rat islets but formed much more acetoacetate. ␤-Hydroxybutyrate augmented insulin secretion in human islets. This information supports previous data that indicate beta cells can use a pathway involving succinyl-CoA:3-ketoacid-CoA transferase and acetoacetyl-CoA synthetase to synthesize and use acetoacetate and suggests human islets may use this pathway more than PC and citrate to form cytosolic acyl-CoAs.Understanding the enzymatic makeup of human pancreatic islets is fundamental to developing strategies for designing artificial beta cells and beta cells differentiated from stem cells as treatments for type 1 diabetes, as well as modulating beta cell metabolism for the treatment of type 2 diabetes. Until recently, most of the information about normal insulin secretion came from studies of rodent islets or clonal cell lines. Although a recent study showed human pancreatic islets respond to insulin secretagogues similarly to rodent islets (1), what is still unknown is whether the use of intracellular pathways of secretagogue metabolism is the same in human islets as in rodent islets and cell lines. During the last few years, human islet preparations from human donors have become more readily available to researchers. By studying the levels of enzymes, the functional units of metabolism, the recent abundant supply of human islets has enabled our laboratory to discover clues suggesting differences in metabolic pathways between pancreatic islets of humans and rodents that have implications for better understanding normal human beta cell physiology.Anaplerosis, the biosynthesis of citric acid cycle intermediates (2), is widely believed to be important for insulin secretion (3). Pyruvate carboxylase (PC) 2 is the key anaplerotic enzyme in this process ...

Mass spectrometry analysis shows the biosynthetic pathways supported by pyruvate carboxylase in highly invasive breast cancer cells

“…Homogenates were prepared and activities of allenzymes except citrate synthase were measured as previously described [15]. The activity of citrate synthase was measured as described in reference [16].…”

Of rodents and men: Species-specific glucose regulation and type 2 diabetes research

“…Glucose metabolism via anaplerosis and cataplerosis plays an important role in insulin secretion by rodent pancreatic β-cells, and metabolic enzymes pyruvate carboxylase and AtP citrate lyase play important roles in rodents. However, the levels and activities of the key anaplerotic enzymes pyruvate carboxylase and ATP citrate lyase are reported to be 80-90% and 60-75% lower in human islets compared to rodent islets, respectively (MacDonald et al, 2011). The role of pyruvate carboxylase in insulin secretion in human islets is also different from rodents: human islets are less dependent on pyruvate carboxylation, with only 15% pyruvate carboxylation activity compared to that in rodents.…”

Evidence for Loss in Identity, De-Differentiation, and Trans-Differentiation of Islet β-Cells in Type 2 Diabetes

“…Further, there is a need for more extensive studies revealing the unique properties regulating the activity of the distinct β-cell populations produced under normal and diabetic circumstances. Given the notable morphological and functional differences between rodent and human islets (Kim et al, 2009; MacDonald et al, 2011; Dai et al, 2012), improved modeling of the human disease process will also move the field forward. A recent report from the Powers group at Vanderbilt University took strides toward addressing this by comparing transplanted mouse and human islets under gluco- or lipo-toxic conditions, revealing significant differences in responses between species (Dai et al, 2016).…”

“…To this end, evidence for this possibility is unclear or even unsupportive, which may in part be due to the relatively recent nature of these observations in mice, but may also reflect differences between species (Kim et al, 2009; MacDonald et al, 2011; Dai et al, 2012). Nonetheless, some promising observations have been reported.…”