Gene C. Webb

Proteolytic processing in the secretory pathway.

Many cellular processes, including embryogenesis (1–4), gene expression (5), cell cycle (6), programmed cell death (7), intracellular protein targeting (8) and endocrine/neural functions (9–13) are regulated by limited proteolysis of precursor proteins (14, 15). These functions are carried out by proteolytic enzyme families that are strategically localized within cells or on cell surfaces (3, 5–7, 9, 10). This review focuses on the serine proteases that process protein precursors (proproteins) traversing the secretory pathway (for recent reviews, see Refs. 9, 11–13, 16–19, 62). The early development of this field is reviewed in Ref. 14.

A Switch From Prohormone Convertase (PC)-2 to PC1/3 Expression in Transplanted α-Cells Is Accompanied by Differential Processing of Proglucagon and Improved Glucose Homeostasis in Mice

OBJECTIVE—Glucagon, which raises blood glucose levels by stimulating hepatic glucose production, is produced in α-cells via cleavage of proglucagon by prohormone convertase (PC)-2. In the enteroendocrine L-cell, proglucagon is differentially processed by the alternate enzyme PC1/3 to yield glucagon-like peptide (GLP)-1, GLP-2, and oxyntomodulin, which have blood glucose–lowering effects. We hypothesized that alteration of PC expression in α-cells might convert the α-cell from a hyperglycemia-promoting cell to one that would improve glucose homeostasis. RESEARCH DESIGN AND METHODS—We compared the effect of transplanting encapsulated PC2-expressing αTC-1 cells with PC1/3-expressing αTCΔPC2 cells in normal mice and low-dose streptozotocin (STZ)-treated mice. RESULTS—Transplantation of PC2-expressing α-cells increased plasma glucagon levels and caused mild fasting hyperglycemia, impaired glucose tolerance, and α-cell hypoplasia. In contrast, PC1/3-expressing α-cells increased plasma GLP-1/GLP-2 levels, improved glucose tolerance, and promoted β-cell proliferation. In GLP-1R−/− mice, the ability of PC1/3-expressing α-cells to improve glucose tolerance was attenuated. Transplantation of PC1/3-expressing α-cells prevented STZ-induced hyperglycemia by preserving β-cell area and islet morphology, possibly via stimulating β-cell replication. However, PC2-expressing α-cells neither prevented STZ-induced hyperglycemia nor increased β-cell proliferation. Transplantation of αTCΔPC2, but not αTC-1 cells, also increased intestinal epithelial proliferation. CONCLUSIONS—Expression of PC1/3 rather than PC2 in α-cells induces GLP-1 and GLP-2 production and converts the α-cell from a hyperglycemia-promoting cell to one that lowers blood glucose levels and promotes islet survival. This suggests that alteration of proglucagon processing in the α-cell may be therapeutically useful in the context of diabetes.

A Switch from PC2 to PC1/3 Expression in Transplanted α-cells is Accompanied by Differential Processing of Proglucagon and Improved Glucose Homeostasis in Mice

OBJECTIVE—Glucagon, which raises blood glucose levels by stimulating hepatic glucose production, is produced in α-cells via cleavage of proglucagon by prohormone convertase (PC)-2. In the enteroendocrine L-cell, proglucagon is differentially processed by the alternate enzyme PC1/3 to yield glucagon-like peptide (GLP)-1, GLP-2, and oxyntomodulin, which have blood glucose–lowering effects. We hypothesized that alteration of PC expression in α-cells might convert the α-cell from a hyperglycemia-promoting cell to one that would improve glucose homeostasis. RESEARCH DESIGN AND METHODS—We compared the effect of transplanting encapsulated PC2-expressing αTC-1 cells with PC1/3-expressing αTCΔPC2 cells in normal mice and low-dose streptozotocin (STZ)-treated mice. RESULTS—Transplantation of PC2-expressing α-cells increased plasma glucagon levels and caused mild fasting hyperglycemia, impaired glucose tolerance, and α-cell hypoplasia. In contrast, PC1/3-expressing α-cells increased plasma GLP-1/GLP-2 levels, improved glucose tolerance, and promoted β-cell proliferation. In GLP-1R−/− mice, the ability of PC1/3-expressing α-cells to improve glucose tolerance was attenuated. Transplantation of PC1/3-expressing α-cells prevented STZ-induced hyperglycemia by preserving β-cell area and islet morphology, possibly via stimulating β-cell replication. However, PC2-expressing α-cells neither prevented STZ-induced hyperglycemia nor increased β-cell proliferation. Transplantation of αTCΔPC2, but not αTC-1 cells, also increased intestinal epithelial proliferation. CONCLUSIONS—Expression of PC1/3 rather than PC2 in α-cells induces GLP-1 and GLP-2 production and converts the α-cell from a hyperglycemia-promoting cell to one that lowers blood glucose levels and promotes islet survival. This suggests that alteration of proglucagon processing in the α-cell may be therapeutically useful in the context of diabetes.

Contrasting patterns of expression of transcription factors in pancreatic α and β cells

Pancreatic α and β cells are derived from the same progenitors but play opposing roles in the control of glucose homeostasis. Disturbances in their function are associated with diabetes mellitus. To identify many of the proteins that define their unique pathways of differentiation and functional features, we have analyzed patterns of gene expression in αTC1.6 vs. MIN6 cell lines by using oligonucleotide microarrays. Approximately 9–10% of >11,000 transcripts examined showed significant differences between the two cell types. Of >700 known transcripts enriched in either cell type, transcription factors and their regulators (TFR) was one of the most significantly different categories. Ninety-six members of the basic zipper, basic helix–loop–helix, homeodomain, zinc finger, high mobility group, and other transcription factor families were enriched in α cells; in contrast, homeodomain proteins accounted for 51% of a total of 45 TFRs enriched in β cells. Our analysis thus highlights fundamental differences in expression of TFR subtypes within these functionally distinct islet cell types. Interestingly, the α cells appear to express a large proportion of factors associated with progenitor or stem-type cells, perhaps reflecting their earlier appearance during pancreatic development. The implications of these findings for a better understanding of α and β cell dysfunction in diabetes mellitus are also considered.