Islet-activating protein blocks glucagon desensitization in intact hepatocytes

Area of Science:

• Biochemistry
• Cellular Biology
• Endocrinology

Background:

• Glucagon is a key hormone regulating blood glucose levels by stimulating cyclic AMP (cAMP) production in hepatocytes.
• Hormonal signaling pathways can be subject to desensitization, reducing cellular responsiveness over time.
• The role of regulatory proteins, particularly inhibitory guanine nucleotide-binding proteins (Gi), in modulating adenylate cyclase activity is crucial.

Purpose of the Study:

• To investigate the effect of islet-activating protein (IAP) from Bordetella pertussis on glucagon-stimulated cyclic AMP (cAMP) production in hepatocytes.
• To elucidate the mechanism by which IAP influences adenylate cyclase activity and its regulation by guanine nucleotides.
• To determine if IAP affects the desensitization of the glucagon signaling pathway in liver cells.

Main Methods:

• Treatment of intact hepatocytes with IAP and subsequent stimulation with glucagon.
• Measurement of intracellular cyclic AMP concentrations under basal and stimulated conditions.
• Analysis of adenylate cyclase activity in isolated liver plasma membranes using forskolin and guanosine triphosphate (GTP).
• Investigation of IAP's effect on the NAD-dependent ribosylation of the Mr-40,000 inhibitory guanine nucleotide-binding protein (Gi).

Main Results:

• IAP treatment significantly increased the ability of glucagon to elevate intracellular cAMP levels in hepatocytes without affecting basal cAMP concentrations.
• IAP enhanced glucagon stimulation of adenylate cyclase in liver cell membranes.
• Elevated GTP, which normally inhibits adenylate cyclase, lost its inhibitory effect in membranes from IAP-treated hepatocytes, indicating Gi inhibition.
• IAP treatment prevented the desensitization of adenylate cyclase activity typically observed with prolonged glucagon exposure.
• IAP mediated the ribosylation of the Mr-40,000 Gi protein in isolated liver plasma membranes.

Conclusions:

• Islet-activating protein potentiates the stimulatory effects of glucagon on hepatocytes by enhancing adenylate cyclase responsiveness.
• IAP inhibits the inhibitory guanine nucleotide-binding protein (Gi), thereby removing a basal inhibitory tone on adenylate cyclase.
• IAP treatment blocks the desensitization of the glucagon signaling pathway, leading to a heightened and sustained cellular response to glucagon.