Endocrine Abstracts

Acromegaly is caused by excessive growth hormone (GH) secretion frequently due to GH-secreting pituitary tumors. Acromegalic tumors (ACRO) display reduced microvascular density versus normal pituitaries, suggesting they are under hypoxic conditions. Under hypoxia, tumors gain a survival advantage by stabilizing the transcription factor known as hypoxia-inducible factor 1 alpha (HIF1a). Western blot screening of ACRO (n=40) revealed significantly higher HIF1a protein levels compared to NFPA (n=18) and normal pituitary (n=6), indicating a role for HIF1a specifically in acromegalic tumorigenesis.

This study investigates the role of HIF1a in ACRO pathophysiology. HIF1a overexpression and hypoxia (1% O₂) increased GH secretion and promoter activity in the GH-secreting GH3 cell line, while knocking down HIF1a blunted these effects. GH transcription is positively regulated through the cAMP cascade, which activates CREB and subsequently GH transcription. HIF1a overexpression in GH3 cells maintained forskolin-induced CREB phosphorylation and increased cAMP responsive element (CRE) transcriptional activity, while HIF1a knockdown abrogated these effects. These effects were abolished by inhibiting CREB, revealing the importance of CREB.

Further analysis revealed that the effects of HIF1a overexpression on GH and CRE transcriptional activity were abrogated when HIF1a was co-expressed with a dominant-negative mutant of PKA, indicating that the target of HIF1a is located within, or upstream of PKA. Real-time PCR screening of PKA-associated proteins revealed the regulatory subunit IIb (PRKAR2B) as a probable HIF1a target gene, as HIF1a overexpression significantly repressed PRKAR2B mRNA expression, while HIF1a knockdown reversed this effect. Furthermore, the stimulatory effects of HIF1a overexpression on GH promoter activity could be suppressed by selectively overexpressing PRKAR2B.

Taken together, our results demonstrate that HIF1a overexpression as it is found in somatotrophinomas is involved in the overactivation of the cAMP pathway transcriptionally repressing the PRKAR2B subunit. Therefore, targeting HIF1a may sensitize tumor cells to pharmacological therapy in patients with acromegaly.