Protein kinase B : the beta version elucidating novel contributions of protein kinase B beta (PKBß/Akt2) to endocrine metabolism, PCOS and cancer

Protein kinase B (PKB/Akt) is a serine/threonine protein kinase that mediates signaling crucial for normal cellular metabolism, proliferation, survival, and differentiation. PKB/Akt is the major effector kinase upon which signaling from cellular receptors via phosphatidylinositol 3,4,5-triphosphate kinase (PI3K) signaling converges.
In mammals, PKB/Akt exists as three isoforms, PKBα/Akt1, PKBβ/Akt2, and PKBγ/Akt3. These isoforms share the same domain structure and over 85% sequence similarity, suggesting these isoforms mediate similar and overlapping functions. However, these three isoforms are encoded by genes on distinct chromosomes and have differential tissue expression, supporting a concept that these isofoms have evolved to mediate specific and unique biological signals. Knockout mouse models of the PKB/Akt isoforms confirm these isoforms do have both redundant and non-redundant, isoform-specific functions. PKBα/Akt1 KO mice are viable but exhibit ~30% perinatal lethality, growth retardation and increased spontaneous apoptosis, strongly implicating PKBα/Akt1 as the major isoform in growth and survival. PKBβ/Akt2 KO mice are viable and of normal size, however they progressively develop a diabetes-like syndrome characterized by insulin resistance and hyperglycemia, illustrating a crucial role for this isoform in transducing signals regulating organism metabolism. PKBγ/Akt3 is viable and normal except for decreased brain and testis size that are the major expression sites of PKBγ/Akt3. This indicates this isoform may have more specialized or subtle functions. This is also supported by compound knockouts that lack PKBγ/Akt3. PKBα/Akt1-PKBγ/Akt3 double knockout mice die at embryonic day12 (E12) with severe growth retardation and developmental defects, whereas PKBβ/Akt2-PKBγ/Akt3 mice are viable but with a reduction in animal size and an enhancement of the single isoform knockout phenotypes. Together this suggests the PKBγ/Akt3 isoform does contribute to normal function of both PKBα/Akt1 and PKBβ/Akt2. Similarly, loss of PKBβ/Akt2 on the background of PKBα/Akt1 deletion enhances the phenotype of PKBα/Akt1, resulting in 100% perinatal lethality and additionally leading to defects in bone and skin development. These observations in mouse models correlate well with human metabolic syndromes and diseases, particularly in insulin resistance/diabetes and cancer that invariably display aberrant PKB/Akt activation.
Accordingly, this work utilized PKBβ/Akt2 null mice to explore defects in metabolism and exploring its contribution to tumour development driven by hyperactivation of the PI3K pathway. We observed that aged PKBβ/Akt2 KO mice, but not wild-type or PKBα/Akt1 KO mice, develop severe ovarian cysts with thecosis and consequent increases in testosterone production. We show that this may reflect an unknown role for PKBβ/Akt2 in regulating testosterone production in the ovary with a potential contribution to the human metabolic disorder Polycystic Ovarian Syndrome (PCOS). PCOS affects 5-10% of women of reproductive age and is the leading cause of infertility. It is characterized by hyperactive leutinizing hormone signaling in ovary, resulting in increased testosterone production and subsequently development of numerous follicular cysts within the ovary. Using a mouse model of PCOS driven by tonic administration of leutinizing hormone, mice lacking PKBβ/Akt2 developed cysts with a threefold increase in size compared to wild-type mice. Furthermore, the contribution of PKBβ/Akt2 to neoplasia was analyzed by utilizing the Pten heterozygous mouse model. Pten acts as the major negative regulator of PI3K signaling and reduction of Pten in mice results in the development of neoplasia in a broad range of organs due to hyperactivation of PKB/Akt signaling. By deletion of PKBβ/Akt2 on this background, its effect on neoplasia formation in multiple organs was assessed. We observed a variety of effects on neoplasia development in various organs, with the most striking being an almost complete inhibition of adrenal medulla pheochomocytomas formation. Pheochromocytoma formation upon Pten loss in mice activates cellular proliferation and transcriptional changes to drive tumour development and progression. This includes increased proliferative signaling via mTORC1 and stimulation of adrenomedullin expression. Pten+/- mice also reflect the clinical setting with increases in catecholamine production and secretion that is observed in ~90% of human pheochromocytoma patients. Analysis of adrenals, illustrated that PKBβ/Akt2 is required for early development of neoplasia and severely hinders growth and progression through attenuating mTORC1 activation and subsequent cellular proliferation. Cellular signaling required for catecholamine production and secretion was also suppressed, reflected in decreased expression of the rate-limiting enzyme required for catecholamine generation: dopamine β-hydroxylase. Adrenomedullin that can trigger increased cAMP production and growth in various tumours, displayed increased expression in Pten+/- adrenals but decreased almost to wild-type levels upon additional deletion of PKBβ/Akt2. These findings indicate that PKBβ/Akt2 exerts an isoform specific role in promoting pheochromocytomas exhibiting hyperactivated PKB/Akt.
The findings from these studies illustrate novel contributions by PKBβ/Akt2 isoform specific signaling to metabolic dysfunction and tumour formation, thereby highlighting the potential of identifying these signaling pathways and PKBβ/Akt2 specific substrates that may represent novel targets for therapeutic intervention.