Integrated or Independent Actions of Metformin in Target Tissues Underlying Its Current Use and New Possible Applications in the Endocrine and Metabolic Disorder Area

Abstract

Metformin is considered the first-choice drug for type 2 diabetes treatment. Actually, pleiotropic effects of metformin have been recognized, and there is evidence that this drug may have a favorable impact on health beyond its glucose-lowering activity. In summary, despite its long history, metformin is still an attractive research opportunity in the field of endocrine and metabolic diseases, age-related diseases, and cancer. To this end, its mode of action in distinct cell types is still in dispute. The aim of this work was to review the current knowledge and recent findings on the molecular mechanisms underlying the pharmacological effects of metformin in the field of metabolic and endocrine pathologies, including some endocrine tumors. Metformin is believed to act through multiple pathways that can be interconnected or work independently. Moreover, metformin effects on target tissues may be either direct or indirect, which means secondary to the actions on other tissues and consequent alterations at systemic level. Finally, as to the direct actions of metformin at cellular level, the intracellular milieu cooperates to cause differential responses to the drug between distinct cell types, despite the primary molecular targets may be the same within cells. Cellular bioenergetics can be regarded as the primary target of metformin action. Metformin can perturb the cytosolic and mitochondrial NAD/NADH ratio and the ATP/AMP ratio within cells, thus affecting enzymatic activities and metabolic and signaling pathways which depend on redox- and energy balance. In this context, the possible link between pyruvate metabolism and metformin actions is extensively discussed.

Keywords: cell metabolism; cell signaling; endocrinology; metabolic diseases; metformin; neuroendocrine tumors; pituitary tumors; pyruvate; pyruvate dehydrogenase complex.

Figure 1

Pyruvate is the product of the glycolytic pathways. Within mitochondria, pyruvate can be oxidized to acetyl-CoA by the pyruvate dehydrogenase (PDH) complex. Alternatively, it can be directed to an anaplerotic reaction catalyzed by pyruvate carboxylase (PC), which generates oxaloacetate. Finally, pyruvate can also be reduced to lactate in the cytosol by lactate dehydrogenase (LDH), This reaction ensures NAD recycling when cells need to maintain a high glycolytic rate.

Figure 2

Metformin decreased the growth and viability of rat pituitary tumor cells in vitro, whereas it did not affect the growth of rat myogenic precursors, which are normal, undifferentiated, and rapidly proliferating cells. Rat pituitary tumor cells and rat myogenic precursors differed significantly in some features of their metabolic profile in basal culture conditions: the reductive activity (reduction of the tetrazolium salt MTT within cells) and the ATP content were lower in pituitary tumor cells vs. myogenic precursors. The two cell types also differed significantly in the PDH complex expression levels (PDHE1α protein level), the S6 ribosomal protein levels, and their response to specific metabolic substrates. In this context, short incubations with pyruvate caused an increase of ATP content in myoblasts, without altering the reductive activity. On the other hand, in rat pituitary tumor cells, pyruvate enhanced the sole reductive activity.

Figure 3

The treatment with metformin negatively affected the ATP production in pituitary tumor cells. Moreover, metformin caused the reversal of the pituitary tumor cell response to pyruvate, as regards the enhancement of the reductive activity (reduction of the tetrazolium salt MTT within cells), and increased the acidification of the extracellular medium. On the other hand, metformin enhanced the energy formation (ATP content) in myogenic precursors, and sustained their growth over prolonged incubations.

Figure 4

Metformin was shown to reduce cell growth and viability in distinct pituitary adenoma cell lines in vitro. Both AMPK-dependent and -independent actions are involved. Metformin induced AMPK phosphorylation and activation in AtT20 corticotroph cells and GH3 lacto-somatotroph cells. Metformin inhibited the IGF-1R/AKT/mTOR pathway in AtT20 cells, and selectively suppressed the EGF-induced mTOR/p70S6 kinase pathway activation in GH3 cells without affecting ERK1/2 phosphorylation downstream of the same growth factor receptor. The AMPK activation can cooperate to the mTOR pathway inhibition by metformin, but AMPK-independent actions cannot be ruled out. In GH3 cells, metformin was shown to reduce the activity of STAT3, while increasing the activity of ATF3, a transcription factor involved in the response to stress conditions. These actions were not AMPK-dependent. The JAK-STAT3 signaling pathway plays a role in tumor cell metabolic reprogramming, and cooperates with other factors to shift cell metabolism towards aerobic glycolysis. A possible functional interaction between metformin and adenylyl cyclase (AC)-activating stimuli was investigated in GH3 cells. No functional antagonism was seen. On the other hand, metformin tended to further increase CREB phosphorylation. The possible involvement of AMPK was not investigated.