Metformin; A character actor in the leptin story!

Our current understanding of metabolic processes tells us that there is more to the control of eating behavior then mere stimulation by hunger and hedonic sensation. How brain in general and hypothalamus in particular interacts with peripheral tissue to control feeding has remained a subject of intensive scientific discussions. Lessons learnt from rare but distinctive mongentic disorders causing extreme obesity have turned the spotlight on hypothalamo-leptin-melanocortin system. And that is how the leptin story began, taking other factors along in its course.[1] Metformin, a biguanide, has been as a first line drug of choice along with lifestyle intervention in the management of type 2 diabetic patients. It seems this molecule has been assigned to play the role of a protagonist in the story of management of T2DM. This superstar status won by a drug discovered half a century ago by Professor Jeanne Sterne is though extraordinary, did not come without struggle. To quote Professor Gerald Slama and Professor Ian Campbell, two pioneers who worked on this molecule extensively through the recent decades, “The metformin journey is one of the discovery, dismissal and re-discovery and its very survival is due to some defining moments over the past 50 years.”[2]

How do we combine these two interesting stories. Do we have a substantial plot to find this wonderful molecule playing an important character role in modifying defect(s) in the leptin actions.

Metformin induced reduction in leptin level in morbidly obese individuals has been proven in several studies, but some others have shown evidence to the contrary.[3-6] Possible explanations for these discrepancies may be the length of treatment and the study population, with obese people showing a decrease in leptin levels after long-term treatment. On metformin, there were linear trends in decrements in weight, girth, waist circumference, waist/hip ratio, insulin, and leptin. Reduction in leptin paralleled the reduction in insulin levels and somewhat greater in those with higher baseline serum leptin.[3] Interestingly, distinct reduction in serum leptin level was observed in non obese healthy individual on metformin, without any reduction in body weight.[4] Hence, the scientific community is trying to look beyond changes in adipocyte morphology in trying to define the role of metformin in leptin biology.[7] More recently, it has been demonstrated that weight loss achieved by metformin was correlated with pretreatment plasma leptin levels. This effect of metformin was paralleled by a stimulation of the expression of the leptin receptor gene (ObRb) in the arcuate nucleus, thus identifying the hypothalamic ObRb as a gene modulated after metformin treatment.[8] It has suggested that the anorectic effects of metformin are potentially mediated via an increase in the central sensitivity to leptin.

Thus, the time is just right and ripe to assign another pioneering role, at least of a character actor, if not the protagonist, for metformin in the story of leptin biology. As scientists travel to the deeper realms of the subject and write and rewrite the story for the medical community at large, the people involved in the editorial task of this issue will continue to derive satisfaction of not missing it out, and of course doing a job on-time.