Keywords

Diabetes, β-cell, hyperlipidemia, palmitate

Abstract

It is estimated that over 370 million people worldwide suffer from diabetes, and this number is increasing rapidly. Normally, β-cells in the pancreas secrete insulin, which is necessary for proper glucose absorption and storage. Both Type 1 and Type 2 diabetes are characterized by decreased functional β-cell mass and insulin production, and increased circulating glucose and fatty acid levels. Diabetics’ pancreata maintain small amounts of functional β-cells, but these surviving cells are damaged and destroyed over time due to the harmful effects of hyperglycemia and hyperlipidemia. These β-cells must adapt to survive. Hence, diabetes is itself a selective process. We have mimicked and analyzed this selection process in vitro by culturing β-cells under conditions of increasingly elevated palmitate concentration applied step-wise, creating distinct cell lines acclimated to various levels of palmitate concentration with corresponding control lines. Through analysis of these lines, we aimed to simulate the gradual progression of hyperlipidemia seen in diabetic patients, and thus determine mechanisms and effects of β-cell adaptation to these conditions. Respiratory control ratios (RCR) suggest that β-cell lines adapted to higher palmitate concentrations do not adversely affect mitochondrial functionality, and Oroboros (O2K) analysis in fact implies increased cellular respiration in these cell lines. Further, DCF analysis reveals decreased free radical formation in treated cells. Importantly, cell counts indicated that proliferation increased when cells were treated with low (0.15 mM) concentrations of sodium palmitate, but decreased at 0.3 mM concentrations and higher. As measured with RT-PCR, expression of transcription factor Nkx6.1, a gene key to β-cell function and survival, follows a similar pattern. Expression of transcription factors Nr4a1 and Nr4a3, conversely, is generally down-regulated for all concentrations of palmitate. These data imply that β-cell proliferation is boosted by low-level exposure to palmitate but inhibited by excessive exposure. Respiratory functionality is not adversely affected by palmitate treatment and may in fact be improved, thus the observed change in proliferation rates may potentially be modulated by expression of key transcription factors such as Nkx6.1, Nr4a1, and Nr4a3. These results represent a significant step towards understanding the causes and effects of the transition that occurs between low and high levels of palmitate adaptation, ultimately promoting investigation into methods by which endogenous β-cells could increase cellular function, survival, and proliferation under hyperlipidemic conditions.