Transport Mechanisms for the Nutritional Supplement β-Hydroxy-β-Methylbutyrate (HMB) in Mammalian Cells

Purpose: β-Hydroxy-β-methylbutyrate (HMB), a nutritional supplement, elicits anabolic activity in muscle. Here we investigated the mechanism of HMB uptake in muscle cells. Methods: Murine muscle cells (C2C12) and human mammary epithelial cells (MCF7) were used for uptake. As HMB is a monocarboxylate, focus was on monocarboxylate transporters, monitoring interaction of HMB with H ⁺ -coupled lactate uptake, and influence of H ⁺ directly on HMB uptake. Involvement of MCT1–4 was studied using selective inhibitors and gene silencing. Involvement of human Na ⁺ /monocarboxylate transporter SMCT1 was also assessed using Xenopus oocytes. Results: H ⁺ -coupled lactate uptake was inhibited by HMB in both mammalian cells. HMB uptake was H ⁺ -coupled and inhibited by lactate. C2C12 cells expressed MCT1 and MCT4; MCF7 cells expressed MCT1–4; undifferentiated C2C12 cells expressed SMCT1. SMCT1 mediated Na ⁺ -coupled HMB transport. Inhibitors of MCT1/4, siRNA-mediated gene silencing, and expression pattern showed that MCT1–4 were responsible only for a small portion of HMB uptake in these cells. Conclusion: HMB uptake in C2C12 and MCF7 cells is primarily H ⁺ -coupled and inhibited by lactate, but MCT1–4 are only partly responsible for HMB uptake. SMCT1 also transports HMB, but in a Na ⁺ -coupled manner. Other, yet unidentified, transporters mediate the major portion of HMB uptake in C2C12 and MCF7 cells.