Hitoshi Shirakawa

By Hsin‐Jung Ho, Hitoshi Shirakawa and Michio Komai

Naturally existing vitamin K consists of vitamins K1 and K2. Menaquinone‐4 (MK‐4), an analog of vitamin K2 and a product of vitamin K1 metabolism, can be detected in several organs, including the testis; however, the function of MK‐4 in these tissues has not been well characterized. Recent studies have suggested that vitamin K is involved in enhancing protein kinase A (PKA) activity in several cell types, thus regulating numerous PKA‐dependent biological processes. To highlight the effect of vitamin K, we focused on its role in the steroidogenic pathway. Experiments on vitamin K–deficient rats revealed a reduced expression of genes involved in the biosynthesis of cholesterol and steroid hormones in the testis. Moreover, compared with control animals, rats fed on MK‐4 diet presented significantly higher testosterone levels in the plasma and testis. These results suggest that vitamin K is involved in the steroidogenic pathway in the testis. Testosterone levels were found to increase in a dose‐dependent manner also in cell‐based experiments upon addition of MK‐4, but such an effect was not observed in vitamin K1 levels. Furthermore, the effect of MK‐4 on testosterone production was abolished by the specific PKA inhibitor H89, thus confirming the regulatory role of MK‐4 on PKA activation. Here, we describe how MK‐4 modulates PKA activation by enhancing intracellular 3′,5′‐cyclic adenosine monophosphate (cAMP) levels in testis‐derived I‐10 cells. The presented evidence supports the role of MK‐4 in cAMP/PKA signaling and steroidogenesis.

Part of the book: Vitamin K2

By Md. Alauddina, Jahidul Islama, Hitoshi Shirakawaa, Takuya Kosekib, Ardiansyahc and Michio Komaia

Rice bran is a byproduct of the rice milling process; it constitutes 10% of rice, with a potential global production of 48 million tons per year. The major portion of this is used as animal feed or discarded as waste material. However, rice bran is attracting attention from researchers because it is widely available, cheap and rich in nutrients such as protein, fat, carbohydrates, bioactive compounds and dietary fiber. Many food‐processing techniques that have improved rice bran resources have been pioneered, such as enzyme treatment and fermentation. We have been investigating the functional role of rice bran since 2003. Our experiments revealed that rice bran and its active compounds, such as γ‐oryzanol, tocopherol, tocotrienol, adenosine and ferulic acid, play a role as a functional food. In this review, we summarize how rice bran is a super food and functional food to illustrate the global interest in rice bran and its functional aspects and medicinal qualities. We also describe the techniques to prepare functional bran and the composition and health benefits of functional bran, which may encourage entrepreneurs to produce rice bran‐based food on a large scale and meet the global demand for super foods and functional foods.

Part of the book: Superfood and Functional Food

By Hitoshi Watanabe, Michael Rose, Yoshinori Kanayama, Hitoshi Shirakawa and Hisashi Aso

Energy homeostasis is maintained by balancing energy intake and energy expenditure. In addition to the central nervous system, several hormones play a key role in energy homeostasis in the whole body. In particular, serotonin is regarded as one of the key regulators of energy homeostasis. Serotonin is unique in that it is able to act in both the brain as a neurotransmitter and the peripheral tissue as a gastrointestinal hormone. In the brain, serotonin is thought of as a pharmacological target for anti-obesity treatments because it greatly inhibits meal size and body weight gain. In contrast, serotonin in the periphery has not been targeted as a strategy for anti-obesity treatment, even though almost all of the serotonin produced in the body is produced in the peripheral tissue. Recently, the peripheral serotonergic signal has been shown to regulate glucose and lipid metabolism through autocrine and paracrine signals in energy homeostasis-related tissues, including the pancreatic β cell, liver, white adipose tissue, brown adipose tissue, and skeletal muscle. Thus, it is possible that the serotonergic system in the peripheral tissue is a new therapeutic target for metabolic disease, including obesity and diabetes. Here, we summarize the role of peripheral serotonin in the regulation of energy homeostasis.

Part of the book: Serotonin