Tikz - Vertical Position Unchanged

I would like to have the graph

\begin{itemize}
\item
$\tikz { \node (a) at (-0.3,-2) [] {U}; \node (b) at (1,-2) [] {V}; \node (c) at (2,-2) [] {W}; \draw (a) edge[left,->] node[above] {$a_1$} (b) (b) edge[->] node[above] {$a_2$} (c); \draw (a) edge[bend right,<-] node[below right] {$a_3$} (b); \draw (a.south) edge[bend right=60,<-] node[below right] {$k_4$} (c); }$
\end{itemize}


in such a way that the line defined by the bottom of the letters $U,V,W$ is an horizontal line passing through the "bullet" of my item.

I tried to put -2 as y-coordinates but it doesn't change anything.

This is what the baseline key is for. I also add an alternative using positioning instead of absolute coordinates.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{positioning}
\begin{document}
\begin{itemize}
\item
\tikz[baseline={(a.base)}] {
\node (a) at (-0.3,-2) [] {U};
\node (b) at (1,-2) [] {V};
\node (c) at (2,-2) [] {W};
\draw (a) edge[left,->] node[above] {$a_1$} (b) (b) edge[->] node[above] {$a_2$} (c);
\draw (a) edge[bend right,<-] node[below right] {$a_3$} (b);
\draw (a.south) edge[bend right=60,<-] node[below right] {$k_4$} (c);
}
\item
\tikz[baseline={(a.base)},node distance=6mm] {
\node (a) {U};
\node[right=of a] (b) {V};
\node[right=of b] (c) {W};
\draw (a) edge[left,->] node[above] {$a_1$} (b) (b) edge[->] node[above] {$a_2$} (c);
\draw (a) edge[bend right,<-] node[below right] {$a_3$} (b);
\draw (a.south) edge[bend right=60,<-] node[below right] {$k_4$} (c);
}
\end{itemize}
\end{document}


your graph is simpler to draw with tikz-cd. it is positioned on item baseline:

\documentclass{article}
\usepackage{tikz-cd}

\begin{document}
\begin{itemize}
\item   \begin{tikzcd}
U \ar[r,"a_1"]
& V \ar[l,bend left,"a_3"] \ar[r,"a_2"]
& W\ar[ll,bend left=60,"k_4"]
\end{tikzcd}
\end{itemize}
\end{document}